Fast lenses, and High ISO

Started 2 months ago | Discussions
EinsteinsGhost
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Re: "fast" is relative
In reply to bobn2, 2 months ago

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

So what? If the speed of `the lens is according to the shutter speed, then I've increased the speed of the lens by increasing the 'sensitivity' (I haven't, by the way) because I can now set a faster shutter speed, have I not?

Yes, you can change ISO and increase or decrease the shutter speed. What does it have to do with whether the lens is fast or slow?

You say that the speed of the lens is determined by the shutter speed it allows you to set. If you increase the ISO it 'allows' you to set a faster shutter speed, therefore under your definition, you have made the lens faster, because now you can set a faster shutter speed.

NO. You don't make a lens faster by increasing ISO.

So you keep saying, but that is not consistent with your definition of lens 'speed', that it allows you to use a fast shutter speed.

Nope. You simply don't get it. Unlike you, I won't run around screaming that my travel zoom is a fast lens when I use the camera at high ISO.

You reduce exposure time value by increasing "sensitivity" of the sensor/film.

You really don't The sensitivity of the sensor never changes. You reduce exposure time value by deciding to use a smaller exposure.

Almost there! Where you didn't want to trust my prediction we were going.

So, why do you shoot Auto ISO?

This is why people like to use the term "Exposure Triangle".

Many people get very misled by the 'Exposure Triangle'. Looks like you are one of them.

Actually the argument in favor of Exposure Triangle looks like this: "If you increase the ISO it 'allows' you to set a faster shutter speed"

Do you agree? I do. But, what is happening there?

Put it another way an f/2 at 200 ISO is as fast as an f/1.4 at 100 ISO because you can set the same shutter speed.

Shutter speed is as fast. An f/2 lens isn't any faster.

You need a new definition of 'fast' then, because you can set the same shutter speed with both.

And yet, you couldn't use a faster shutter speed because you claim having reached a limit on your lens being f/4.5 .

so the shutter speed can be twice as fast to achieve same brightness level in the image. Your change in exposure is only due to change in "sensitivity" of the media (we've discussed that several times before, haven't we? Funny though, you seem to be opposed to the idea of playing with exposure with ISO changes)

I don't know what you mean. I don't remember ever having expressed opposition to 'playing with exposure with ISO changes'.

Trust me, you will be going there sometime very soon.

Why should I trust you when you say I have said things that I haven't?

You don't have to trust me.

I trust you to be wrong.

You don't have a choice.

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Bob

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Bob

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EinsteinsGhost
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Re: "the light"?
In reply to bobn2, 2 months ago

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

Albert Silver wrote:

tko wrote:

Remember that F4.0 is considered kind of slow on FF, but is equal to F2.0 on M43rds, which is considered "fast."

That's not entirely accurate. You are describing the depth of field equivalence, from one sensor to the next, not the light. f/2 on a m43rds may have the depth of field of f/4 on a full-frame, but the light will still be f/2.

What do you mean by "the light will still be f/2"? For a given scene luminance, shutter speed, and lens transmission, the density of light (exposure) projected on the sensor will be the same regardless of format, but not the total amount of light projected on the sensor.

Total light does not matter for correct exposure. It does not change the time value for which an exposure is made for a specific aperture value.

A competent mFT photographer with an EM10 + 12-40 / 2.8 shoots a scene at 25mm f/5.6 1/100 ISO 400. What settings would result

in the "correct exposure" if they had instead been using FF with a 6D + 24-70 / 2.8 VC?

For same exposure, any competent photographer would use f/5.6, 1/100 and ISO 400 on ANY format.

Would a competent photographer always use f/5.6, 1/100 and ISO 400 then?

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Bob

No. The choice depends on scene. We're talking about a comparison of exposure here (don't deflect).

I'm not deflecting. The issue is how a competent photographer would select exposure.

And, a competent photographer would know that f/5.6, 1/100 at ISO 400 indicates a set of exposure parameters that is not bound by format.

That's not an answer. The photographer could choose f/2.8, 1/100, ISO 200; f/2.8, 1/50, ISO 100; f/11, 1/100, ISO 800; f/11, 1/200, ISO 1600 etc. Different exposures which lead to the same final image brightness. So which one should the competent photographer be choosing?

Same exposure as f/5.6, 1/100s, ISO 400. In fact, that is exactly what you arrived at with several possible combinations (something you don't seem to get in your own arguments above). Instead, there you claim, only one set of values provides you with "maximum" exposure.

