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Author Topic: 12, 14 or 16 bits?  (Read 11094 times)
cunim
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« Reply #20 on: February 20, 2011, 08:33:34 AM »
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Bill, my hat is off to someone who understands DR.  I have an amusing story about regression lines that .....  Nah, what am I thinking.  Nothing to do with photography.

Peter
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ErikKaffehr
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« Reply #21 on: February 20, 2011, 09:23:45 AM »
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Hi,

I'm most thankful for Emil Martinec and Bill chiming in and putting some things right. The topic was not really intended to discuss DR but to shed some light on the usefulness of bits. DxO is measuring DR so I felt it was a good starting point on usefulness of bits. My suggestions on measuring DR was intended to illustrate that measuring that is no wizardry. I was not really aware of the issues that Bill mentioned.

Best regards
Erik

Bill, my hat is off to someone who understands DR.  I have an amusing story about regression lines that .....  Nah, what am I thinking.  Nothing to do with photography.

Peter
« Last Edit: February 20, 2011, 01:31:43 PM by ErikKaffehr » Logged

bjanes
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« Reply #22 on: February 20, 2011, 11:16:50 AM »
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I'm most thankful for Emil Martinec and Bill chiming in and putting some things right. The topic was not really intended to discuss DR but to shed some light on the usefulness of bits. DxO is measuring DR so I felt it was a good starting point on usefulness of bits. My suggestions on measuring DR was intended to illustrate that measuring that is no wizardry. I was not really aver of the issues that Bill mentioned.

Yes, but the number of bits needed to encode a raw image depends on the DR. DXO rates the DR of the new Pentax 645 at 11.37 EV (screen). According to the Kodak spec sheet the full well for the KAF 40000 used in that camera is 42,000 e- and the read noise is 13 e-. This would give an engineering DR of 11.7 EV, consistent with DR of 70.2 dB reported on the spec sheet (1 EV is approximately equal to 6 dB). Thus, 12 bits should be sufficient for this camera and 14 bits are not really needed. The useful photographic DR would be less than 11.37 EV reported by DXO, since a S:N of 1 would give poor shadow detail, and one could probably get by with fewer bits.

For a daylight exposure, the red and blue channels would be considerably below saturation and the demosiaced DR would be worse. Some photographers place a magenta filter over the lens to hold back some of the green and obtain a better exposure balance for the channels, but with modern cameras I don't think this is worthwhile. Usually, DR is calculated from green channels.

DXO rates the Nikon D7000 DR (screen) at 13.35 EV. The DPR review gives a DR of 9.2 EV. They are using a Stouffer wedge and are apparently using a demosaiced RGB image with a "defined 'black point' (about 2% luminance) or the signal-to-noise ratio drops below a predefined value (where shadow detail would be swamped by noise), whichever comes first". They are using the camera with a picture control which is not linear and has a black point well above 0. 2% luminance in a linear raw file is 5.64 EV below clipping, so they are obviously working with a gamma encoded file. What a joke Smiley Smiley
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ErikKaffehr
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« Reply #23 on: February 20, 2011, 01:42:24 PM »
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Hi,

The starting point on this was really that someone stated that Leica DMR was 16 bits, and I wanted to demonstrate that 16 bit is not really useful with todays technology. I learned a lot from this discussion.

Best regards
Erik


Yes, but the number of bits needed to encode a raw image depends on the DR. DXO rates the DR of the new Pentax 645 at 11.37 EV (screen). According to the Kodak spec sheet the full well for the KAF 40000 used in that camera is 42,000 e- and the read noise is 13 e-. This would give an engineering DR of 11.7 EV, consistent with DR of 70.2 dB reported on the spec sheet (1 EV is approximately equal to 6 dB). Thus, 12 bits should be sufficient for this camera and 14 bits are not really needed. The useful photographic DR would be less than 11.37 EV reported by DXO, since a S:N of 1 would give poor shadow detail, and one could probably get by with fewer bits.

