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Author Topic: Will Michael revisit ETTR?  (Read 111004 times)

Schewe

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Re: Will Michael revisit ETTR?
« Reply #60 on: August 21, 2011, 06:20:47 pm »

Hans,

Go back and read what Eric wrote: "ACR's Exposure and Brightness are essentially the same, except for the handling of highlights.  Exposure will hard-clip the highlights, whereas Brightness will not.  For example, if you push Exposure to +1, then anything that was in the top stop will get clipped off -- gone -- similar to digital exposure.  In contrast, if you push Brightness from +50 to +100, then anything that was in the top stop will get smoothly rolled off (instead of clipped off) -- similar to film."

The big difference between Exposure and Brightness is the fact that + Brightness gets rolled off so it won't clip while + Exposure is designed to clip. There are differences between -Brightness and -Exposure as well, -Exposure has a degree of highlight clipping reduction while -Brightness shifts the midtone down more.

As far as Eric giving you more...well he's one of the top ACR/LR engineers working directly with Thomas Knoll. Exactly how much he can say about anything can only be determined by him. He also indicated somewhere on one of the many (too many) threads about ETTR that you can get more info on how ACR/LR works by downloading and reading the DNG SDK available here.
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Ray

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Re: Will Michael revisit ETTR?
« Reply #61 on: August 21, 2011, 07:39:45 pm »


 ETTR is about maximizing signal-to-noise.  That's all. 


Halleluiah, we can finally put this myth about the number of levels in the highlights to rest. If Michal were to remove that concept from his otherwise fine post, I would be in complete agreement. My opinion may not carry much weight, but the opinions of experts like yourself and others do.

An excellent point!

Regards,

Bill


I think we're engaging in more than semantics here, Bill. One can't disagree with Eric that ETTR is about maximizing signal-to-noise, but the 'That's all' bit is misleading in my view.

A more precise definition would be, "Setting the exposure to maximise signal-to-noise whilst simultaneously retaining full detail in all highlights in the scene that one considers important."

The process of merging to HDR is also all about maximising signal-to-noise, and often one of the 3 or 5 or 7 or 9 different exposures taken for that purpose, will likely be an ETTR. However, the exposure that maximises signal-to-noise will be an obvious overexposure.

It's very easy to maximise signal-to-noise. Just overexpose, and the more overexposure the better, as regards signal-to-noise. From a strictly logical perspective, all overexposures could be considered as ETTRs, if you wish to add 'That's all'.


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hjulenissen

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Re: Will Michael revisit ETTR?
« Reply #62 on: August 22, 2011, 04:31:50 am »

It's very easy to maximise signal-to-noise. Just overexpose, and the more overexposure the better, as regards signal-to-noise. From a strictly logical perspective, all overexposures could be considered as ETTRs, if you wish to add 'That's all'.
You could consider clipping to be some kind of noise (undesired modification of the image as projected onto the sensor). But your point still stands: if artistic goals (DOF, motion blur) allows, you should expose so hot as to maximize signal level compared to noise level, while avoiding exposing so hot as to clip bright parts of the scene that you want accurately captured.

-h
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bjanes

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Re: Will Michael revisit ETTR?
« Reply #63 on: August 22, 2011, 10:58:25 am »

The process of merging to HDR is also all about maximising signal-to-noise, and often one of the 3 or 5 or 7 or 9 different exposures taken for that purpose, will likely be an ETTR. However, the exposure that maximises signal-to-noise will be an obvious overexposure.

It's very easy to maximise signal-to-noise. Just overexpose, and the more overexposure the better, as regards signal-to-noise. From a strictly logical perspective, all overexposures could be considered as ETTRs, if you wish to add 'That's all'.

That depends on how you define signal. Shown below are results from the Nikon D3 with duplicate flat fields of the green1 channel taken for sensor analysis. The shot noise is determined by subtracting the two flat fields after adding an offset to one field to prevent negative values. The resultant value is the shot noise for two frames, so one divides by the square root of 2 to obtain the noise for one image. Sigma (standard deviation) is shown for the individual values for each pair and Sigma 3 is the standard deviation for one frame. One notes that the noise increases as one goes from frames 55 to 39 and the SNR decreases as expected. However, when the sensor saturates, the noise is clipped and the SNR reached very high values. However, this is not useful SNR, since the signal has been clipped. The same trend applies for total noise rather than shot noise.

