Will Michael revisit ETTR?

[kwalsh]

Ray,

Since you are the one out in left field on this one perhaps rather than challenging other people to provide examples to "prove" to you that fundamental image and signal processing concepts are in fact true you could take the time to do them yourself.  No one here is served by wasting their time illustrating the same point over and over only to have you plug your ears.  If you want to understand something take the time to explore it yourself since clearly no one is going to convince you of anything.  If you own a camera and a RAW converter you've already got all the tools you need.

Ken

[Ray]

Do the number of 'levels' and 'distinguishable tones' in reply 85 look the same to you?

Of course they don't. There is obviously a distinction to be made between what's recorded and visually distinguishable, and what's recorded but not visually distinguishable.

What I've asked you to provide is a comparison whereby increasing the exposure, and consequently the SNR, does not simultaneously increase the number of distinguishable levels where such greater numbers of different levels do actually exist in the scene being photographed, and also exist in that part of the tonal range where the eye is able to perceive such increase in the number of levels.

If you can do this, then I would concede that ETTR is all about SNR and not about levels.

[Ray]

Ray,

Since you are the one out in left field on this one perhaps rather than challenging other people to provide examples to "prove" to you that fundamental image and signal processing concepts are in fact true you could take the time to do them yourself.  No one here is served by wasting their time illustrating the same point over and over only to have you plug your ears.  If you want to understand something take the time to explore it yourself since clearly no one is going to convince you of anything.  If you own a camera and a RAW converter you've already got all the tools you need.

Ken

You're joking, aren't you? I've done hundredes of tests over the years comparing such issues with different exposures, different cameras and different ISOs.

The most recent set of experiments I made was when I took delivery of the D7000, having bought the camera because I was intrigued by its high DR capability. I took dozens of test shots ranging from full ETTR exposures to 14 stops underexposure.

With each stop of increase in exposure, from 14 stops underexposed, I've observed an increase in the number of distinguishable levels of detail, where detail exists in the real scene. Where detail doesn't exist, as in a plain wall, then of course one doesn't expect to see an increase in the number of 'real' tones, merely a reduction in the number of spurious tones, ie, noise.

[ejmartin]

What I've asked you to provide is a comparison whereby increasing the exposure, and consequently the SNR, does not simultaneously increase the number of distinguishable levels where such greater numbers of different levels do actually exist in the scene being photographed, and also exist in that part of the tonal range where the eye is able to perceive such increase in the number of levels.

If you can do this, then I would concede that ETTR is all about SNR and not about levels.

The discussion has been about the assertion in Michael's posts on ETTR that the benefit derives from the increase in available raw levels, not from the increase in distinguishable tonalities resulting from increased S/N.  I don't know why you are trying to change the basis of the discussion.  Of course increasing the SNR increases the number of distinguishable tonal values; that was my point all along, so of course I won't be providing you a counterexample.  The issue was always whether the image quality improvement had anything to do with the number of raw levels used to represent the image, and I have provided a wide variety of counterexamples to that assertion.  Since you have a D7000, I suggest you try my proposed experiment in reply #98.

[Ray]

The discussion has been about the assertion in Michael's posts on ETTR that the benefit derives from the increase in available raw levels, not from the increase in distinguishable tonalities resulting from increased S/N.  I don't know why you are trying to change the basis of the discussion.  Of course increasing the SNR increases the number of distinguishable tonal values; that was my point all along, so of course I won't be providing you a counterexample.  The issue was always whether the image quality improvement had anything to do with the number of raw levels used to represent the image, and I have provided a wide variety of counterexamples to that assertion.  Since you have a D7000, I suggest you try my proposed experiment in reply #98.

Emil,
The argument is really about semantics. I'm not in any fundamental disagreement with you. I just see things from a slightly different perspective, perhaps.

The initial effect of increasing exposure is an improvement in SNR. There no doubt about it.

The consequences of this increase in SNR is that more detail (or a greater number of tones or levels) become apparent, no longer obscured by noise.

From my very pragmatic perspective, this increase in detail is what's important because I know if it's lost, or buried too deep in the noise, it can't be retrieved.

However, if there's noise present where no detail exists, then any good noise reduction program can fix the problem. One can have a severely underexposed shot of a lady's face and make it appear as smooth as a baby's bottom, with a good noise reduction program, but at the expense maybe of losing a few eyelashes.

It's the increase in levels, where it can be seen, which is the main benefit of ETTR for us pragmatic photographers.

[kwalsh]

You're joking, aren't you?

No I really wasn't.

I've done hundredes of tests over the years comparing such issues with different exposures, different cameras and different ISOs.

Then you shouldn't have any trouble doing a few more.