For a daylight exposure, the red and blue channels would be considerably below saturation and the demosiaced DR would be worse. Some photographers place a magenta filter over the lens to hold back some of the green and obtain a better exposure balance for the channels, but with modern cameras I don't think this is worthwhile. Usually, DR is calculated from green channels.

DXO rates the Nikon D7000 DR (screen) at 13.35 EV. The DPR review gives a DR of 9.2 EV. They are using a Stouffer wedge and are apparently using a demosaiced RGB image with a "defined 'black point' (about 2% luminance) or the signal-to-noise ratio drops below a predefined value (where shadow detail would be swamped by noise), whichever comes first". They are using the camera with a picture control which is not linear and has a black point well above 0. 2% luminance in a linear raw file is 5.64 EV below clipping, so they are obviously working with a gamma encoded file. What a joke Smiley Smiley
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NikoJorj
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« Reply #24 on: February 20, 2011, 02:51:27 PM »
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[...] The useful photographic DR would be less than 11.37 EV reported by DXO, since a S:N of 1 would give poor shadow detail, and one could probably get by with fewer bits.
Yes, but...wouldn't reduce too far the number of bits actually makes more noise, due to quantization noise kicking in?

I say that mostly because when truncating bits of an image (as with Guillermo's example I linked in the beginning of this discussion, or see here if you can read french for some more), the first effect is in noise ; the noise gets a bit uglier (more colorful maybe?), well before some posterization occurs in the tonalities of the image itself.
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Dick Roadnight
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« Reply #25 on: February 24, 2011, 01:10:58 PM »
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Eric…
I know your were trying to be helpful, and you were trying to persuade me that you were right, while not realizing that you were barking up the wrong tree... but I now realize why you were confused.

In the ideal world, in a studio, when you have control over the lighting, 9.7 stops is adequate, and you expose for the highlight and avoid shadow noise.

In the real world, for the landscape photographer 15.7 stops DR can be useful for shadow detail… as I said above…

If 16 bits gives you shadow and highlight detail and colour, then that is a real benefit - even if the MTF res is not so high. To deny this would be like insisting that lens manufacturers specify image circle diameters that give the same res as the centre of the lens.

If, in the shadows, 1 pixel in 4 or 10 captures a photon, and the software interpolates that to a soothe shade of some colour.. that too is a benefit: a trade-off between res and noise.

…"fake" pixels were counted as noise? What you call fake pixels are interpolation, and there is a difference between interpolated data (signal) and noise.

the  lower you set the SNR yardstick, the higher the DR.

Note I am talking about pixels per photon, not photons per pixel… this is where you got confused about subtracting 3 stops instead of adding them.

You could interpolate where the light level is one photon per 500 pixels… but that would be low res or no res, and not very useful.

You said:
The figures are measurement of the noise but doesn't describe the look of the noise. Some noise is more ugly.

In the shadows it is better to have noise/data that looks OK, even if it is low-res.
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ErikKaffehr
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« Reply #26 on: February 24, 2011, 01:58:29 PM »
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Hi,

Sorry for barking...

The issues are very well explained in Emil's (ejmartin) postings and also the postings of Bill (bjanes).

This posting by Emil is specially recommended: http://www.luminous-landscape.com/forum/index.php?topic=51510.msg424143#msg424143

Regarding the interpolation issue I don't agree. But, there is an advantage to having more pixels. This is discussed at some depth here:
http://www.dxomark.com/index.php/en/Our-publications/DxOMark-Insights/More-pixels-offset-noise!
or in this article on Luminous Landscape:
http://peter.vdhamer.com/2010/12/05/dxomarksensor/

To me it seems that we have some interesting developments in CMOS sensor recently, with Pentax K5 and Nikon D7000 making best use of Sony's latest technology. That improvement is probably mostly achieved by reducing readout noise. Shot noise, which is caused by photon statistics is said to be less disturbing.