In ETTR exposing, when the DR of the scene is greater than that of the sensor, SNR will increase with increasing exposure until the sensor saturates. Further exposure will clip the highlights but the DR will remain constant with further exposure as one stop of shadows is gained for each loss of a stop in highlights. The SNR of the overexposed shadow values will be improved. This is the basis for HDR photography.

Regards,

Bill
« Last Edit: August 22, 2011, 11:08:55 am by bjanes »
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madmanchan

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Re: Will Michael revisit ETTR?
« Reply #64 on: August 22, 2011, 04:21:56 pm »

A more precise definition would be, "Setting the exposure to maximise signal-to-noise whilst simultaneously retaining full detail in all highlights in the scene that one considers important."

The process of merging to HDR is also all about maximising signal-to-noise, and often one of the 3 or 5 or 7 or 9 different exposures taken for that purpose, will likely be an ETTR. However, the exposure that maximises signal-to-noise will be an obvious overexposure.

It's very easy to maximise signal-to-noise. Just overexpose, and the more overexposure the better, as regards signal-to-noise. From a strictly logical perspective, all overexposures could be considered as ETTRs, if you wish to add 'That's all'.

Yes, but I don't consider a clipped (saturated) signal to be useful.   ;)  

It is up to the photographer to decide what is "important" to retain in the highlights.  For example, it may be fine (even desirable) to clip some specular reflections to pure white -- no detail recorded in those areas.  In other cases, it may be desirable to keep the full range of highlights.  This may make it difficult or impossible to increase the exposure further.

So, to clarify: the motivation behind ETTR is all about maximizing signal-to-noise, but in the field ETTR cannot always be used for all situations.
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Eric Chan

Ray

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Re: Will Michael revisit ETTR?
« Reply #65 on: August 22, 2011, 07:27:26 pm »

Yes, but I don't consider a clipped (saturated) signal to be useful.   ;)  

It is up to the photographer to decide what is "important" to retain in the highlights.  For example, it may be fine (even desirable) to clip some specular reflections to pure white -- no detail recorded in those areas.  In other cases, it may be desirable to keep the full range of highlights.  This may make it difficult or impossible to increase the exposure further.

So, to clarify: the motivation behind ETTR is all about maximizing signal-to-noise, but in the field ETTR cannot always be used for all situations.


We seem to be confusing the 'objective' with the 'subjective', which is why I would consider my definition to be more precise because it includes both the objective and the subjective, ie. ""Setting the exposure to maximise signal-to-noise whilst simultaneously retaining full detail in all highlights in the scene that one considers important."

What one considers important is a subjective assessment. Setting exposure to maximise the total number of levels captured in the scene could be considered as the basic, objective definition of ETTR, which is a good starting point.

The obvious example, which I've used before somewhere, is a scene of wildlife in the shade of a tree with significant areas of sky visible through the branches. One photographer thinks the sky (its detail or its color) is sufficiently interesting to retain, and increases his shutter speed accordingly. Another photographer decides to use a slower shutter speed to reduce noise in the main subject which he considers is the bird or wildlife, at the risk perhaps of failing to freeze movement.

It makes little sense from an objective perspective to describe both exposures, which may differ significantly, an ETTR. From an objective perspective, the shot that retains detail in the sky, to point of near clipping, is an ETTR. The other shot is a deliberate overexposure for esthetic or subjective reasons.
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Bryan Conner

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Re: Will Michael revisit ETTR?
« Reply #66 on: August 23, 2011, 02:47:54 am »

Maybe we should officially christen the term "Subjective ETTR".  But, this a purely subjective suggestion....or is it? :o
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Ray

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Re: Will Michael revisit ETTR?
« Reply #67 on: August 23, 2011, 03:48:31 am »

Maybe we should officially christen the term "Subjective ETTR".  But, this a purely subjective suggestion....or is it? :o

Ha! The lack of understanding of the difference between the subjective and the objective is perhaps the major contributor to all the troubles in the world.

I would prefer it if the definition of ETTR (Extra Terrestrial Tryannosaurus Rex) would confine itself to objective facts, ie, a full exposure short of clipping the highlights, excluding specral highlights which by definition are very intense and cover a very small area of the frame.

