Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions
Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

bobn2 wrote:

Beachcomber Joe wrote:

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same. A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

Completely wrong. What matters is the total number of photons captured. Differences in exposure can always be compensated by simply changing the relationship between exposure and output grey scale. The total light locks in a noise pattern in the image, which can't be altered apart from noise reduction, which will usually lose you image detail. Those wishing to maximise image quality deal in total light. Of course, if you stick to one sensor size, there is no difference between total light and exposure.

Some people may not have known what I meant by the term technobabble.  Thank you for providing such an excellent example.

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Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

Great Bustard wrote:

Beachcomber Joe wrote:

Lee Jay wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

Well, then it's always been thought of wrong. f-stop measures light intensity (illuminance) not light gathering ability (total light). Yes, they go together if the sensor size is constant.

That is certainly true if that is the way you choose to define the two terms. I happen to be a photographer so, like most photographers, my definitions are based on real world and may be less specific than those in a physics lab. In my world light gathering ability is understood to be the ability to put a certain amount of light on a specific area. What you choose to call illuminance.

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format.

Yes, it does. A bright light on a small surface means a relatively small amount of total light compared with a dimmer light over a much larger surface. Total light controls image quality (noise) and noise controls how fast your shutter speed can go.

You may continue to play with, among other things, semantics. The light gathering ability of a photographic lens is measured by the amount of light per unit of area.

Sure. But the same light per unit of area, in terms of the visual properties of the photo, has different effects on different formats.

- SNIP -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

pavi1 Veteran Member • Posts: 6,868
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Because it is Babel.

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Just another Canon shooter
Just another Canon shooter Veteran Member • Posts: 4,691
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

bobn2 wrote:

Beachcomber Joe wrote:

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same. A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

Completely wrong. What matters is the total number of photons captured. Differences in exposure can always be compensated by simply changing the relationship between exposure and output grey scale. The total light locks in a noise pattern in the image, which can't be altered apart from noise reduction, which will usually lose you image detail. Those wishing to maximise image quality deal in total light. Of course, if you stick to one sensor size, there is no difference between total light and exposure.

Some people may not have known what I meant by the term technobabble. Thank you for providing such an excellent example.

More technobabble here. And this is the real stuff.

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Erik Magnuson Forum Pro • Posts: 12,247
Re: Let's correct it, then.

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term.  A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

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Erik

Tim Tucker Senior Member • Posts: 1,337
Re: Whoops!

Lee Jay wrote:

Tim Tucker wrote:

Are we just talking about noise generation in digital sensors here?

No, we're talking about the noise that's in the light itself (due to the quantum nature of light). And the signal to noise ratio that such noise generates on its own is equal to the square root of the total number of photons. More SNR = good, therefore more light = good. More light comes from either brighter light (faster f-stop) or more capture area (bigger sensor). Given the same f-stop, a sensor with 16 times more sensor area will capture 16 times as much light and therefore have four times the signal to noise ratio, which is the same as if the small sensor had been using an f-stop four stops faster with the same shutter speed.

IOW, FF 1/60th f/8 = 4x-crop 1/60th f/2, as far as signal to noise ratio goes.

Now that's not something that I've ever thought of before.

But I can see that you are right, QED backs this up and I will take your word on the maths.

Lee Jay Forum Pro • Posts: 55,372
Re: Let's correct it, then.

Tim Tucker wrote:

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

No, all f/2 lenses transmit the same light intensity (illuminance), but the total light is the illuminance multiplied by the sensor area, so different sensor sizes collect different total amounts for the same exposure.

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Lee Jay

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Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

Erik Magnuson wrote:

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term. A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

Eh? Total amount IS a precise term. Measure your volume in litres and you have a precise answer. But that is not what I asked.

Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

Lee Jay wrote:

Tim Tucker wrote:

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

No, all f/2 lenses transmit the same light intensity (illuminance), but the total light is the illuminance multiplied by the sensor area, so different sensor sizes collect different total amounts for the same exposure.

Thank you, this is now getting clearer.

Lee Jay Forum Pro • Posts: 55,372
Re: Whoops!

Tim Tucker wrote:

Lee Jay wrote:

Tim Tucker wrote:

Are we just talking about noise generation in digital sensors here?

No, we're talking about the noise that's in the light itself (due to the quantum nature of light). And the signal to noise ratio that such noise generates on its own is equal to the square root of the total number of photons. More SNR = good, therefore more light = good. More light comes from either brighter light (faster f-stop) or more capture area (bigger sensor). Given the same f-stop, a sensor with 16 times more sensor area will capture 16 times as much light and therefore have four times the signal to noise ratio, which is the same as if the small sensor had been using an f-stop four stops faster with the same shutter speed.

