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

Started Jul 24, 2014 | Discussions
bobn2
bobn2 Forum Pro • Posts: 64,355
Re: Focal length 35mm equivalent, but not F-stop?

Tim Tucker wrote:

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?

Of course, a good artist knows her medium and uses it to the best effectiveness. That is why matters like 'equivalence' are important for those that truly want to know their medium and how to use it. It also matters in selecting ones equipment, for there's no point spending the extra money of a 36MP FF camera if you're never going to use it in a situation where it gives you a better result than a 1/2.4" compact.

So, on your point about 'fantastic images', of course there is a valid argument that all modern films and digital camera produce images as fantastic as anyone could want, but that doesn't stop people buying better, and this web site is all about people buying better.

So, how does 'total light' relate to 'highest image quality? Simply because it tells you, so far as your operation of the camera  controls are concerned, how much noise or grain (depending on digital or film) will be in the image. And in the end, all else being equal, noise or grain is the major metric of image quality.

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Bob

Great Bustard Forum Pro • Posts: 44,125
Re: Focal length 35mm equivalent, but not F-stop?

Tim Tucker wrote:

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?

Sure. But do you not agree that higher exposures for a given camera, no matter what camera you are using, result in higher IQ, all else equal, so long as you don't blow the highlights?

That's because more light fell on the sensor (film). The thing is, though, that the same exposure on different formats results in more light falling on the sensor (film) of the larger format, and thus the higher IQ (all else equal).

What matters from a photographic standpoint is the image you produce.

I don't think anyone disputes that. It's the IQ of the resulting photo as it relates to the amount of light that made up the photo that is being discussed.

And isn't there an argument that working within the limitations of your media produces the creativity and the strength of the image?

There is, indeed, such an argument. There are many who use cell phones as their cameras of choice, and I've seen excellent photos come from them. That said:

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

But what, exactly, is IQ, and what does it have to do with the "success" of a photo? The first step in defining "IQ" is to make the distinction between "image quality" and a "quality image". Many would take it as a given that if we have two photos of the same scene with the same composition then, all else equal as well, the photo with "higher IQ" would be "more successful". That is, the photo with "higher IQ", for example, would place better in a photo competition, would be more likely to sell, would sell for a higher price, etc. For sure, this may certainly be true for a large number of photos, such as a landscape photo displayed at a huge size. But it is important to acknowledge that there is a class of photography where image quality, as opposed to a quality image, is all but irrelevant (please see these outstanding photos, for example).

Furthermore, while one system may yield "higher IQ" than another, those differences may not be large enough to make any significant difference in the appeal of the photo, depending on the QT (quality threshold) of the viewer, the scene itself, the size at which the photo is displayed, and how closely it is scrutinized (see here for an interesting example of this point), and the processing applied to the photo. In other words, it is not merely whether System A has "higher IQ" than System B, but under what conditions it has higher IQ (and, indeed, which has "higher IQ" may flip-flop, depending on the conditions), and if the IQ is "enough higher" to make any significant difference.

For some photographers, IQ may be the most important aspect of photography. For others, it may play no role at all or simply be an added plus. But it is time well spent to reflect on just how important IQ is to our own photography, given that IQ is, at best, merely a means to achieving a quality image, and, at worst, completely irrelevant to the photo.

With all the disclaimers said about the relevance of IQ to the "success" of the photo, we can discuss what IQ is. The attributes of IQ include, but are not limited to:

Attributes of IQ do not include: subject, composition, focus accuracy, DOF, etc., which are attributes of system operation, available lenses, artistic design, and/or photographer skill. Of course, it's important to note that operational differences, such as focus accuracy, can have a substantial effect on the ability to capture a "high IQ" image. The "overall IQ" of a photo is a function of how the viewer subjectively weighs the individual objective components of IQ, which often depend greatly on the scene and how large the photo is displayed.

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

Great Bustard wrote:

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".

Makes a lot of sense, and much what I thought. But it seems to be a lot of hard work in comparing properties of cameras of different sensor sizes when they are overridden by far more fundamental properties. Thanks again GB.

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

Lee Jay wrote:

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.

LOL I would like to say that I only ever made correct exposures, but that may need correcting as well.

flakin stephen Contributing Member • Posts: 752
Re: Whoops!