Why complicate things with a multitude of different subjective assessments which can have no ultimate resolution.

If you want to blow highlights, such as a sky, because it's of less importance than the rest of the image, then let's call it an 'overexposure for esthetic reasons'.
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bjanes

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Re: Will Michael revisit ETTR?
« Reply #68 on: August 23, 2011, 08:49:43 am »

What one considers important is a subjective assessment. Setting exposure to maximise the total number of levels captured in the scene could be considered as the basic, objective definition of ETTR, which is a good starting point.

The old myth regarding the number of levels dies hard. As Eric and Emil Martinec have pointed out it is the SNR that benefits from ETTR. Due to dithering by noise, the highlights have many fewer levels than presumed from a purely mathematical analysis, but also do not show posterization because of the noise.

The obvious example, which I've used before somewhere, is a scene of wildlife in the shade of a tree with significant areas of sky visible through the branches. One photographer thinks the sky (its detail or its color) is sufficiently interesting to retain, and increases his shutter speed accordingly. Another photographer decides to use a slower shutter speed to reduce noise in the main subject which he considers is the bird or wildlife, at the risk perhaps of failing to freeze movement.

It makes little sense from an objective perspective to describe both exposures, which may differ significantly, an ETTR. From an objective perspective, the shot that retains detail in the sky, to point of near clipping, is an ETTR. The other shot is a deliberate overexposure for esthetic or subjective reasons.

Since the writings of Bruce Fraser, the rationale of ETTR has always been to place highlights in which one wishes to preserve image detail near the clipping point. If you want to preserve the sky in your situation, you place the sky highlights near clipping. If shadow detail is more important and the DR of the scene exceeds that of the camera, then one will have to allow the highlights to clip or else take 2 or more exposures. The principles of ETTR remain the same, but subjective decisions govern the exposure. In both cases one gives maximal exposure to achieve the intended effect.

Regards,

Bill
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Ray

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Re: Will Michael revisit ETTR?
« Reply #69 on: August 23, 2011, 12:59:32 pm »


Since the writings of Bruce Fraser, the rationale of ETTR has always been to place highlights in which one wishes to preserve image detail near the clipping point. If you want to preserve the sky in your situation, you place the sky highlights near clipping. If shadow detail is more important and the DR of the scene exceeds that of the camera, then one will have to allow the highlights to clip or else take 2 or more exposures. The principles of ETTR remain the same, but subjective decisions govern the exposure. In both cases one gives maximal exposure to achieve the intended effect.

Regards,

Bill


So what's new? What's all the fuss about?
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hjulenissen

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Re: Will Michael revisit ETTR?
« Reply #70 on: August 23, 2011, 01:58:04 pm »

So what's new? What's all the fuss about?
Good question. Exposing "hot" is common sense for anyone doing digital recordings of any kind of data. For people that have only used film cameras, this could be new. For people relying on automatic camera exposure algorithms, or in-camera histograms, knowledge about the impresise feedback and built-in margins could be new.

-h
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madmanchan

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Re: Will Michael revisit ETTR?
« Reply #71 on: August 23, 2011, 02:06:45 pm »

While the basic concept of ETTR is objective, my view is that the practical application of ETTR is nearly always subjective to some degree.  In many common situations, some pixels will get clipped regardless of the choice of exposure (e.g., bright light source in the scene).  To be effective, a camera algorithm that implemented ETTR would need to have some algorithm (or possibly user control) to decide the clip threshold, e.g., the difference between clipping .01% and 1% can be huge in some scenes.
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Eric Chan

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Re: Will Michael revisit ETTR?
« Reply #72 on: August 23, 2011, 03:13:09 pm »

While the basic concept of ETTR is objective, my view is that the practical application of ETTR is nearly always subjective to some degree.  In many common situations, some pixels will get clipped regardless of the choice of exposure (e.g., bright light source in the scene).  To be effective, a camera algorithm that implemented ETTR would need to have some algorithm (or possibly user control) to decide the clip threshold, e.g., the difference between clipping .01% and 1% can be huge in some scenes.
Agree with the differences that would occur; I just want us to have more options that we have right now.  I'm reading up on uniWB and maybe it's a decent solution but it does require some work to implement it (just as testing your metering system and JPG histogram to calibrate that for the real world).
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Ray

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Re: Will Michael revisit ETTR?
« Reply #73 on: August 24, 2011, 01:21:18 am »

The old myth regarding the number of levels dies hard. As Eric and Emil Martinec have pointed out it is the SNR that benefits from ETTR. Due to dithering by noise, the highlights have many fewer levels than presumed from a purely mathematical analysis, but also do not show posterization because of the noise.