IOW, FF 1/60th f/8 = 4x-crop 1/60th f/2, as far as signal to noise ratio goes.

Now that's not something that I've ever thought of before.

But I can see that you are right, QED backs this up and I will take your word on the maths.

You can read about it here:

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

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Lee Jay

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Erik Magnuson Forum Pro • Posts: 12,247
Re: Let's correct it, then.

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term. A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

Eh? Total amount IS a precise term. Measure your volume in litres and you have a precise answer. But that is not what I asked.

It was the answer to your first question and use of "amount."   All f/2 lenses do not transmit the same amount of light or my f/2 cell phone lens would transmit as much light as a 200mm f/2 lens.  Even if I managed to put a 24x36mm sensor behind the call phone cam lens, it would not help much.  What the lens transmits and what the sensor collects may be related, but they are not the same thing.

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Erik

bobn2
bobn2 Forum Pro • Posts: 64,270
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

bobn2 wrote:

Beachcomber Joe wrote:

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same. A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

Completely wrong. What matters is the total number of photons captured. Differences in exposure can always be compensated by simply changing the relationship between exposure and output grey scale. The total light locks in a noise pattern in the image, which can't be altered apart from noise reduction, which will usually lose you image detail. Those wishing to maximise image quality deal in total light. Of course, if you stick to one sensor size, there is no difference between total light and exposure.

Some people may not have known what I meant by the term technobabble. Thank you for providing such an excellent example.

Technobabble it might be, but it is true technobabble, and you have no way of refuting it. So, when you  'Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.' you're still 100% wrong. Choose to call what you're incapable of understanding 'technobabble' if you like, it doesn't make you any less wrong.

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Bob

JBurnett Contributing Member • Posts: 835
Re: A Camera Club Perspective

Erik Magnuson wrote:

JBurnett wrote:

  • I would estimate that 35 of them purchased their APS-C cameras with the kit lens, and had no concept of equivalencies at the time.

YMMV. In my club 35mm equivalent focal lengths are mentioned often. If your members had compact cameras previously, I'll bet almost none of them could have told you the actual focal length of the lenses. If anyone said "mm" instead of "3x", they would have used equivalence.

A good point. In my former club in the big city, a larger percentage would be technologically savvy. The average age would be younger as well.

  • Maybe 10 understand that there is a difference in depth of field between their camera and cameras with larger or smaller sensors. Only 3 would know where to go to calculate that equivalence (or would ever care to do so).

Again, I'll bet most actually do know this intuitively -- it's obvious when they use their cell phone camera. Or when they compare the results to their previous compact camera.

Well, hopefully, there will be more, as I and another member are doing presentations on DOF and ISO this coming season. And the impact of sensor size will be addressed. The difficulty will be making the presentation short and entertaining enough to keep everyone awake.

  • Most members understand that if a light meter says f/4 at 1/60 at ISO 200 on one camera, they could use the same settings on a different camera

Since you say they mostly have similar cameras, that's reasonable. But would you tell the compact camera user to use ISO 3200 @ f/4 like the APS-C kit users?

Not at all. Given the relatively low light your example suggests, I would suggest shooting with the largest aperture and the lowest ISO possible. If a tripod could be used (assuming it was available and appropriate), then base ISO and use the camera's self-timer to release the shutter. If handheld, then I might ask about the need to capture action, the "equivalent focal length"; and the existence of image stabilization. From these I might suggest a reasonable shutter speed, and set ISO to give an appropriate exposure. Of course, there may be situations where the resulting image would be unsatisfactory (and a discussion about camera limitations/sensor size would ensue).

As I said, about half the members have already figured out the higher-ISO greater noise relationship (which I will cover in more detail this year). And we'll cover DOF in more detail in the coming season as well. But, in general, we don't speak about actual numbers when talking about pictures. There's more chat about "I used a wide aperture for a shallower depth of field" or "I used a fast shutter speed to capture the action" or "The image is noisier because I had to bump my ISO higher to get a fast enough shutter speed for hand-held. When someone says "Yeah, but why can't I get that shallow depth of field on my camera?", then it may be time to talk DOF and sensor size.

  • Only one person in the club really cares about equivalencies, because he shoots both full-frame and APS-C bodies at the same time.

Or if he has to talk to the APS-C users.

Nah. He only talks to me (a micro-4/3 user) about stuff like equivalencies. And when we do talk tech, everyone glazes over moves away.

  • The majority of time I've been drawn into a discussion about equivalencies, it has been a newer member who wants to move from a P&S to a DSLR or mirrorless. Equivalent focal lengths to what they see with their existing camera makes sense to them.