Great Bustard wrote:

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".

In fairness, the OP's questions was about lenses. You could have a 40mm f2.8 lens designed for APS-C, 35mm, or medium format. Even though the density of light on the sensor would be the same, each would transmit a different total amount of light, as the area it will illuminate is designed to be larger for the larger formats.

Absolutely everything else could be exactly the same (f stop, aperture size, noodle points). The variables are all to do with sensor size, so are irrelevant to a question about lenses. Surely measuring photons by the proxy of noise levels (or vice-versa) is a minority interest ?

cheers,

Flakey

-- hide signature --

flakey

Great Bustard Forum Pro • Posts: 44,125
Re: Whoops!

Tim Tucker wrote:

Great Bustard wrote:

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".

Makes a lot of sense, and much what I thought.

Pleased to hear it!

But it seems to be a lot of hard work in comparing properties of cameras of different sensor sizes when they are overridden by far more fundamental properties.

You mean something like this?

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

Of course, sensors are not equally efficient (although often, but not always, close for a given generation), nor are all lenses proportionally sharp, have the same color, bokeh, distortion, or flare characteristics, nor do all sensors have the same pixel count, CFA, and AA filter.

.

.

.

This is not to say that photos that are not Equivalent may not look more similar to photos that are Equivalent. For example, let's say we have an old FF DSLR and a modern mFT camera that has a much more efficient sensor. We may find that a photo at 25mm f/2.8 1/100 ISO 400 with the mFT camera may look more similar to a photo at 50mm f/4 1/100 ISO 800 on the FF DSLR than an Equivalent photo at 50mm f/5.6 1/100 ISO 1600, even though the f/4 photo on the FF DSLR has less DOF than the mFT photo, since the f/5.6 photo is much more noisy due to the less efficient sensor, and noise may matter more than DOF, depending on the scene and how large the photo is displayed. On the other hand, sensors of the same generation are usually pretty close in terms of efficiency (see here for quite a few examples). Still, there are other issues to consider, such as color and distortion (although these two can usually be corrected for in processing), bokeh, lens flare, etc..

So while differences in the technology may well make for differences that matter more than one or more of the parameters of Equivalence, Equivalent photos will typically be the closest in appearance (more so, of course, when the sensors are of the same generation). The larger the photo is displayed, the more extreme the processing, and the lower the amount of light that makes up the photo, the more obvious the role that differences in technology will play.

Thanks again GB.

My pleasure!

bobn2
bobn2 Forum Pro • Posts: 64,355
Re: Let's correct it, then.

Lee Jay wrote:

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.

Here's a thought on this 'correct' exposure nonsense. Let's go back to film, since it is film based methodology which has confused people. Suppose we added to the equivalence requirements the condition 'equal granularity', and were looking at equivalence between 110 (roughly Four Thirds) and 135 (what we'd now call 'full frame'). If we wanted equal granularity in the image enlarged to the same output size, we'd want the same number of grains in each shot, which means that the 135 grains would need to be four times the area. Since each grain requires two photons to reduce it, both shots would need the same number of photons to expose. Thus the exposure of the 135 would need to be one quarter of that for the 110,  or in other words, the ISO of the 135 film would need to be four times higher (which it would be in any case, since the grains were four times larger in area, so it would produce the same density for one quarter the number of photons).

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Bob

Great Bustard Forum Pro • Posts: 44,125
Re: Whoops!

flakin stephen wrote:

In fairness, the OP's questions was about lenses. You could have a 40mm f2.8 lens designed for APS-C, 35mm, or medium format. Even though the density of light on the sensor would be the same, each would transmit a different total amount of light, as the area it will illuminate is designed to be larger for the larger formats.

Absolutely everything else could be exactly the same (f stop, aperture size, noodle points). The variables are all to do with sensor size, so are irrelevant to a question about lenses. Surely measuring photons by the proxy of noise levels (or vice-versa) is a minority interest ?

We are not "measuring photons by the proxy of noise levels (or vice-versa)" -- we are saying that the more light that makes up a photo, the less noisy it will be, and that the same exposure on different formats results in more light falling on the sensor of the larger format.  Even more specifically, the same total amount of light falls on the sensor for all systems for a given DOF and shutter speed.

