Discussion of 'Equivalence'

[simsurace]

Forgot the link!!

https://www.fastrawviewer.com/blog/how-to-use-the-full-dynamic-range-of-your-camera

Thanks for providing that link.
A few years ago I figured out a similar methodology after realizing at what value the camera places a grey card in spot metering, and being puzzled.
I realized that Nikon is not doing what is optimal for data capture, but what probably minimizes the complaints from clueless customers (or those coming from film that have a clue).
It is nice to have such a nice writeup for reference.

The article reinforces my gut feeling of what an optimal exposure for digital is.

[Les Olson]

I think what the exposure slider 'should' do is apply a linear scaling to the RAW levels before the translation to the color space occurs. Can you determine that from the book?

Why would the perceptual effects apply after you have adjusted the brightness?

Because the brightness is not reduced equally at all luminances, so colours that were only slightly less bright in the original are a lot less bright after adjustment. 

No.  Adobe is reluctant to say much about the algorithms, because that is proprietary.  Their bias is that you should look at the picture and adjust the sliders until you like what you see and not worry your cute little head about the details.  The important point is that whatever the "exposure" slider does, it does not change exposure, so that compensating for over-exposure is much more complicated than it looks. 

[David H. Hartman]

Nikon is following an ANSI standard. Here is more...

Meters Don't See 18% Gray by Thom Hogan

and...

... It turned out that Bob [Shanebrook] was the man who signed off on the 18% value during the 1979 revision. At that time a number of people at Kodak wanted to change the card to 12.5 % reflectance to simplify the work of photographer who were using the cards. They were all ready to make the change when Ansel Adams got wind of the coming change. As Bob tells the story, Ansel Adams came to Rochester and basically camped out in one of the offices and said he would not leave until they agreed to keep the Gray Card 18% because it matched one of the Zones in his Zone System. To get rid of him the folks at Kodak gave in and left the card 18%. When we revised the card in 1999 we left it 18% because the manufacturing technology was known for producing 18% and there was no budget to do the R & D which would have been necessary to change it. ...

--A clip from Bob Shell's Talk delivered to NECCC on July 12th and 14th, 2002

[simsurace]

Because the brightness is not reduced equally at all luminances, so colours that were only slightly less bright in the original are a lot less bright after adjustment. 

No.  Adobe is reluctant to say much about the algorithms, because that is proprietary.  Their bias is that you should look at the picture and adjust the sliders until you like what you see and not worry your cute little head about the details.  The important point is that whatever the "exposure" slider does, it does not change exposure, so that compensating for over-exposure is much more complicated than it looks.

For me it is not at all intuitive to think of the transformation in the color space, but in the linear Raw level space it is clear what should happen. Exposure slider should be more or less like a digital post-hoc adjustment of ISO.

On RAW converters that are fully documented (like RawTherapee) this is exactly what the exposure slider does.

This is different from brightness or luminance, which refer to the HSB, HSL, or Lab spaces.

[David H. Hartman]

I'm interested why you think that it is not possible (given the right equipment) to make two nearly identical photograph using different formats. What fundamental limit are you thinking of?

OK, here is a chart. I hope the formulas behind it are accurate...

http://asklens.com/howmuchblur/#compare-1.5x-70mm-f1.7-and-1x-105mm-f2.5-and-1.5x-70mm-f1.4-and-1x-105mm-f1.4-and-1.5x-85mm-f2-on-a-0.9m-wide-subject

I own a 105/2.5 and a couple of 105/2.8 lenses so I can have what's shown. I don't know of any 70/1.7, 1.8 or 1.4 lens. There is a 75/1.4 ASP lens for Leica M but I know of none in Nikon F bayonet.

I can have the same blur circles with a 105/2.5 or 2.8 on FX but not the same perspective with an 85/2.0 on DX. To have the framing of the subject the same I'll have to move back and accept flatter perspective.

If I had a 70 or 75/1.7 or 1.4 I could have similar blur circles but the lens would have to give good results wide open or stopped down less than a stop. To do this the lens would probably have to be aspheric. Would the bokeh be as good as the 105/2.5 or 105/2.8 lenses? The DoF would be different. Maybe not in a bad way. Anyway in a short search I didn't find a fast 70 or 75mm lens in F-Bayonet. If there is it would be priced out of my reach.