The differences in noise or DOF never comes up?

It depends. Often noise or the desire to have more control of DOF (eg. a shallow depth of field for portraits) is at the root of considering a larger-sensored camera. And, inevitably, the user wants a camera and lens(es) to cover the same range as the compact. And when that compact is a bridge superzoom, it's going to be a longer chat.

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Erik

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Great Bustard Forum Pro • Posts: 44,083
Intentional obfuscation?

D Cox wrote:

Great Bustard wrote:

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

It does.

The number of photons arriving on each square mm is not affected by the size of the sensor. This is the whole point of using f numbers rather than apertures in mm -- a small camera such as 35mm and a large camera such as 10x8 inch, using the same film, need the same exposure time at the same f number.

The intensity of the light is the same.

Would you say that two pots filled to the same depth have the same amount of water? Then why would you say that the same density of light falling on the sensor means the same amount of light on the sensor?

To do so seems like an exercise in intentional obfuscation, as the conversation between Lee Jay and Tim Tucker below demonstrates.

Likewise for different focal lengths on the same camera (ignoring lens faults such as vignetting).

???

The difference with digital is that the design of sensors varies greatly, so you are not using "the same film".

Sensors of the same generation are usually rather close in terms of efficiency:

http://www.dpreview.com/forums/post/51782704

Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

Erik Magnuson wrote:

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term. A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

Eh? Total amount IS a precise term. Measure your volume in litres and you have a precise answer. But that is not what I asked.

It was the answer to your first question and use of "amount." All f/2 lenses do not transmit the same amount of light or my f/2 cell phone lens would transmit as much light as a 200mm f/2 lens. Even if I managed to put a 24x36mm sensor behind the call phone cam lens, it would not help much. What the lens transmits and what the sensor collects may be related, but they are not the same thing.

Sematics, as corrected by Lee Jay (my original statement above). Lenses for different formats do not have the same coverage therefore the total amount of light will not be the same but the light/square metre of sensor size (transmitted?) is the same at f2. A Super Angulon 90mm at f8 will produce the correct exposure regardless of whether I put 4"x5" film or 35mm film behind it.

Great Bustard Forum Pro • Posts: 44,083
Whoops again!

D Cox wrote:

Great Bustard wrote:

Wheatfield7 wrote:

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Um, it is absolutely correct.

It is perfectly correct.

It is not.

The illuminance is the same, and therefore the exposure time needed is the same (if the sensors match in sensitivity).

The same exposure time will result in 16x as much light falling on a FF sensor at 50mm f/2 than a 4x sensor at 12.5mm f/2 for a given scene luminance.

Illuminance determines both exposure time and noise levels, for a given sensor.

Totally wrong.

(Different sensors may have different noise levels at the same exposure. Technology keeps on improving.)

It is a gross misunderstanding to believe that the reason larger sensors are less noisy for a given exposure is because they are more efficient than smaller sensors.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

This is correct, assuming the "amount of light" is measured in lux (i.e. lumens per square meter), which is the appropriate unit.

The exposure (density of light falling on the sensor) is measured in lux · seconds, or, equivalently for a given luminosity function, photons / mm². The total amount of light falling on the sensor is measured in lumen · seconds, or, equivalently for a given luminosity function, photons.

jonas ar
jonas ar Contributing Member • Posts: 745
Re: Focal length 35mm equivalent, but not F-stop?

Lee Jay wrote:

Donald B wrote:

if what you say is true show us all some pics to back up your knowledge ?

Factor of almost 40 difference in sensor area.

Same shutter speed and actual f-stop, not even close to the same image:

Same shutter speed and equivalent f-stop, almost exactly the same image:

So simple and yet so difficult to grasp for many. Really odd.

Great Bustard Forum Pro • Posts: 44,083
Re: Whoops!

Tim Tucker wrote:

Great Bustard wrote:

Wheatfield7 wrote:

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Um, it is absolutely correct.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

Incorrect.

Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

Yes, they do. The fact of the matter is that 16x as much light will fall on the sensor for a given perspective, framing, and shutter speed with 50mm f/2 on FF as will fall on the sensor at 12.5mm f/2 on 4x.

- SNIP -

WHOA!!! Hang on there. I don't quite understand the point here, is this different with digital than it was with film?

Same.

Let's say I have one lens and I put it on my FF camera and take a picture at a shutter and aperture combination X1.

If I take the same lens and put it on a camera with a smaller sensor then it still transmits the same amount of light and the correct exposure would still be X1 or an equivelant of X1. However the smaller sensor would collect less of this light. If I used film it would be a simple crop to obtain the equivalent photo. The exposure would be the same. Yes?