Regarding MFDBs, I have nothing against MFDBs. I'm even considering buying one, now and than. Problem is expense and weight. I have difficulty carrying my DSLR gear on flights adding MFDB stuff would not make my gear lighter. Also I'm the kind of person who is using many lenses.

Best regards
Erik
Eric…
I know your were trying to be helpful, and you were trying to persuade me that you were right, while not realizing that you were barking up the wrong tree... but I now realize why you were confused.

In the ideal world, in a studio, when you have control over the lighting, 9.7 stops is adequate, and you expose for the highlight and avoid shadow noise.

In the real world, for the landscape photographer 15.7 stops DR can be useful for shadow detail… as I said above…

If 16 bits gives you shadow and highlight detail and colour, then that is a real benefit - even if the MTF res is not so high. To deny this would be like insisting that lens manufacturers specify image circle diameters that give the same res as the centre of the lens.

If, in the shadows, 1 pixel in 4 or 10 captures a photon, and the software interpolates that to a soothe shade of some colour.. that too is a benefit: a trade-off between res and noise.

…"fake" pixels were counted as noise? What you call fake pixels are interpolation, and there is a difference between interpolated data (signal) and noise.

the  lower you set the SNR yardstick, the higher the DR.

Note I am talking about pixels per photon, not photons per pixel… this is where you got confused about subtracting 3 stops instead of adding them.

You could interpolate where the light level is one photon per 500 pixels… but that would be low res or no res, and not very useful.

You said:
The figures are measurement of the noise but doesn't describe the look of the noise. Some noise is more ugly.

In the shadows it is better to have noise/data that looks OK, even if it is low-res.

« Last Edit: February 24, 2011, 02:23:00 PM by ErikKaffehr » Logged

BJL
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« Reply #27 on: February 24, 2011, 04:16:05 PM »
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In the real world, for the landscape photographer 15.7 stops DR can be useful for shadow detail… as I said above…
Dick, wat you say might well be true about the virtues of an imagined digital camera delivering a signal with 16 significant bits, but the point of Erik and others in this thread is that there is as of now now such camera: only some that provide sixteen buts, but with at most the first 12 or 13 being significant, the rest being noise from the sensor and other electronic components. A simple software hack involving a random number generator could convert camera phone output to 16 bits, or even 64, but ...
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ondebanks
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« Reply #28 on: February 25, 2011, 09:20:15 AM »
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even my lowly Hasselblad H3D2-50 (which I have upgraded to an H4D-60) had a "real world" Dynamic range of 15.7 stops - thank you for the information.

Dick,

You've used this 15.7 stops figure a couple of times in this thread - the first time I saw it I assumed it must be a typo, but then you repeated it separately.

Your H3D2-50 uses a Kodak KAF-50100 sensor. Kodak publishes a very clear data sheet on it, containing the following information:
Read noise = 12.5 electrons
Full well depth = 40,300 electrons
Dynamic Range = 70.2 dB

Now from the readnoise and full well depth figures I calculate DR = 70.2 dB (exactly verifying Kodak's own DR figure) and DR = 11.7 stops.

Now you claim that Hasselblad turned an 11.7 stop sensor into a 15.7 stop digital back!?   Roll Eyes
Hasselblad are good; but they're not that good!

Let's play with that notion for a moment. The only way that DR=15.7 stops could be done, with that full well depth, is if the read noise is reduced to 0.75 electrons. At present, sub-electron readout noise is far, far beyond any standard CCD technology. L3-CCDs with very high electron-multiplication gain and pipeline thresholding can achieve it (see Andor Technologies for example; I built a high speed astronomical photometer called GUFI around one of their iXon units) - but these are small, very specialist sensors, and their DR is still only around 12 stops when operated at such high E-M gain.

I think that if a regular Kodak CCD had achieved sub-electron readout noise, we'd have heard all about it, very loudly and proudly, from Kodak!

Needless to say, I am intrigued. How did you arrive at the 15.7 stop figure in the first place?