If you're going to say, "Well, duh!", then congratulations -- you already knew! 

edhannon
edhannon Senior Member • Posts: 1,761
Both are an unecessary crutch

35mm equivalent focal length is a crutch invented to help us old f@*ts to transition from 35mm film to APS-C digital.  It helped us transition from film by knowing what lens to use to get a particular FOV in APS-C using our experience from 35mm film..

However, like learning a second language, real proficiency only comes when you think in the new format.

Same applies to DoF - one needs to learn what DoF will result with the lens/f-stop/sensor size you have - not one that you are currently familiar with.

This transition is easier for those of us who used both 35mm and medium format film. I never did translate lens focal length or f/stop when I switched between my Pentax 35mm film camera and my 500C Hasselblad.  I learned to think 35mm when I shot with the Pentax and 6x6 when I shot with the Hassey.

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pidera Contributing Member • Posts: 927
Exactly (nt)

It's as easy as that.

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

bobn2 wrote:

Lee Jay wrote:

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.

Here's a thought on this 'correct' exposure nonsense. Let's go back to film, since it is film based methodology which has confused people. Suppose we added to the equivalence requirements the condition 'equal granularity', and were looking at equivalence between 110 (roughly Four Thirds) and 135 (what we'd now call 'full frame'). If we wanted equal granularity in the image enlarged to the same output size, we'd want the same number of grains in each shot, which means that the 135 grains would need to be four times the area. Since each grain requires two photons to reduce it, both shots would need the same number of photons to expose. Thus the exposure of the 135 would need to be one quarter of that for the 110, or in other words, the ISO of the 135 film would need to be four times higher (which it would be in any case, since the grains were four times larger in area, so it would produce the same density for one quarter the number of photons).

Ah, but grain is a function of the film emulsion and development (development produces the grain not exposure), but is also affected by focus and over/underexposure. If you scan you have artefacts of noise, nyquist considerations, sharpening artefacts, if you print then you have development and over developed/under exposed/temperature etc. considerations. Noise in film is not translated into the final image in the same way that digital noise is.

The point being that it as a pointless argument because it doesn't apply to grain size. I know this is supposed to be an analogy but this is getting out of hand. Stick to the facts.

gandalfII Senior Member • Posts: 1,952
Re: Focal length 35mm equivalent, but not F-stop?

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. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

Sometimes but not always the faster lens was top of the line and optically superior; eg nikkor35/1.4 and 20/2.8 were better than their f/2 and f/3.5 counterparts.  I paid the weight and cost premiums but nearly always shot them at about f/5.6.

Great Bustard Forum Pro • Posts: 44,125
Equivalence is about format independent parameters.

edhannon wrote:

35mm equivalent focal length is a crutch invented to help us old f@*ts to transition from 35mm film to APS-C digital. It helped us transition from film by knowing what lens to use to get a particular FOV in APS-C using our experience from 35mm film..

However, like learning a second language, real proficiency only comes when you think in the new format.

Same applies to DoF - one needs to learn what DoF will result with the lens/f-stop/sensor size you have - not one that you are currently familiar with.

This transition is easier for those of us who used both 35mm and medium format film. I never did translate lens focal length or f/stop when I switched between my Pentax 35mm film camera and my 500C Hasselblad. I learned to think 35mm when I shot with the Pentax and 6x6 when I shot with the Hassey.

Equivalence does not require comparing to FF -- you can use the equivalence ratio (crop factor) between any two formats.  For example, the equivalence ratio between mFT and 1.6x is 1.25.

Of course, if you are not comparing formats, then there's no reason to be thinking of Equivalence.

Lee Jay Forum Pro • Posts: 55,392
Sure seems simple to me.

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.

And even with these examples some people deny the math is correct. Strange.
--
Lee Jay

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bobn2
bobn2 Forum Pro • Posts: 64,355
Re: Let's correct it, then.

Tim Tucker wrote:

bobn2 wrote:

Lee Jay wrote:

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.