The most recent set of experiments I made was when I took delivery of the D7000, having bought the camera because I was intrigued by its high DR capability. I took dozens of test shots ranging from full ETTR exposures to 14 stops underexposure.

Awesome, you should already have your answer then.  If you want more, you've got the best camera around to perform the tests Emil suggested.

With each stop of increase in exposure, from 14 stops underexposed, I've observed an increase in the number of distinguishable levels of detail, where detail exists in the real scene. Where detail doesn't exist, as in a plain wall, then of course one doesn't expect to see an increase in the number of 'real' tones, merely a reduction in the number of spurious tones, ie, noise.

Yep, that jives with what everyone is saying and what you've been saying as well as far as SNR goes.

The final step to your question about RAW levels is a trivial one with your D7000.  Take one exposure at 0 eV.  Take two exposures at -1 EV, average the two RAW files, do +1 EV compensation on the averaged RAW file.  Take four exposures at -2 EV, average the four RAW files, do +2 EV on the averaged RAW file.  Are there any differences in the mid tones of these three scenarios?  If yes, RAW levels matter, if no RAW levels don't matter only SNR.  If you can't average RAW files on your own the application PhotoAcute will do it for you and has a free trial download with a bit of water marking you can work around for this test.

Let us know what you find out.

Ken

P.S. Apologies if I've been missing what you are asking in all the semantics - the thread is confusing to say the least.

[Ray]

The final step to your question about RAW levels is a trivial one with your D7000.  Take one exposure at 0 eV.  Take two exposures at -1 EV, average the two RAW files, do +1 EV compensation on the averaged RAW file.  Take four exposures at -2 EV, average the four RAW files, do +2 EV on the averaged RAW file.  Are there any differences in the mid tones of these three scenarios?  If yes, RAW levels matter, if no RAW levels don't matter only SNR.  If you can't average RAW files on your own the application PhotoAcute will do it for you and has a free trial download with a bit of water marking you can work around for this test.

Let us know what you find out.

Why would I want to do that? There are all sorts of procedures one could adopt in order to get a particular result. I'm only concerned with what's of benefit in the field or the studio. I'm not a laboratory technician.

I once spent some time testing the stacking feature in Photoshop Extended. The principle is, one takes a number of shots of a static scene, say 5 or 8. When the images are stacked using the appropriate mode in Photoshop, the 'best' pixels from each frame are automatically selected to result in an image with lower noise and better detail. The improvement is very obvious at high ISO, with a reduction in noise and a corresponding increase in detail (number of 'real image' levels) equivalent to about 2 stops, maybe a bit less.

The issue for me is, in what circumstances would this technique be useful in the field. I could find only one circumstance where this might be useful, and that was when the scene was static, I had no tripod, and the light was poor, requiring use of a high ISO for a reasonably fast shutter speed to freeze camera movement.

If I were really careful, with steady hand, perhaps leaning against a tree, I could take about 5  shots at say ISO 1600 and 1/25th, and get a 'stacked' result roughly equivalent to a single ISO 400 shot taken on a tripod at a shutter speed of 1/6th.

P.S. Apologies if I've been missing what you are asking in all the semantics - the thread is confusing to say the least.

Apologies accepted, but why are you confused? Noise is a problem because it obscures detail (read levels). If there's no detail to be obscured, then ETTR is surely less of a concern because any unwanted, visible noise can be easily removed in noise reduction programs, or even the noise reduction slider in ACR.

[sandymc]

If yes, RAW levels matter, if no RAW levels don't matter only SNR.

We already know the answer to that, based on literally hundreds of thousands of exposures by some of the most picky IQ conscious photographers in the world. Shoot a Leica M9 in compressed mode, and your mid range f/stop (zone 5) has 14 raw levels. Yes, that's not a mistake, 14 levels. Gamma encoded, sure, so those levels are optimally distributed in the zone, but still only 14 discrete raw levels. Uncompressed you have 256 raw levels in the same zone. Nobody that I know of has ever been able to demonstrate that in a practical shooting situation the M9's uncompressed image has visibly better IQ than the compressed image.

I don't know whether Emil's numbers for the P65 are right or not, but they demonstrate a basic point - if you can only distinguish 10 tones in a zone, then whether you encode those 10 tones into 20 raw levels or 1000 raw levels make no difference to visible IQ at all.

It's probably possible to construct scenarios where raw levels matter, if you tried hard enough. But in a real world situation shooting a modern camera with a real sensor with real noise? Raw levels don't matter.

Sandy

[hjulenissen]

The issue for me is, in what circumstances would this technique be useful in the field. I could find only one circumstance where this might be useful, and that was when the scene was static, I had no tripod, and the light was poor, requiring use of a high ISO for a reasonably fast shutter speed to freeze camera movement.