Back when Nikon didn't have an FX camera people recommended a 105/2.5 for portraits. They recommended a 50/1.8, 1.4 or 1.2. The 105mm is a longer lens for DX that for FX. The 50mm is a shorter lens on DX compared to a 105mm on FX.

The only solution for me for DX was an 85/2.0 as that's what I own. I also own an 85/1.4 AIS but it's lube contaminated.

On DX I can have the blurring I want but not the perspective I want. I can have the perspective I want but not the blurring I want. I can't have both together.

It doesn't make sense to me to compare with crop factors and all. Equivalence doesn't work for me. One should lean what a 75mm lens does on 4x5 and a 105mm lens does on FX and what a 300/4.0 lens does on DX and go from there.

Dave Hartman

[David H. Hartman]

I though my Nikon F2As was mis-calibrated until I read about the 12~13% ANSI standard for meters. All of my Nikons in the film era were calibrated that way.

Anyway it's all Ansel Adam's fault.

But what I've been puzzled with is that some people propose 'exposing to the left' for digital.

What I hate is a camera that gives you ETTR and ETTL at the same time, my D2H for example.

I'm not sure the last paragraph made any sense so I clipped it. I'll have another cup of coffee and hope for the best.

Dave Hartman

[simsurace]

We have covered a lot of ground in this thread. Let me try to give a summary of positions expressed. I will just list the statements, without repeating the arguments for or against a particular thesis. Even though I think that not all of the positions are equally defensible or relevant, I will leave it to the reader to read the full discussion as well as other material and make up his/her mind about it.

My opinion is that all of the technical criticisms have been debunked (that's why I list them separately). However, this has not been actively acknowledged in all cases, and therefore I think that the discussion is not fully settled. In the interest of not being confusing about issues that should, in my opinion, have a clear objective answer, it would be ideal if we could reach some form of consensus.

As for the meta-level issues; I think that no consensus is required since they can be left to personal preference.

Please let me know if you want to add something or if you think that something is unfairly represented.
I will afterwards also add this to the opening post for new people that come across this thread.

Equivalence

Meta-level criticism

• It is difficult to put into practise because of the limited availability of lenses in certain formats.
• Photographers do not shoot equivalent pictures on different formats.
• Photographers do not get better by reading about equivalence.
• Equivalence does not teach us anything new that wasn't known from basic photographic theory.
• Equivalence was invented by (certain) camera manufacturers in order to promote their product.
• Equivalence should not be called 'Equivalence'.
• It does not account for perceptual thresholds, i.e. two images might not be equivalent, but indistinguishable to most viewers.
• It is confusing.

Technical criticism (debunked)

• Equivalence contradicts basic photographic theory.
• In order for two pictures to be equivalent, they have to be exactly the same (i.e. the criteria are too restrictive).
• Empty magnification is not accounted for.

Arguments in favor

• It is a reformulation of conventional photographic theory that is more suited to digital capture.
• It can be useful as a conceptual tool to evaluate gear choices.
• It can be useful for learning photography.

Photographic dynamic range

Meta-level criticism

• It is a metric that does not correlate with what human viewers find pleasing.
• It should not be called 'dynamic range'
• It is strange that a property of the sensor should be related to output size.
• (related) It is a normalized measure that removes degrees of freedom, and is therefore confusing.
• It is a circular definition that is used to prove a pre-conceived statement.
• Dynamic range is an irrelevant property of camera gear.

Technical criticism (debunked)

• The PDR values are not measured.
• It does not predict what can be achieved on an output medium with limited dynamic range.

Arguments in favor

• It is a useful metric that is received positively from many photographers.
• It is superior to DXOMark's 'print' DR metric.
• By being normalised to output size, it allows for apples-to-apples comparisons.
• It can be used to predict the maximum tonal image quality (noise etc.) that is achievable, given suitable output media and viewing conditions.

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Re-reading my own answers, I was again reminded that I haven't been completely clear on the distinction between photon noise and read noise. If there is anything to clarify, please ask.

[Les Olson]

• In order for two pictures to be equivalent, they have to be exactly the same (i.e. the criteria are too restrictive).

The problem is not that things can be "equivalent" despite being only "nearly identical" but that you will not define "nearly identical" otherwise than "what makes the answer come out how I want it to".       