Here's a thought on this 'correct' exposure nonsense. Let's go back to film, since it is film based methodology which has confused people. Suppose we added to the equivalence requirements the condition 'equal granularity', and were looking at equivalence between 110 (roughly Four Thirds) and 135 (what we'd now call 'full frame'). If we wanted equal granularity in the image enlarged to the same output size, we'd want the same number of grains in each shot, which means that the 135 grains would need to be four times the area. Since each grain requires two photons to reduce it, both shots would need the same number of photons to expose. Thus the exposure of the 135 would need to be one quarter of that for the 110, or in other words, the ISO of the 135 film would need to be four times higher (which it would be in any case, since the grains were four times larger in area, so it would produce the same density for one quarter the number of photons).

Ah, but grain is a function of the film emulsion and development

Film emulsion, yes. A film works because an incoming photon liberates a photoelectron which reduces a molecule of silver chloride to an atom of silver. At least two silver atoms are required to catalyse the chemical reduction process which will turn the whole halide crystal to silver. These need to be at the same locality in the grain, so the grain needs an 'electron trap' so that the silver atoms are likely to occur at the same place. The process of increasing the quantum efficiency of emulsions involved (chemically) engineering better electron traps and also manipulating the shape of the crystal so that it presented a bigger surface area (compared with its overall size). So, that's much the same as digital, films improved in QE as emulsion formulations improved, but for a given formulation the basic change in speed depended on grain size, basically how much silver got produced per photon. So in the end 'film speed' was a function of QE and grain size, it could be increased either by increasing QE or grain size. So, my comment was assuming film of similar QE, where the speed would be controlled by grain size. So far as development goes, the silver atoms catalyse reduction in the sense that they speed it up. Leave the halide in a reducing agent, its all going to become silver in the end. In the end, development has to be controlled so that the proportion of exposed halide to the proportion of halide that gets reduced anyway favours the development of a good quality image. So more development (by means of a more aggressive developer, longer development time or higher temperature) increases the cahnces of all exposed grains being developed out (thus increasing 'speed') but also increases the chance of non-exposed grains being developed, this providing 'fog' in unexposed parts of the image, and reducing the overall DR.

(development produces the grain not exposure),

No, manufacture produces the grain. It is in the film. Exposure and development only determines which grains are turned into silver. Development can also effect the shape of the final developed grain.

but is also affected by focus

No

and over/underexposure.

Also no. What can happen is that printing to compensate for a thin or thick negative makes the grain in the final image more apparent.

If you scan you have artefacts of noise, nyquist considerations, sharpening artefacts, if you print then you have development and over developed/under exposed/temperature etc. considerations. Noise in film is not translated into the final image in the same way that digital noise is.

The starting process is exactly the same, a photon releases a photoelectron. The difference is because film is more digital than digital is. While a pixel will continue to accumulate charge so long as its FWC is not exceeded, and can thus count tens of thousands of photons, a silver halide grain can only ever count a few photons (that's why film is non linear, as exposure increases the chance of any photon finding an unreduced grain reduces). So, in the end, what I said is true. Keep the same film technology and development, and to produce the same granularity with 135 as 110 you need a film with four times the grain area, which will have four times the ISO. And I can be allowed 'same film technology and development' if the f2=f2=f2 brigade can be allowed to insist on the same film.

The point being that it as a pointless argument because it doesn't apply to grain size.

It certainly does, you need to brush up on your film chemistry.

I know this is supposed to be an analogy but this is getting out of hand. Stick to the facts.

I am, not my fault if you've got the wrong impression of the facts.

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Bob

mostlyboringphotog Veteran Member • Posts: 9,049
Re: What ordinary people want

Chikoo wrote:

mostlyboringphotog wrote:

Chikoo wrote:

Chikoo wrote:

Gerry Winterbourne wrote:

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?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

A person who has grown up using 35mm camera, and loves the way his f2.8 lens provide a nice bokeh, and sees the new camera in the store which is smaller and lighter, and boasts the same focal length and zoom range + F-stop, will be more than happy to purchase that camera. Only to find out that it is not the same. He is not getting the same quality of picture as his 35mm f2.8 lens gave him for the last 30 years.

That is deceptive to say the least.

This is like stating the hp of a car with a 4 cylinder engine as a ratio of power to weight and sell it saying it is the same or better than the large v8 he currently drives.

This was true even when "I, Roman" ruled the world - that's why they warned "Caveat Emptor".

So if I have a 35mm camera and a m43, will the same exact focal length, fstop give me the same picture given the ISO/DR/SNR is the same on both?