Shooting a person/animal in low light using a stand, to improve the IQ of the background?

-h

[bjanes]

It's the increase in levels, where it can be seen, which is the main benefit of ETTR for us pragmatic photographers.

Ray,

You are truly a bitter-ender .  How are you quantifying all those levels that are overlapped by noise and how do you know that the improvement in IQ is due to increased levels and not merely due to improved SNR?

Bill

[Guillermo Luijk]

It's the increase in levels, where it can be seen, which is the main benefit of ETTR for us pragmatic photographers.

I have been hesitating about two possible answers to this, so I'll answer twice:

ANSWER #1
Have you ever _seen_ any increase in levels thanks to ETTR?

ANSWER #2

[Ray]

Ray,

You are truly a bitter-ender .  How are you quantifying all those levels that are overlapped by noise and how do you know that the improvement in IQ is due to increased levels and not merely due to improved SNR?

Bill

Ah! Good question, Bill. I think I've already agreed that the improvement in IQ, when increasing exposure, is a result of an improvement in SNR, but such improvement is expressed (or viewed to be relevant and useful) as a discernment in the number of different 'real' tones, or levels in the converted image.

In the absence of sufficiently different levels (detail and texture) in the scene being photographed, one would expect to notice a reduction in levels or tones, that is, a reduction in spurious levels or tones, described as noise, unless one is viewing detail in the upper tonal region where noise may not be a problem and the number of levels recorded is far greater than the eye can discern in any case.

I've raised the question, is it possible to have an image with reduced 'visible' noise, as a result of a greater exposure, which does not allow for the 'visual' discernment of a greater number of levels or tones, when such tones exist in the scene being photographed.

If the answer is 'no', then I think we would have to agree that ETTR is all about improvement in SNR and increased number of levels. No? 

[digitaldog]

What I've asked you to provide is a comparison whereby increasing the exposure, and consequently the SNR, does not simultaneously increase the number of distinguishable levels where such greater numbers of different levels do actually exist in the scene being photographed, and also exist in that part of the tonal range where the eye is able to perceive such increase in the number of levels.

I don’t think Michael ever implied that ETTR provides more levels right? I think the point made originally was there are fewer levels in the shadow area of the (linearly) captured data and that by implementing ETTR, you end up with the cleanest (less noise) levels where so few levels originate.

The initial effect of increasing exposure is an improvement in SNR. There no doubt about it.
The consequences of this increase in SNR is that more detail (or a greater number of tones or levels) become apparent, no longer obscured by noise.

I think all of us are in agreement in those comments right? Noise has a tone/level, its non image data (noise, gunk, crap) and doesn’t provide anything useful in terms of showing us what we hoped to capture.

Did we get sidetracked with semantics with the idea that ETTR provides more levels?

[Ray]

I have been hesitating about two possible answers to this, so I'll answer twice:

ANSWER #1
Have you ever _seen_ any increase in levels thanks to ETTR?

Ha!   All the time, whenever I increase exposure. It's wonderful. But I'm referring to differences in real tones and levels that exist in the scene. The increase in such levels is of course most apparent in the shadows, as you know.

[bjanes]

I don’t think Michael ever implied that ETTR provides more levels right? I think the point made originally was there are fewer levels in the shadow area of the (linearly) captured data and that by implementing ETTR, you end up with the cleanest (less noise) levels where so few levels originate.

I think all of us are in agreement in those comments right? Noise has a tone/level, its non image data (noise, gunk, crap) and doesn’t provide anything useful in terms of showing us what we hoped to capture.

Did we get sidetracked with semantics with the idea that ETTR provides more levels?

Well, he starts out with the passage:

A typical consumer DSLR recording 12 bits per sensel is able to record up to 4,098 separate tonal values.

    If we assume a 10 stop dynamic range this is how this data is distributed...
        The brightest stop = 2048 tonal values
        The next brightest stop = 1024 tonal values
        The next brightest stop = 512 tonal values
        The next brightest stop = 256 tonal values
        The next brightest stop = 128 tonal values
        The next brightest stop = 64 tonal values
        The darkest stop = 32 tonal values
    As can be seen, each stop from the brightest to the darkest contains half of the data of the one preceding it.

This helps explain why noise is seen most in the darkest areas of a file. In the brightest areas there is a lot of data and so the noise floor (which is always present) only represents a small percentage of the total signal (or data). In the darker areas, where data is sparse, ever-present noise becomes easily visible.

This implies that noise has something do to with the number of levels. Later in the essay the does a SNR analysis using photon shot noise as a limiting factor. However, he never quite comes around to the fact that the number of discrete and distinguishable levels is limited by noise throughout the image from highlights to shadows (disregaring read noise) rather than the number of levels in the raw file. Much confusion would have been eliminated had he left out that passage and gone directly to SNR analysis.