For example: we all know that 35mm on DX is equivalent  = nearly identical to 50mm on FX.  But 35mm x 1.5 = 52mm (to two significant figures).  And the DX sensor is not 24 x 16mm and the FX sensor is not 36 x 24mm.  DX sensors have been as small as 23.1 x 15.4mm (D3000) but are currently 23.5 x 15.7mm (D500 and D7500; the D7200 is 23.5 x 15.6mm), FX is currently 35.9 x 23.9mm.  So the DX crop factor is not 1.5, it is (currently) 1.53. To two significant figures, 35mm x 1.53 = 54mm.

But on FX at 2m 50mm at f/1.4 has the same DoF as 54mm at f/1.8!  So if a one stop difference is so important when DX is compared to FX, why can you ignore a 2/3 stop difference and say 35mm on DX is equivalent to 50mm on FX? 

Another example: if I use the 105/2.5 for a portrait the DX equivalent is 70mm (105/1.5; 105/1.53 = 69mm).  DoF is the same for FX with 105mm at f/2.8 and 3m as for DX with 75mm at f/2.8 at 2.9m.  By what non-arbitrary criterion are the DX framing and perspective not "nearly identical" to the FX framing and perspective?  Of course, if I am using the 70-200/2.8G VRI its actual focal length at "70mm" and 3m is 75mm, which I would never know if someone had not tested it because its successor had marked focus breathing (http://www.bythom.com/nikkor-70-200-VR-II-lens.htm).  Many lenses have actual focal lengths 3% or so different from the values they are designated by, even at infinity: if you can ignore that, how can you justify saying that 75mm is not nearly identical to 70mm?   

[bclaff]

...
As for the meta-level issues; I think that no consensus is required since they can be left to personal preference.

Please let me know if you want to add something or if you think that something is unfairly represented.
I will afterwards also add this to the opening post for new people that come across this thread.

Photographic Dynamic Range (PDR)

Meta-level criticism

• 1) It is a metric that does not correlate with what human viewers find pleasing.

Feedback over the years from many photographers contradicts this criticism.

• 2) It should not be called 'dynamic range'

Dynamic range is generically a logarithmic value of a maximum or a minimum; PDR meet this criteria.

• 3) It is strange that a property of the sensor should be related to output size.

It isn't, not related to a fixed output size; it's related to visual acuity which in turn forms a COC, not unlike DOF calculations.

• 4) (related) It is a normalized measure that removes degrees of freedom, and is therefore confusing.

Not sure why that's confusing provided it is stated (which it is).

• 5) It is a circular definition that is used to prove a pre-conceived statement.

Don't understand the circularity. There is no pre-conceived statement by the author. Are you referring to the misuse but those who don't understand the measure?

• 6) Dynamic range is an irrelevant property of camera gear.

I accept that some people think so but feedback from others (on other fora, by email etc.) would indicate otherwise.


----------

Re-reading my own answers, I was again reminded that I haven't been completely clear on the distinction between photon noise and read noise. If there is anything to clarify, please ask.

I added my views above in the quote.

Regarding read noise and photon noise; there is a huge difference.

Read noise is a variance is the accuracy of reading the pixel value; it affects shadow performance and establishes (EDR, DxOMark) or influences (PDR) dynamic range.

Photon noise is a property of light that increases with the amount of light; it becomes relevant at mid to high light level and has nothing to do with dynamic range.

[Jack Dahlgren]

Dynamic range is generically a logarithmic value of a maximum or a minimum; PDR meet this criteria.

The issue with coopting an accepted term in the same domain is that it leads to confusion. For example, if we use aperture in the generic sense as an opening to refer to the mount size of a particular camera and then called it "photographic aperture", people would get confused. Certainly the measure is interesting and has some effect, but it would just be odd to overload the term in that way.

[bclaff]

The issue with coopting an accepted term in the same domain is that it leads to confusion. For example, if we use aperture in the generic sense as an opening to refer to the mount size of a particular camera and then called it "photographic aperture", people would get confused. Certainly the measure is interesting and has some effect, but it would just be odd to overload the term in that way.