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67gtonr
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Re: Fast lenses, and High ISO
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

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EinsteinsGhost
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Re: Fast lenses, and High ISO
In reply to 67gtonr, 2 months ago

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

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67gtonr
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Re: Fast lenses, and High ISO
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

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EinsteinsGhost
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Re: Fast lenses, and High ISO
In reply to 67gtonr, 2 months ago

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

No, the lens via SB works as if it is a 35/1 lens. But, if you put 35mm lens on APS-C, you get (approx) field of view of 50mm lens on FF. The DOF equivalence is f/1.5. However, exposure is determined for f/1.

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67gtonr
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Re: Fast lenses, and High ISO
In reply to 67gtonr, 2 months ago

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

I see what your saying, that the lens & booster gave you the same results as using a 35/1 lens on your NEX

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67gtonr
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Re: Fast lenses, and High ISO
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

No, the lens via SB works as if it is a 35/1 lens. But, if you put 35mm lens on APS-C, you get (approx) field of view of 50mm lens on FF. The DOF equivalence is f/1.5. However, exposure is determined for f/1.

My last reply was being typed as this was being posted. I was initially reading it that the booster was giving an image w/ a 35mmFOV and a F1 equivalent picture and that is not how I understood Speedboosters to work.

So does the auto exposure meter for the f1 correctly or do you adjust it manually?

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EinsteinsGhost
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Re: Fast lenses, and High ISO
In reply to 67gtonr, 2 months ago

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

No, the lens via SB works as if it is a 35/1 lens. But, if you put 35mm lens on APS-C, you get (approx) field of view of 50mm lens on FF. The DOF equivalence is f/1.5. However, exposure is determined for f/1.

My last reply was being typed as this was being posted. I was initially reading it that the booster was giving an image w/ a 35mmFOV and a F1 equivalent picture and that is not how I understood Speedboosters to work.

So does the auto exposure meter for the f1 correctly or do you adjust it manually?

Auto works (you can use the camera in any mode).

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67gtonr
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Re: Fast lenses, and High ISO
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

No, the lens via SB works as if it is a 35/1 lens. But, if you put 35mm lens on APS-C, you get (approx) field of view of 50mm lens on FF. The DOF equivalence is f/1.5. However, exposure is determined for f/1.

My last reply was being typed as this was being posted. I was initially reading it that the booster was giving an image w/ a 35mmFOV and a F1 equivalent picture and that is not how I understood Speedboosters to work.

So does the auto exposure meter for the f1 correctly or do you adjust it manually?

Auto works (you can use the camera in any mode).

Thanks, good to know.

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Great Bustard
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Re: You forgot something.
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

You're wrong about that, unless you mean that it is also wrongly used for DOF based arguments. Fast is about speed (faster the lens, shorter the exposure time for identical conditions).

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

Obviously, you do not understand ISO. Why would a FF photographer feel compelled to shoot the same ISO as the mFT photographer?

I don't see a reason for a FF photographer feeling compelled to shoot an ISO 100 shot at ISO 3200.

Neither do I:

http://www.josephjamesphotography.com/equivalence/#purpose

If one system can take a photo that another system cannot, and that results in a "better" photo, then, of course, we would do so.

I see both photographers to shoot at the lowest possible ISO and in this case, it will be the base ISO.

It's fair to say that if f/2 meters at 1/4000, and thus f/4 would meter for 1/1000, then, sure, 1/1000 is more than likely to be "fast enough" in almost all circumstances. What happens with f/2 meters at 1/400? Will f/4 at 1/100 be "fast enough"?

Please tell us about the exposure differences here as they relate to the visual properties of the photo:

  • 50mm f/2 1/100 ISO 400 on mFT
  • 100mm f/2 1/100 ISO 400 on FF
  • 100mm f/4 1/100 ISO 1600 on FF

Same exposure on first and second (f/2, 1/100s, ISO 400). Lower exposure in third by two stops.

That is correct! However, you forgot to tell us how this relates to the visual properties of the photo.

The visual property that an exposure is all about is brightness of the scene. For same brightness, same exposure is expected. With higher ISO (third bullet above), you're doing just that, increasing brightness by two stop to compensate for reduced exposure by two stops.

So what we see here, really, is that exposure is merely part of the equation:

  • Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)
  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)
  • Total Light (photons) = Exposure (photons / mm²) · Effective Sensor Area (mm²)
  • Total Light Collected (electrons) = Total Light (photons) · QE (electrons / photon)
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Great Bustard
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Re: You missed the question.
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

A competent mFT photographer with an EM10 + 12-40 / 2.8 shoots a scene at 25mm f/5.6 1/100 ISO 400. What settings would result in the "correct exposure" if they had instead been using FF with a 6D + 24-70 / 2.8 VC?

For same exposure...