Regards,

Bill

[Ray]

Did we get sidetracked with semantics with the idea that ETTR provides more levels?

I think so. I'm making a distinction between the recording of real levels in the scene, as opposed to spurious levels defined as noise.
I have a few images which are drastically underexposed, yet contain more levels than some ETTR exposures. The levels take the form of pattern noise which looks like a coarse jute fabric, quite detailed.

See attached image. With a greater exposure, that sky would have lost a great number of different levels.  

[sandymc]

Did we get sidetracked with semantics with the idea that ETTR provides more levels?

Actually, it's not a sidetrack, it's central to the ETTR argument - without the alleged benefits from the additional raw levels, ETTR is a waste of time outside of certain special situations (synthesizing a lower ISO, optimizing read noise versus sensor noise for some cameras). But the general case for ETTR just vanishes without levels because you can get the S/N advantages of ETTR by just changing ISO, and exposing normally.

Sandy

[ejmartin]

I don’t think Michael ever implied that ETTR provides more levels right? I think the point made originally was there are fewer levels in the shadow area of the (linearly) captured data and that by implementing ETTR, you end up with the cleanest (less noise) levels where so few levels originate.

Did we get sidetracked with semantics with the idea that ETTR provides more levels?

The quibble is that in both posts, Michael puts the following statement -- which is misleading at best -- front and center:

A typical consumer DSLR recording 12 bits per sensel is able to record up to 4,098 separate tonal values.
    * If we assume a 10 stop dynamic range this is how this data is distributed...
          o The brightest stop = 2048 tonal values
          o The next brightest stop = 1024 tonal values
          o The next brightest stop = 512 tonal values
          o The next brightest stop = 256 tonal values
          o The next brightest stop = 128 tonal values
          o The next brightest stop = 64 tonal values
          o The darkest stop = 32 tonal values
    * As can be seen, each stop from the brightest to the darkest contains half of the data of the one preceding it.

and a little bit later:

The reason why we want to expose every shot that we take with the data as far to the right of the histogram as possible is because that's where the data is! It also is where the visible noise isn't. The visible noise is lurking in the darker stops.

Since both increasing the exposure and increasing the ISO move the histogram to the right, one is led to conclude that they are equally good according to this criterion, and this is simply false, as one sees from the correct analysis in terms of S/N.  The conflating of S/N and number of raw levels is all over Michael's writing on this subject, and it has done a tremendous disservice to the photography community.  For instance, for many cameras (including all MFDB's; I showed which ones in post #90), ETTR doesn't make sense above base ISO -- you are better off at or near base ISO* even if the maximum exposure you can tolerate leaves the histogram to the left.  With these cameras, the data has the same shadow/midtone quality at the lower ISO, with the histogram to the left, and more headroom for accurately capturing highlights, relative to the same maximum tolerable exposure at a higher ISO where the histogram would be more to the right.

*Since MFDB's typically use 16-bit data output from a system with 12-bit DR, the data is already overspecified and so base ISO is ample.

Consequently, the whole section of Michael's most recent post entitled "Welcome to the 21st Century" is misguided for the above class of cameras.  For such cameras, the proper optimization is not an 'ETTR mode' which sets the exposure on the basis of (a user specified) percentage of clipped pixels.  Rather, one wants a 'maximize exposure' mode.  How would this work for this class of cameras?  It would act much like "safety shift" does on Canon DSLRs -- it sets the ISO to base; the user specifies their max tolerated exposure by setting Tv and Av according to their motion blur and DoF requirements (ie as in Manual mode); and if a specified threshold percentage of pixels is clipped, the exposure is reduced.  This functionality is rather different from the proposals in Michael's post, unless the exposure set is such that the safety shift is always invoked.

[digitaldog]

Well, he starts out with the passage:
        The darkest stop = 32 tonal values
    As can be seen, each stop from the brightest to the darkest contains half of the data of the one preceding it.

This helps explain why noise is seen most in the darkest areas of a file. In the brightest areas there is a lot of data and so the noise floor (which is always present) only represents a small percentage of the total signal (or data). In the darker areas, where data is sparse, ever-present noise becomes easily visible.[/i]

This implies that noise has something do to with the number of levels.

The key word there is implies. He doesn’t say as far as I can see, that ETTR alters the numbers of levels, only provides cleaner (less noise) in those levels.

[Ray]

Shooting a person/animal in low light using a stand, to improve the IQ of the background?

-h

How does that work? If one takes half a dozen shots over a period of one second, the person or animal will likely have moved. The person or animal will be blurred.