I disagree. I consider the term dynamic range as generic (logarithm of a ratio of high to low) and always needing to be put into context.
The careless use of "dynamic range" (unqualified) is what leads to confusion.
On other (unnamed) boards it's common to distinguish between Engineering Dynamic Range (EDR), my Photographic Dynamic Range (PDR), DxOMark Landscape Dynamic Range, etc. to avoid confusion.
FWIW, on spec sheets EDR is often in dB (more common in engineering, hence the name) and PDR (and the like) are in EV (or photographic stops).

[Andrea B.]

The careless use of "dynamic range" (unqualified) is what leads to confusion.

Agreed! I am going to try to be more careful myself about using the terms (EDR, PDR, LDR) correctly.

[simsurace]

The problem is not that things can be "equivalent" despite being only "nearly identical" but that you will not define "nearly identical" otherwise than "what makes the answer come out how I want it to".       

Ok, I'm confused by this terminology. Is 'nearly identical' related to the perceptual thresholds?
I do not have particular answers in mind. What would those answers be? I think that it is not particularly useful if I proclaim a certain value for perceptual threshold because 1) it might not apply to all viewers, and 2) I think that the conceptual framework of equivalence can be equipped with arbitrary perceptual thresholds, depending on application, i.e. it is a free parameter of the model.

If you fit the parameter to each individual comparison, of course the model will produce nonsense. Maybe that's what you are getting at. But if you set the parameter once for all comparisons, depending on your purposes, you can apply the model and get useful predictions.

For example: we all know that 35mm on DX is equivalent  = nearly identical to 50mm on FX.  But 35mm x 1.5 = 52mm (to two significant figures).  And the DX sensor is not 24 x 16mm and the FX sensor is not 36 x 24mm.  DX sensors have been as small as 23.1 x 15.4mm (D3000) but are currently 23.5 x 15.7mm (D500 and D7500; the D7200 is 23.5 x 15.6mm), FX is currently 35.9 x 23.9mm.  So the DX crop factor is not 1.5, it is (currently) 1.53. To two significant figures, 35mm x 1.53 = 54mm.

But on FX at 2m 50mm at f/1.4 has the same DoF as 54mm at f/1.8!  So if a one stop difference is so important when DX is compared to FX, why can you ignore a 2/3 stop difference and say 35mm on DX is equivalent to 50mm on FX? 

Another example: if I use the 105/2.5 for a portrait the DX equivalent is 70mm (105/1.5; 105/1.53 = 69mm).  DoF is the same for FX with 105mm at f/2.8 and 3m as for DX with 75mm at f/2.8 at 2.9m.  By what non-arbitrary criterion are the DX framing and perspective not "nearly identical" to the FX framing and perspective?  Of course, if I am using the 70-200/2.8G VRI its actual focal length at "70mm" and 3m is 75mm, which I would never know if someone had not tested it because its successor had marked focus breathing (http://www.bythom.com/nikkor-70-200-VR-II-lens.htm).  Many lenses have actual focal lengths 3% or so different from the values they are designated by, even at infinity: if you can ignore that, how can you justify saying that 75mm is not nearly identical to 70mm?

I don't follow all your calculations, in particular those about DOF (did you take into account secondary magnification?).
Are you saying that because of generous evaluation, i.e. "70mm and 75mm yield very similar angle of view that I will consider, for my purposes, to be the same", they are indistinguishable? In practise, there are caveats because of the differences between nominal and actual focal length, focus breathing, distortion, etc. But all caveats aside, 75mm and 70mm are not exactly equivalent because it is a difference that can be measured or noticed (there will be things included in the 70mm shot that are not included in the 75mm shot). Again, if you set your thresholds of 'significantly different' to generous levels, you will simply ignore those measurable differences.

-----

I think that the criticism by Bjørn that you responded to was saying something else, but I may still be misunderstanding him.
His statement was that the list of criteria that are enumerated by Joseph James are too restrictive. To me, this means that if two pictures are the same to some precision, then they must have been shot with the same parameters on the same format under the same conditions, i.e. they must be the same shot.

My view is that because of the dimensionless nature of the digitally recorded images, given a model (any model) of the complete imaging chain (from the scene to the final resized image), and given two final images that are the same to arbitrary precision (even more so if perceptual thresholds are present), you cannot be certain that they were shot with the same parameters, i.e. they might have been shot with different parameters. This allows us (in theory) to get the same shot with different formats and parameters adjusted accordingly.