I didn't ask about the setting for the same exposure -- I asked about the settings that resulted in the "correct exposure".

Tell me what you mean by "correct exposure".

That's what I was hoping you would answer.

...any competent photographer would use f/5.6, 1/100 and ISO 400 on ANY format. This, of course assumes same shooting conditions, transmissive properties of the lens, and metering used in respective cameras.

Why would a competent photographer necessarily use the same exposure with different formats?

Because a competent photographer knows exposure is not dependent on format. It is dependent on: ISO, Aperture and Shutter values for a given scene brightness. That is it.

So, for a given scene luminance, f/2 1/100 ISO 400 and f/4 1/100 ISO 1600 have the same exposure? That's not what you said in another post. You said that the f/4 photo had two stops lower exposure.

So, in what way does the density of light falling on the sensor matter more than the total amount of light falling on the sensor?

The answer to the "correct exposure" question should make it easier to answer the question immediately above.

Only if you knew what "correct exposure" is. Can you quantify and describe it?

That's what I was asking you. Here, I'll repeat the question:

A competent mFT photographer with an EM10 + 12-40 / 2.8 shoots a scene at 25mm f/5.6 1/100 ISO 400. What settings would result in the "correct exposure" if they had instead been using FF with a 6D + 24-70 / 2.8 VC?

Note that I am asking for the settings for the "correct exposure" (whatever you think makes that exposure "correct" -- that's the reason for the question), not the settings for the same exposure.

Correct exposure in a comparison would entail, same exposure.

Ah -- so if f/2 1/100 ISO 400 on mFT gives the "correct exposure" then f/2 1/100 ISO 400 on FF would also give the "correct exposure"? So, if you used a different exposure on FF, then it would be "incorrect"?

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Great Bustard
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Aside from DOF and noise...
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

I don't care about different people. I only care about why I would call a lens fast.

...why do you care about the "speed" of a lens?

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bobn2
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Re: "the light"?
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

Albert Silver wrote:

tko wrote:

Remember that F4.0 is considered kind of slow on FF, but is equal to F2.0 on M43rds, which is considered "fast."

That's not entirely accurate. You are describing the depth of field equivalence, from one sensor to the next, not the light. f/2 on a m43rds may have the depth of field of f/4 on a full-frame, but the light will still be f/2.

What do you mean by "the light will still be f/2"? For a given scene luminance, shutter speed, and lens transmission, the density of light (exposure) projected on the sensor will be the same regardless of format, but not the total amount of light projected on the sensor.

Total light does not matter for correct exposure. It does not change the time value for which an exposure is made for a specific aperture value.

A competent mFT photographer with an EM10 + 12-40 / 2.8 shoots a scene at 25mm f/5.6 1/100 ISO 400. What settings would result

in the "correct exposure" if they had instead been using FF with a 6D + 24-70 / 2.8 VC?

For same exposure, any competent photographer would use f/5.6, 1/100 and ISO 400 on ANY format.

Would a competent photographer always use f/5.6, 1/100 and ISO 400 then?

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No. The choice depends on scene. We're talking about a comparison of exposure here (don't deflect).

I'm not deflecting. The issue is how a competent photographer would select exposure.

And, a competent photographer would know that f/5.6, 1/100 at ISO 400 indicates a set of exposure parameters that is not bound by format.

That's not an answer. The photographer could choose f/2.8, 1/100, ISO 200; f/2.8, 1/50, ISO 100; f/11, 1/100, ISO 800; f/11, 1/200, ISO 1600 etc. Different exposures which lead to the same final image brightness. So which one should the competent photographer be choosing?

Same exposure as f/5.6, 1/100s, ISO 400.

No, they are all different exposures, one stop apart.

In fact, that is exactly what you arrived at with several possible combinations

As I said, they are all different exposures. So, your problem here is arguing from ignorance. You won't get much further unless you learn some photographic basics.

(something you don't seem to get in your own arguments above). Instead, there you claim, only one set of values provides you with "maximum" exposure.

Only one set here does give 'maximum' exposure, f/2.8, 1/50 ISO 100. Why wouldn't you use that? We don't have enough information to know - maybe not enough DOF at f/2.,8, maybe too much camera shake at 1/50 - assuming that the competent photographer chose f/5.6, 1/100 for a good reason. Still, maybe you should go and learn some basic photography before we continue. First, what 'exposure' means.

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Re: "fast" is relative
In reply to EinsteinsGhost, 2 months ago

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

So what? If the speed of `the lens is according to the shutter speed, then I've increased the speed of the lens by increasing the 'sensitivity' (I haven't, by the way) because I can now set a faster shutter speed, have I not?