Ordinary people want photos in which their spouse look good and their kids don't look blurry because they were running around. If you have 35mm camera and m43, you don't need to ask the question and if you do (no condescension intended as we all need to learn at least once and someone like me, many time but I digress...) there are far better ways to learn than looking for the "equivalent aperture" on a spec sheet. That is my opinion.

maybe they should not print any specs. Just try and buy if you like your wife in the picture eh?

Now your are being passive aggressive... How about just the facts like FL and maximum aperture and the sensor size, etc, nothing artificial added

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mostlyboringphotog Veteran Member • Posts: 9,049
Re: Focal length 35mm equivalent, but not F-stop?

Lee Jay wrote:

Lee Jay wrote:

I also use equivalence to make purchase decisions, and decisions about what equipment to carry into certain situations. Sometimes I use it for determining what settings I want to use in some situations.
--
Lee Jay

"I also use equivalence to make purchase decisions,..." I like to challenge this sentiment.

I say that you have not chosen m43 because m43 f/2 is equivalent to f/4 on FF. You know FF can do f/2 as well as f/4.

Also, I will say that you have not chosen FF because f/4 is equivalent to f/2 on m43.

And so without the actual comparison of the real images that you're after, this equivalence is meaningless to make the purchase decision.

Nope...wrong. I use equivalence to plot lens (and format) performance envelopes, just like DPReview has started doing in their compact camera reviews (except mine are better). In fact they started doing that because of my repeated suggestions over several years.
--
Lee Jay

Now I'm being passive aggressive - so you don't look at the images from potential equipment you want to buy? You buy FF because f/4 is equivalent to f/2 on m43?

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Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

bobn2 wrote:

So, how does 'total light' relate to 'highest image quality? Simply because it tells you, so far as your operation of the camera controls are concerned, how much noise or grain (depending on digital or film) will be in the image. And in the end, all else being equal, noise or grain is the major metric of image quality.

Oh wow, I see I haven't kept up with the times. Last year in the fantasy world where all else is equal resolution was the metric du jour of the technobabblers, a couple of months ago it was dynamic range, now you tell us it is noise. Any chance that you guys will ever consider subject matter, other than test charts and brick walls, and composition as being of some importance?

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bobn2
bobn2 Forum Pro • Posts: 64,355
Re: Focal length 35mm equivalent, but not F-stop?

mostlyboringphotog wrote:

Lee Jay wrote:

Lee Jay wrote:

I also use equivalence to make purchase decisions, and decisions about what equipment to carry into certain situations. Sometimes I use it for determining what settings I want to use in some situations.
--
Lee Jay

"I also use equivalence to make purchase decisions,..." I like to challenge this sentiment.

I say that you have not chosen m43 because m43 f/2 is equivalent to f/4 on FF. You know FF can do f/2 as well as f/4.

Also, I will say that you have not chosen FF because f/4 is equivalent to f/2 on m43.

And so without the actual comparison of the real images that you're after, this equivalence is meaningless to make the purchase decision.

Nope...wrong. I use equivalence to plot lens (and format) performance envelopes, just like DPReview has started doing in their compact camera reviews (except mine are better). In fact they started doing that because of my repeated suggestions over several years.
--
Lee Jay

Now I'm being passive aggressive - so you don't look at the images from potential equipment you want to buy? You buy FF because f/4 is equivalent to f/2 on m43?

You buy FF because when you see that f/4 is equivalent to f/2 on mFT, you realise that you have two extra stops in hand, if you need them. If you're never going to need them, then FF is a waste.

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Bob

bobn2
bobn2 Forum Pro • Posts: 64,355
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

bobn2 wrote:

So, how does 'total light' relate to 'highest image quality? Simply because it tells you, so far as your operation of the camera controls are concerned, how much noise or grain (depending on digital or film) will be in the image. And in the end, all else being equal, noise or grain is the major metric of image quality.

Oh wow, I see I haven't kept up with the times. Last year in the fantasy world where all else is equal resolution was the metric du jour of the technobabblers, a couple of months ago it was dynamic range, now you tell us it is noise. Any chance that you guys will ever consider subject matter, other than test charts and brick walls, and composition as being of some importance?

Nice argument in favour of Lomography. If it's all the subject and the quality of the equipment doesn't count for anything, then this whole site is a dead duck.

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Bob

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