(If you disagree with this, what would be a model where it is always possible to say that if two images look the same, they must have been shot with the same parameters on the same format?)

In practise, there are limits imposed by availability of gear and physical limits (e.g. diffraction limits, maximum theoretical numerical aperture, etc.).
To me, it makes an important difference whether something is possible in theory, but sometimes may be hard to carry out precisely in practise, or whether it is impossible even in theory. YMMV.

Perceptual thresholds make it 'easier' to achieve this, not harder. If viewing conditions are very poor, all images look the same. Even under good but not exceptional viewing conditions, we get away with things like focal lengths not being the same by fractions of a mm, the entrance pupil not being at the same location, minor variations in exposures due to the tolerance of the shutter and aperture mechanism, etc.

Note that this is not the definition of equivalence that Joseph James is using, but a generalization thereof. Joseph James uses quite a simple model of imaging (but not too simple to be meaningless) and subdivides the comparison into a list of criteria, which makes his definition easier to use in practise.

An example for how this principle works was shown in the background blur calculation in Reply #184.

[simsurace]

I added my views

above in the quote.

Regarding read noise and photon noise; there is a huge difference.

Read noise is a variance is the accuracy of reading the pixel value; it affects shadow performance and establishes (EDR, DxOMark) or influences (PDR) dynamic range.

Photon noise is a property of light that increases with the amount of light; it becomes relevant at mid to high light level and has nothing to do with dynamic range.[/list]

Thanks for adding your views. I fully agree.

Thanks also for reminding us of the difference between read noise and photon noise. I will add some notes to my previous posts where I got momentarily confused about the two.

[Les Olson]

Ok, I'm confused by this terminology. Is 'nearly identical' related to the perceptual thresholds?

No.  It is simply pointing out that "nearly identical" means nothing.  It is like the often heard inanity that humans and chimpanzees are nearly identical because they have 98% (or whatever the number is) of their DNA the same.  But 98% is only "nearly identical" to 100% if you already think chimpanzees and humans are "nearly identical", and if you think humans and chimpanzees are quite different 98% is quite different to 100%.

In science, one common meaning of "equivalent" would be "the same within measurement error".  In the case of lens focal length, eg, the measurement error is +/- 3%, so the precision of DoF calculations cannot be greater than that, so DoF within +/- 3% must be "equivalent".  In everyday photography, some measurements have much larger error - subject distance, eg.  You may blithely say "suppose I am 3m away", but that distance is, in practice, rarely measured at all, let alone to +/- 3%.  And measurement errors accumulate: if the focal length is measured to +/- 3%, and the distance is measured to +/- 3%, the DoF is accurate to +/- 18% - because DoF is proportional to the square of distance and inversely proportional to the square of focal length. (This is leaving aside the issue of significant figures, and the indefensible practice of calculating DoF to four or five significant figures when the CoC has one).

So, is your definition of "equivalent", "within measurement error"?  If so, you will know what all the errors are, so you can tell us. 

Another common meaning of "equivalent" would be that the difference is less than is practically important.  My professional expertise is in medicine, so I will use a medical example.  In the pre-antibiotic era, 3% of previously healthy people with pneumococcal pneumonia died.  With penicillin, none die.  So you have to treat 100/3 = 33 previously healthy people with pneumonia with penicillin to prevent one death.  This is called the "number needed to treat", or NNT.  Everyone can set the NNT they care about for themselves, but most ordinary people regard numbers over a few hundred as not worth bothering about, even for serious outcomes.  So if you have to give 500 people treatment A compared to treatment B to prevent one day off work, the two treatments are "equivalent" (there are plenty of commonly used treatments with NNT to prevent one death in the thousands - 5000 for taking a statin if you are a woman under 50 with no known heart disease, eg).  Thresholds of worthwhileness come in here, in photography as they do in medicine - the difference being that you can't get a D5 on the NHS, like you can a statin, so there is no need for a public concensus. 

So, what is your definition of a meaningful difference, and, if you want everyone else to accept it, where did it come from?  (If it is for your personal use only, you can choose whatever you like - but it would help if you said what it was).  I  am not saying that 70mm and 75mm are not different: I am asking why, if 50 and 54mm are not different, how come 70mm and 75mm are?