Yes, you can change ISO and increase or decrease the shutter speed. What does it have to do with whether the lens is fast or slow?

You say that the speed of the lens is determined by the shutter speed it allows you to set. If you increase the ISO it 'allows' you to set a faster shutter speed, therefore under your definition, you have made the lens faster, because now you can set a faster shutter speed.

NO. You don't make a lens faster by increasing ISO.

So you keep saying, but that is not consistent with your definition of lens 'speed', that it allows you to use a fast shutter speed.

Nope. You simply don't get it. Unlike you, I won't run around screaming that my travel zoom is a fast lens when I use the camera at high ISO.

Well you should be, according to your own definition of 'fast'. I don't use your definition, so I don't have that problem. Remember we are here because you claimed that yours was the only definition of fast. Well, it had better not be because it's a pretty poor definition, as we've seen.

You reduce exposure time value by increasing "sensitivity" of the sensor/film.

You really don't The sensitivity of the sensor never changes. You reduce exposure time value by deciding to use a smaller exposure.

Almost there! Where you didn't want to trust my prediction we were going.

So, why do you shoot Auto ISO?

By and large, I don't. I haven't found an Auto ISO which can be configured to do the right thing with respect to the exposure of the shot.

This is why people like to use the term "Exposure Triangle".

Many people get very misled by the 'Exposure Triangle'. Looks like you are one of them.

Actually the argument in favor of Exposure Triangle looks like this: "If you increase the ISO it 'allows' you to set a faster shutter speed"

That's not an argument in favour of the 'Exposure Triangle'. Firstly the 'triangle' geometry doesn't work. Secondly, the effect of increasing ISO is very different from the effect of increasing exposure. Increasing ISO resally doesn't 'allow' you to use a faster shutter speed, it just signifies that you've decided to use a lower exposure, so yiou can set a faster shutter speed. So now we come to the next question, why might one not allow oneself to use a smaller exposure?

Do you agree?

No

I do.

That doesn't surprise me at all.

But, what is happening there?

What has happened is that your lack of knowledge of the basics of photography has allowed you to be deceived by a popular but flawed mnemonic graphic into thinking that the ISO control is 'allowing' you to use a smaller exposure,when all that is happened is that you have decided to use a smaller exposure and used the ISO control to set the camera up for that exposure.

Put it another way an f/2 at 200 ISO is as fast as an f/1.4 at 100 ISO because you can set the same shutter speed.

Shutter speed is as fast. An f/2 lens isn't any faster.

You need a new definition of 'fast' then, because you can set the same shutter speed with both.

And yet, you couldn't use a faster shutter speed because you claim having reached a limit on your lens being f/4.5 .

Of course I could have set a faster shutter speed, but that would have reduced exposure. In manual, I can set any combination of values that I like. Looks to me like you camera is controlling you, rather than you controlling your camera.

so the shutter speed can be twice as fast to achieve same brightness level in the image. Your change in exposure is only due to change in "sensitivity" of the media (we've discussed that several times before, haven't we? Funny though, you seem to be opposed to the idea of playing with exposure with ISO changes)

I don't know what you mean. I don't remember ever having expressed opposition to 'playing with exposure with ISO changes'.

Trust me, you will be going there sometime very soon.

Why should I trust you when you say I have said things that I haven't?

You don't have to trust me.

I trust you to be wrong.

You don't have a choice.

I have a choice whether or not to trust you, though if I trust you to be wrong, I know that my trust will be rewarded amply.

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Mahmoud Mousef
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Re: Fast lenses, and High ISO
In reply to Chikoo, 2 months ago

Chikoo wrote:

That said, should they be called Fast Lenses or Shallow Lenses?

I would call mine: CatEye!

http://en.wikipedia.org/wiki/Cat_senses#Sight

The tapetum and other mechanisms give the cat a minimum light detection threshold up to seven times lower than that of humans. Variation in color of cats' eyes in flash photographs is largely due to the reflection of the flash by the tapetum.

I can't wait for sensors to progress beyond what we have now, making fast lenses less necessary for some shooting scenarios.

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Re: You forgot something.
In reply to Great Bustard, 2 months ago

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

You're wrong about that, unless you mean that it is also wrongly used for DOF based arguments. Fast is about speed (faster the lens, shorter the exposure time for identical conditions).

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

Obviously, you do not understand ISO. Why would a FF photographer feel compelled to shoot the same ISO as the mFT photographer?

I don't see a reason for a FF photographer feeling compelled to shoot an ISO 100 shot at ISO 3200.

Neither do I:

http://www.josephjamesphotography.com/equivalence/#purpose

If one system can take a photo that another system cannot, and that results in a "better" photo, then, of course, we would do so.