Bill


Really! Do you mean it's possible to have an unprocessed ettr RAW file with the resultant benefit of a better SNR but without the concomitant increase in the number of levels that one expects from an ETTR? I never realised that.

I always had some vague notion that an ETTR exposure, as compared to an underexposure, would allow a greater number of levels to be recorded.

Of course, I understand whether or not such increased levels can be seen is another issue. In the midtones and shadows, they no doubt can be seen. In the upper tones, many of the levels may to be too similar for the eye to distinguish.

Is this a myth then?
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Ray

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Re: Will Michael revisit ETTR?
« Reply #74 on: August 24, 2011, 01:53:28 am »

While the basic concept of ETTR is objective, my view is that the practical application of ETTR is nearly always subjective to some degree.  In many common situations, some pixels will get clipped regardless of the choice of exposure (e.g., bright light source in the scene).  To be effective, a camera algorithm that implemented ETTR would need to have some algorithm (or possibly user control) to decide the clip threshold, e.g., the difference between clipping .01% and 1% can be huge in some scenes.

Indeed! And herein lies the problem; deciding how much clipping is best. An automatic exposure which is accurate from an ETTR perspective, resulting in a RAW histogram on the review screen that takes the place of a UNIWB adjustment, whilst simultaneously providing a realistic and natural rendition of the picture taken, would be of some benefit.

But as soon as we get back to user-controlled adjustments made on the basis of subjective assessments of the brightness characteristics of the scene, such as trying to assess whether the area of sky visible through the branches of a tree constitutes 1% or 5% or 10% of the image, we are likely to fail to capture the moment through too much stuffing around.

One might as well just use the spot-meter technique directed at the brightest part of the scene one wants to preserve, then make the appropriate increase in exposure.

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Bart_van_der_Wolf

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Re: Will Michael revisit ETTR?
« Reply #75 on: August 24, 2011, 03:48:52 am »

Really! Do you mean it's possible to have an unprocessed ettr RAW file with the resultant benefit of a better SNR but without the concomitant increase in the number of levels that one expects from an ETTR? I never realised that.

The number of levels is fixed upon quantization by the ADC. The lower exposure levels may suffer from noise to the point that they are hard to discriminate in a single exposure. It's solely the improved S/N ratio that will seemingly add levels, but that is just the result of the improved S/N ratio. The levels were there all the time, it's just that some may not be very useful.

Quote
I always had some vague notion that an ETTR exposure, as compared to an underexposure, would allow a greater number of levels to be recorded.


They are recorded anyway, but may look swamped by noise, especially when underexposed. Improving the Poisson noise statistics by increasing the photon count is all it takes to improve the discrimination. Increasing the exposure time without clipping relevant highlights is one method to achieve that, stacking and averaging multiple exposures of scenes without moving objects is another. It's all about improving the photon statistics.

Cheers,
Bart
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Ray

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Re: Will Michael revisit ETTR?
« Reply #76 on: August 24, 2011, 05:24:47 am »

The number of levels is fixed upon quantization by the ADC. The lower exposure levels may suffer from noise to the point that they are hard to discriminate in a single exposure. It's solely the improved S/N ratio that will seemingly add levels, but that is just the result of the improved S/N ratio. The levels were there all the time, it's just that some may not be very useful.
 

They are recorded anyway, but may look swamped by noise, especially when underexposed. Improving the Poisson noise statistics by increasing the photon count is all it takes to improve the discrimination. Increasing the exposure time without clipping relevant highlights is one method to achieve that, stacking and averaging multiple exposures of scenes without moving objects is another. It's all about improving the photon statistics.