What I'm getting at is that if you put film behind each of the lenses you discuss then with the exposure of X1 you will get correct exposure for all the film, just the coverage of the different lenses would change, yes? (the total light hitting the film would be the exposure - X1 times the total area exposed).

Here we are:

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

Given four cameras, one with...

  • ...an mFT (4/3) sensor,
  • ...another with a 1.6x sensor,
  • ...another with a 1.5x sensor,
  • ...and another with a FF sensor...

...and...

  • ...a photo of a scene from the same position with the same focal point and the same settings (e.g. 25mm f/1.4 1/200 ISO 400) with all cameras,
  • ...the photos cropped to the same framing as the photo from the mFT (4/3) camera,
  • ...and the photos are displayed at the same size...

...then the resulting photos will be Equivalent. In addition, if...

  • ...all the sensors are equally efficient, then all the photos will also have the same noise,
  • ...the pixels are all the same size, the AA filter the same strength, and the lens is the same sharpness, then all the photos will also have the same detail,
  • ...the exact same lens is used and the sensors are of the exact same design with the exact same size pixels, AA filter, CFA, and processing...

... then the photos will not merely be Equivalent, but be identical.

Are we just talking about noise generation in digital sensors here?

Works for film and digital.

(Not forgetting that the aperture marking system is not limited to digital camera lenses, and it measures transmittance and not DoF. DoF is not even an absolute science as certain assumptions are made about enlargement and viewing distance.)

Next up:

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

Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor:

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:

  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same shutter speed (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same DOF and shutter speed.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.
  • The same total light will result in the same noise for equally efficient sensors (regardless of pixel count and regardless of the ISO setting).
  • If the 25mm lens at f/1.4 is 2x as sharp as the 50mm lens at f/2.8, 1.33x as sharp as the 33mm lens at f/1.8, 1.25x as sharp as the 31mm at f/1.8, the sensors have the same pixel count, and the AA filter introduces the same blur, then all systems will also resolve the same detail.
  • Other elements of IQ, such as bokeh, color, distortion, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".
Tim Tucker Senior Member • Posts: 1,337
Re: Focal length 35mm equivalent, but not F-stop?

bobn2 wrote:

Beachcomber Joe wrote:

bobn2 wrote:

Beachcomber Joe wrote:

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same. A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

Completely wrong. What matters is the total number of photons captured. Differences in exposure can always be compensated by simply changing the relationship between exposure and output grey scale. The total light locks in a noise pattern in the image, which can't be altered apart from noise reduction, which will usually lose you image detail. Those wishing to maximise image quality deal in total light. Of course, if you stick to one sensor size, there is no difference between total light and exposure.

Some people may not have known what I meant by the term technobabble. Thank you for providing such an excellent example.

Technobabble it might be, but it is true technobabble, and you have no way of refuting it. So, when you 'Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.' you're still 100% wrong. Choose to call what you're incapable of understanding 'technobabble' if you like, it doesn't make you any less wrong.

I'm following this with interest, and learning some useful bits and bobs, such as how sensors work.

But I am a bit perplexed as to how "total light" and "highest image quality" relate to good image. In the 20th century film produced fantastic images, even early digital cameras produced fantastic images. In fact some of the images produced are still fantastic, so if you used the same cameras could you not produce fantastic images today?

What matters from a photographic standpoint is the image you produce. And isn't there an argument that working within the limitations of your media produces the creativity and the strength of the image?

Lee Jay Forum Pro • Posts: 55,372
Re: Let's correct it, then.

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term. A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

Eh? Total amount IS a precise term. Measure your volume in litres and you have a precise answer. But that is not what I asked.

It was the answer to your first question and use of "amount." All f/2 lenses do not transmit the same amount of light or my f/2 cell phone lens would transmit as much light as a 200mm f/2 lens. Even if I managed to put a 24x36mm sensor behind the call phone cam lens, it would not help much. What the lens transmits and what the sensor collects may be related, but they are not the same thing.

Sematics, as corrected by Lee Jay (my original statement above). Lenses for different formats do not have the same coverage therefore the total amount of light will not be the same but the light/square metre of sensor size (transmitted?) is the same at f2. A Super Angulon 90mm at f8 will produce the correct exposure regardless of whether I put 4"x5" film or 35mm film behind it.

More semantics...

"A Super Angulon 90mm at f8 will produce the same exposure regardless of whether I put 4"x5" film or 35mm film behind it."

It may or may not be "correct", whatever that means.

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Lee Jay

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