I see both photographers to shoot at the lowest possible ISO and in this case, it will be the base ISO.

It's fair to say that if f/2 meters at 1/4000, and thus f/4 would meter for 1/1000, then, sure, 1/1000 is more than likely to be "fast enough" in almost all circumstances. What happens with f/2 meters at 1/400? Will f/4 at 1/100 be "fast enough"?

Please tell us about the exposure differences here as they relate to the visual properties of the photo:

  • 50mm f/2 1/100 ISO 400 on mFT
  • 100mm f/2 1/100 ISO 400 on FF
  • 100mm f/4 1/100 ISO 1600 on FF

Same exposure on first and second (f/2, 1/100s, ISO 400). Lower exposure in third by two stops.

That is correct! However, you forgot to tell us how this relates to the visual properties of the photo.

The visual property that an exposure is all about is brightness of the scene. For same brightness, same exposure is expected. With higher ISO (third bullet above), you're doing just that, increasing brightness by two stop to compensate for reduced exposure by two stops.

So what we see here, really, is that exposure is merely part of the equation:

  • Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)

Not exactly. Luminous exposure, which is what we use in photography, is measured in luminous flux times time, which is luminous energy. Not quite the same thing as 'photons / mm². If we were radiologists using radiometric exposure, then we'd be measuring that in W/m², and it would become clearer that it is a power density, and integrated over time it becomes an energy density, which can then be related to a photon count, given some assumed distribution of the photon energies. The relationship between radiometric and luminous exposure is that luminous exposure is weighted by the luminosity function, that is, includes only visible energy. So, while our cameras do in fact work as photon counters, that is an approximation tio what they should be doing photographically (film cameras are also photon counters, they just count photons in pairs and their QE is rather low).

  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)

No, no, no. You don't get 'brightness' by 'amplifying' exposure. The luminance of a viewed image depends on what you're using to view it. The luminosity of your TV,monitor or projector or the strength of the light that you're using to view a print. It also continues to provide light as long as you view it, it's luminous energy is not limited by the luminous energy in the exposure. So, not 'amplification' at all. This is the root of the 'ISO' misunderstanding.  So, the output of`the photo isn't a luminance. In film days it was a 'density', in digital days it's a file value denoting a grey scale. It simply represents the value from black to whiter than white that we want this exposure to represent (white is set a bit grey so we can have convincing light sources and specular reflections in our photos, if what was white, they'd just look white). Generally rather than 'amplification' it is a 'mapping', which  maps the set of exposures which the sensor measures to a set of grey scale values which will include gamma correction and very likely a film like S- curve.

  • Total Light (photons) = Exposure (photons / mm²) · Effective Sensor Area (mm²)
  • Total Light Collected (electrons) = Total Light (photons) · QE (electrons / photon)
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Great Bustard
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Exposure and brightness.
In reply to bobn2, 2 months ago

bobn2 wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

You're wrong about that, unless you mean that it is also wrongly used for DOF based arguments. Fast is about speed (faster the lens, shorter the exposure time for identical conditions).

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

Obviously, you do not understand ISO. Why would a FF photographer feel compelled to shoot the same ISO as the mFT photographer?

I don't see a reason for a FF photographer feeling compelled to shoot an ISO 100 shot at ISO 3200.

Neither do I:

http://www.josephjamesphotography.com/equivalence/#purpose

If one system can take a photo that another system cannot, and that results in a "better" photo, then, of course, we would do so.

I see both photographers to shoot at the lowest possible ISO and in this case, it will be the base ISO.

It's fair to say that if f/2 meters at 1/4000, and thus f/4 would meter for 1/1000, then, sure, 1/1000 is more than likely to be "fast enough" in almost all circumstances. What happens with f/2 meters at 1/400? Will f/4 at 1/100 be "fast enough"?

Please tell us about the exposure differences here as they relate to the visual properties of the photo:

  • 50mm f/2 1/100 ISO 400 on mFT
  • 100mm f/2 1/100 ISO 400 on FF
  • 100mm f/4 1/100 ISO 1600 on FF

Same exposure on first and second (f/2, 1/100s, ISO 400). Lower exposure in third by two stops.

That is correct! However, you forgot to tell us how this relates to the visual properties of the photo.

The visual property that an exposure is all about is brightness of the scene. For same brightness, same exposure is expected. With higher ISO (third bullet above), you're doing just that, increasing brightness by two stop to compensate for reduced exposure by two stops.