Cheers,
Bart

A wonderful exercise in obfuscation, Bart. Can I nominate you for a prize?  ;D

The question is simple. Does an increase in exposure, up to the point of clipping of highlights, result in an increase in the number of levels recorded? Yes, No or Maybe?

If maybe, then please decribe the circumstances.
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ejmartin

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Re: Will Michael revisit ETTR?
« Reply #77 on: August 24, 2011, 08:31:59 am »


The question is simple. Does an increase in exposure, up to the point of clipping of highlights, result in an increase in the number of levels recorded? Yes, No or Maybe?

If maybe, then please decribe the circumstances.

As people have been saying (repeatedly) the issue is S/N ratio, not the number of levels.  At a given illumination level (number of photons = S), the noise goes as sqrt(S), so the S/N = sqrt(S).  You improve S/N by capturing more photons -- raising the exposure.  Regardless how many levels there are, two tones are distinguishable (on average) if they are spaced more than the noise.  So there are roughly sqrt(S2)-sqrt(S1) distinguishable tones in the exposure range between S1 and S2.  If you increase the exposure by a stop, you increase this number of distinguishable tones by a factor sqrt(2)~1.4, even though the number of levels in the raw data has doubled.  Similarly, if you took the same picture with a D700 in 12-bit mode and 14-bit mode, the 14-bit capture would have 4x more levels in the raw data, but the same number of distinguishable tones, because at the same exposure the same number of photons is captured.  It's the S/N, not the number of levels.

There are corrections to the above analysis due to read noise, that are only important at low exposure (deep, deep in the shadows at low ISO, creeping upward into higher zones as the ISO is raised).
« Last Edit: August 24, 2011, 08:33:42 am by ejmartin »
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emil

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Re: Will Michael revisit ETTR?
« Reply #78 on: August 24, 2011, 09:46:54 am »

As people have been saying (repeatedly) the issue is S/N ratio, not the number of levels.

Indeed and a good point to reinforce. But if we look at the distribution of levels in a linear encoded file, we see half of all that data in the first stop of highlight and on the other end, the smallest number of levels. The question is, how do we describe in a sentence or two the relationship of those fewer levels in the last stop, the noise that results there with a higher or lower S/N ratio? In the classic example shown on this site and originally by Bruce Fraser, the last stop of shadow detail had 16 levels (as opposed to the first at 2048). Its a good point that no matter the exposure, its still 16 levels, how do we define the relationship and effect of these fewer levels, noise and S/N?

Quote
A wonderful exercise in obfuscation, Bart. Can I nominate you for a prize?
In the original ETTR article here, Ian Lyons is quoted as saying: The ideal exposure ensures that you have maximum number of levels describing your image without loosing important detail in the highlights. The closer you get to this ideal then the more of those levels are being used to describe your shadows. How about working on a sentence that takes all of the above into account and explains the relationship and effect of these fewer levels, noise and S/N.

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bjanes

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Re: Will Michael revisit ETTR?
« Reply #79 on: August 24, 2011, 10:20:58 am »

As people have been saying (repeatedly) the issue is S/N ratio, not the number of levels.  ...

If you increase the exposure by a stop, you increase this number of distinguishable tones by a factor sqrt(2)~1.4, even though the number of levels in the raw data has doubled.  Similarly, if you took the same picture with a D700 in 12-bit mode and 14-bit mode, the 14-bit capture would have 4x more levels in the raw data, but the same number of distinguishable tones, because at the same exposure the same number of photons is captured.  It's the S/N, not the number of levels.

Data from DXO can be used to confirm Emil's analysis. I will use the Phase One P65+ as an example, since it is generally regarded as one of the highest performers. The tonal range is the number of discrete gray levels that can be discerned in the image. From the DXO graph and derived table, one can see that the tonal range decreases by 0.5 bits for each doubling of ISO. Doubling of ISO decreases the number of photons by a factor of 2 and the number of levels by sqrt(2). At base ISO the P65+ has a tonal range of 8.64 bits or 399 levels.

The difference in the number of levels between ISO of 100 and ISO 50 reveals by subtraction that the brightest f/stop used for the base ISO of 50 contains 117 levels, far fewer than the 33,000 levels in the brightest f/stop of the 16 bit capture.

Regards,

Bill

« Last Edit: August 24, 2011, 10:32:26 am by bjanes »
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