So what we see here, really, is that exposure is merely part of the equation:

  • Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)

Not exactly. Luminous exposure, which is what we use in photography, is measured in luminous flux times time, which is luminous energy. Not quite the same thing as 'photons / mm². If we were radiologists using radiometric exposure, then we'd be measuring that in W/m², and it would become clearer that it is a power density, and integrated over time it becomes an energy density, which can then be related to a photon count, given some assumed distribution of the photon energies. The relationship between radiometric and luminous exposure is that luminous exposure is weighted by the luminosity function, that is, includes only visible energy. So, while our cameras do in fact work as photon counters, that is an approximation tio what they should be doing photographically (film cameras are also photon counters, they just count photons in pairs and their QE is rather low).

We discussed this a while back:

http://www.dpreview.com/forums/post/39448295

So are you saying, for example, that one billion red photons falling on the sensor does not result in the same exposure as one billion blue photons falling on the sensor? If a room were illuminated by a 100W red LED, would the camera meter differently than if it the room were illuminated by a 100W blue LED?

  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)

No, no, no. You don't get 'brightness' by 'amplifying' exposure. The luminance of a viewed image depends on what you're using to view it. The luminosity of your TV,monitor or projector or the strength of the light that you're using to view a print. It also continues to provide light as long as you view it, it's luminous energy is not limited by the luminous energy in the exposure. So, not 'amplification' at all. This is the root of the 'ISO' misunderstanding. So, the output of`the photo isn't a luminance. In film days it was a 'density', in digital days it's a file value denoting a grey scale. It simply represents the value from black to whiter than white that we want this exposure to represent (white is set a bit grey so we can have convincing light sources and specular reflections in our photos, if what was white, they'd just look white). Generally rather than 'amplification' it is a 'mapping', which maps the set of exposures which the sensor measures to a set of grey scale values which will include gamma correction and very likely a film like S- curve.

When I talk about "brightness", I was thinking in terms of the nominal values for the color channels in the image file.  For example, (200, 200, 200) is "brighter" than (100, 100, 100). Now, I'm sure the mapping is non-linear, so I'm not saying that (200, 200, 200) is twice as bright as (100, 100, 100). But that's what I meant by "brightness".  (Note that "amplification" does not necessarily mean "gain", although an analog gain could be part of the amplification).

For example, if we took a photo of a white wall at f/4 1/100 ISO 400 on mFT and f/8 1/100 ISO 1600 on FF, and the ISOs were calibrated the same, then if a pixel for mFT read (100, 100, 100), it would read the same on FF. On the other hand, if the FF photo were taken at f/8 1/100 ISO 400, then the nominal values for the color channels in the image file would be lower -- "less bright".

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bobn2
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Re: Exposure and brightness.
In reply to Great Bustard, 2 months ago

Great Bustard wrote:

bobn2 wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

You're wrong about that, unless you mean that it is also wrongly used for DOF based arguments. Fast is about speed (faster the lens, shorter the exposure time for identical conditions).

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

Obviously, you do not understand ISO. Why would a FF photographer feel compelled to shoot the same ISO as the mFT photographer?

I don't see a reason for a FF photographer feeling compelled to shoot an ISO 100 shot at ISO 3200.

Neither do I:

http://www.josephjamesphotography.com/equivalence/#purpose

If one system can take a photo that another system cannot, and that results in a "better" photo, then, of course, we would do so.

I see both photographers to shoot at the lowest possible ISO and in this case, it will be the base ISO.

It's fair to say that if f/2 meters at 1/4000, and thus f/4 would meter for 1/1000, then, sure, 1/1000 is more than likely to be "fast enough" in almost all circumstances. What happens with f/2 meters at 1/400? Will f/4 at 1/100 be "fast enough"?

Please tell us about the exposure differences here as they relate to the visual properties of the photo:

  • 50mm f/2 1/100 ISO 400 on mFT
  • 100mm f/2 1/100 ISO 400 on FF
  • 100mm f/4 1/100 ISO 1600 on FF

Same exposure on first and second (f/2, 1/100s, ISO 400). Lower exposure in third by two stops.

That is correct! However, you forgot to tell us how this relates to the visual properties of the photo.

The visual property that an exposure is all about is brightness of the scene. For same brightness, same exposure is expected. With higher ISO (third bullet above), you're doing just that, increasing brightness by two stop to compensate for reduced exposure by two stops.

So what we see here, really, is that exposure is merely part of the equation:

  • Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)

Not exactly. Luminous exposure, which is what we use in photography, is measured in luminous flux times time, which is luminous energy. Not quite the same thing as 'photons / mm². If we were radiologists using radiometric exposure, then we'd be measuring that in W/m², and it would become clearer that it is a power density, and integrated over time it becomes an energy density, which can then be related to a photon count, given some assumed distribution of the photon energies. The relationship between radiometric and luminous exposure is that luminous exposure is weighted by the luminosity function, that is, includes only visible energy. So, while our cameras do in fact work as photon counters, that is an approximation tio what they should be doing photographically (film cameras are also photon counters, they just count photons in pairs and their QE is rather low).

We discussed this a while back:

http://www.dpreview.com/forums/post/39448295

So are you saying, for example, that one billion red photons falling on the sensor does not result in the same exposure as one billion blue photons falling on the sensor? If a room were illuminated by a 100W red LED, would the camera meter differently than if it the room were illuminated by a 100W blue LED?

It should. Exposure is, as you know, measured in lux seconds. Read about what the 'lux' is here: http://en.wikipedia.org/wiki/Lux. Specifically it is lumens per square meter. Lumens here: http://en.wikipedia.org/wiki/Lumen_(unit)

Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all electromagnetic waves emitted, independent of the eye's ability to perceive it.

The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.

http://en.wikipedia.org/wiki/Luminosity_function

Photopic (black) and scotopic (green) luminosity functions.[c 1] The photopic includes the CIE 1931 standard[c 2] (solid), the Judd–Vos 1978 modified data[c 3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data[c 4] (dotted). The horizontal axis is wavelength in nm.

So, indeed green and red (or blue) light is differently weighted with respect to exposure.

  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)

No, no, no. You don't get 'brightness' by 'amplifying' exposure. The luminance of a viewed image depends on what you're using to view it. The luminosity of your TV,monitor or projector or the strength of the light that you're using to view a print. It also continues to provide light as long as you view it, it's luminous energy is not limited by the luminous energy in the exposure. So, not 'amplification' at all. This is the root of the 'ISO' misunderstanding. So, the output of`the photo isn't a luminance. In film days it was a 'density', in digital days it's a file value denoting a grey scale. It simply represents the value from black to whiter than white that we want this exposure to represent (white is set a bit grey so we can have convincing light sources and specular reflections in our photos, if what was white, they'd just look white). Generally rather than 'amplification' it is a 'mapping', which maps the set of exposures which the sensor measures to a set of grey scale values which will include gamma correction and very likely a film like S- curve.

When I talk about "brightness", I was thinking in terms of the nominal values for the color channels in the image file. For example, (200, 200, 200) is "brighter" than (100, 100, 100). Now, I'm sure the mapping is non-linear, so I'm not saying that (200, 200, 200) is twice as bright as (100, 100, 100). But that's what I meant by "brightness". (Note that "amplification" does not necessarily mean "gain", although an analog gain could be part of the amplification).

The word 'amplification' implies that it is linear (unless you qualify that it isn't) and the proposition that is is unitless implies that you are getting the same thing out as goes in. That is false. The photographic reproduction chain falls short of the viewers eye. It isn't a case of reproducing the light that was incident on the sensor - that is the mistake of thinking that makes people believe that somehow ISO is a magic light amplifier. Best to steer clear from any such misleading terminology. There is no 'amplification' needed - because the output from the photographic process is the analogue of what was in film days a 'density' - simply a filter value from letting no light through (or reflecting no light) to letting some arbitrary amount greater than zero through. Film ISOs are defined in terms of exposure to produce a given density and digital ISOs in terms of exposure to produce a given file value. It absolutely is not either 'gain' or 'amplification'. As I said, once you start thinking that way, confusion results, so best not to propagate such a confusing

For example, if we took a photo of a white wall at f/4 1/100 ISO 400 on mFT and f/8 1/100 ISO 1600 on FF, and the ISOs were calibrated the same, then if a pixel for mFT read (100, 100, 100), it would read the same on FF. On the other hand, if the FF photo were taken at f/8 1/100 ISO 400, then the nominal values for the color channels in the image file would be lower -- "less bright".

Yes, because ISO 1600 maps one quarter of the exposure to 12.7% (or 100% depending on the ISO you want to use) than does ISO 400. There is no 'amplification', just a different mapping, or 'scaling' if you like. The important thing is what's coming out isn't light, so the idea that there is some unitless 'gain' or 'amplification' in there is dead wrong.

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Great Bustard
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Re: Exposure and brightness.
In reply to bobn2, 2 months ago

bobn2 wrote:

Great Bustard wrote:

bobn2 wrote:

Not exactly. Luminous exposure, which is what we use in photography, is measured in luminous flux times time, which is luminous energy. Not quite the same thing as 'photons / mm². If we were radiologists using radiometric exposure, then we'd be measuring that in W/m², and it would become clearer that it is a power density, and integrated over time it becomes an energy density, which can then be related to a photon count, given some assumed distribution of the photon energies. The relationship between radiometric and luminous exposure is that luminous exposure is weighted by the luminosity function, that is, includes only visible energy. So, while our cameras do in fact work as photon counters, that is an approximation tio what they should be doing photographically (film cameras are also photon counters, they just count photons in pairs and their QE is rather low).

We discussed this a while back:

http://www.dpreview.com/forums/post/39448295

So are you saying, for example, that one billion red photons falling on the sensor does not result in the same exposure as one billion blue photons falling on the sensor? If a room were illuminated by a 100W red LED, would the camera meter differently than if it the room were illuminated by a 100W blue LED?

It should. Exposure is, as you know, measured in lux seconds. Read about what the 'lux' is here: http://en.wikipedia.org/wiki/Lux. Specifically it is lumens per square meter. Lumens here: http://en.wikipedia.org/wiki/Lumen_(unit)

Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all electromagnetic waves emitted, independent of the eye's ability to perceive it.

The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.

http://en.wikipedia.org/wiki/Luminosity_function

Photopic (black) and scotopic (green) luminosity functions.[c 1] The photopic includes the CIE 1931 standard[c 2] (solid), the Judd–Vos 1978 modified data[c 3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data[c 4] (dotted). The horizontal axis is wavelength in nm.

So, indeed green and red (or blue) light is differently weighted with respect to exposure.

Would it be correct to say that the density of light falling on the sensor (photons / mm²) is mapped into the exposure via the luminosity function?

  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)

No, no, no. You don't get 'brightness' by 'amplifying' exposure. The luminance of a viewed image depends on what you're using to view it. The luminosity of your TV,monitor or projector or the strength of the light that you're using to view a print. It also continues to provide light as long as you view it, it's luminous energy is not limited by the luminous energy in the exposure. So, not 'amplification' at all. This is the root of the 'ISO' misunderstanding. So, the output of`the photo isn't a luminance. In film days it was a 'density', in digital days it's a file value denoting a grey scale. It simply represents the value from black to whiter than white that we want this exposure to represent (white is set a bit grey so we can have convincing light sources and specular reflections in our photos, if what was white, they'd just look white). Generally rather than 'amplification' it is a 'mapping', which maps the set of exposures which the sensor measures to a set of grey scale values which will include gamma correction and very likely a film like S- curve.

When I talk about "brightness", I was thinking in terms of the nominal values for the color channels in the image file. For example, (200, 200, 200) is "brighter" than (100, 100, 100). Now, I'm sure the mapping is non-linear, so I'm not saying that (200, 200, 200) is twice as bright as (100, 100, 100). But that's what I meant by "brightness". (Note that "amplification" does not necessarily mean "gain", although an analog gain could be part of the amplification).

The word 'amplification' implies that it is linear (unless you qualify that it isn't) and the proposition that is is unitless implies that you are getting the same thing out as goes in. That is false. The photographic reproduction chain falls short of the viewers eye. It isn't a case of reproducing the light that was incident on the sensor - that is the mistake of thinking that makes people believe that somehow ISO is a magic light amplifier. Best to steer clear from any such misleading terminology. There is no 'amplification' needed - because the output from the photographic process is the analogue of what was in film days a 'density' - simply a filter value from letting no light through (or reflecting no light) to letting some arbitrary amount greater than zero through. Film ISOs are defined in terms of exposure to produce a given density and digital ISOs in terms of exposure to produce a given file value. It absolutely is not either 'gain' or 'amplification'. As I said, once you start thinking that way, confusion results, so best not to propagate such a confusing

For example, if we took a photo of a white wall at f/4 1/100 ISO 400 on mFT and f/8 1/100 ISO 1600 on FF, and the ISOs were calibrated the same, then if a pixel for mFT read (100, 100, 100), it would read the same on FF. On the other hand, if the FF photo were taken at f/8 1/100 ISO 400, then the nominal values for the color channels in the image file would be lower -- "less bright".

Yes, because ISO 1600 maps one quarter of the exposure to 12.7% (or 100% depending on the ISO you want to use) than does ISO 400. There is no 'amplification', just a different mapping, or 'scaling' if you like. The important thing is what's coming out isn't light, so the idea that there is some unitless 'gain' or 'amplification' in there is dead wrong.

OK, this is a little more work. Tell me how you like this:

A certain number of photons fall on a pixel releasing a certain number of electrons which generates a charge. A gain may, or may not, be applied to this charge as a function of the ISO setting on the camera. The charge is then converted into a digital number by the ADC (Analog to Digital Converter). The RAW converter (or in-camera JPG engine) processes groups of digital numbers into RGB values for the image file where the ISO setting on the camera maps these values so that they have the brightness that the corresponds to the exposure and ISO setting.

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