Ray


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cunim
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« Reply #29 on: February 25, 2011, 09:55:13 AM »
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Other problem is that at some point lens flare, blooming, reflections across the CCD etc. become limiting.  You can have 16 bits of SNR from the camera electronics package, but that is measured using localized detector illumination generated by an ideal optic.  In actual practice, the intrascene DR is limited by optical flare from brighter regions or electronic contamination from detector areas that contain bright data.  In my experience, the electronic contamination is much more pronounced with CMOS and the like than with CCDs.

In other words, you can achieve a true 16 bit precision only by using an ideal optic and only in images that contain a relatively narrow brightness range. 

Fortunately, I don't think this limitation matters very much in photography.  After all, the cameras are not actually precise enough to resolve 16 bit data, so why obsess?  In contrast, a technical imager deals with these issues with on a routine basis.  A 16 bit liquid nitrogen cooled camera reading out a 1 kHz does you no good if your lens has any flare at all - and all lenses have some.  In fact, it is usually only possible to achieve such high precision levels with intrinsically luminous targets (fluorescent, bioluminescent, astronomical) where we can use very aggressive filters to remove everything except the bandwidth of interest. 

I would love to hear experiences from the science users on this board but the discussion is probably not very relevant to making photographs.

Peter
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ErikKaffehr
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« Reply #30 on: February 25, 2011, 10:44:27 AM »
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Ray,

The DxO figures are calculated for SNR = 1, which I think is a pretty conventional figure. Also, DxO normalizes DR to 8 MPixels assuming a constant print size. A suggestion I made was that a criterion of SNR = 8 could be used instead and DR would be reduced by about three steps. So if the DxO figure for DR would be 12.7 eV a DR value for SNR = 8 would probably be errand 12.7 - 3 = 9.7 step. I know it's an oversimplification but in my view not a terrible one. For some reason Dick interpreted my writing as meaning 15.7 stops. So that figure is coming from me, although I stated it to be approximately 9.7 stops.

Thank you very much for elaborating on the issue!

Best regards
Erik


Dick,

You've used this 15.7 stops figure a couple of times in this thread - the first time I saw it I assumed it must be a typo, but then you repeated it separately.

Your H3D2-50 uses a Kodak KAF-50100 sensor. Kodak publishes a very clear data sheet on it, containing the following information:
Read noise = 12.5 electrons
Full well depth = 40,300 electrons
Dynamic Range = 70.2 dB

Now from the readnoise and full well depth figures I calculate DR = 70.2 dB (exactly verifying Kodak's own DR figure) and DR = 11.7 stops.

Now you claim that Hasselblad turned an 11.7 stop sensor into a 15.7 stop digital back!?   Roll Eyes
Hasselblad are good; but they're not that good!

Let's play with that notion for a moment. The only way that DR=15.7 stops could be done, with that full well depth, is if the read noise is reduced to 0.75 electrons. At present, sub-electron readout noise is far, far beyond any standard CCD technology. L3-CCDs with very high electron-multiplication gain and pipeline thresholding can achieve it (see Andor Technologies for example; I built a high speed astronomical photometer called GUFI around one of their iXon units) - but these are small, very specialist sensors, and their DR is still only around 12 stops when operated at such high E-M gain.

I think that if a regular Kodak CCD had achieved sub-electron readout noise, we'd have heard all about it, very loudly and proudly, from Kodak!

Needless to say, I am intrigued. How did you arrive at the 15.7 stop figure in the first place?

Ray



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bjanes
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« Reply #31 on: February 26, 2011, 11:36:12 AM »
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The DxO figures are calculated for SNR = 1, which I think is a pretty conventional figure. Also, DxO normalizes DR to 8 MPixels assuming a constant print size. A suggestion I made was that a criterion of SNR = 8 could be used instead and DR would be reduced by about three steps. So if the DxO figure for DR would be 12.7 eV a DR value for SNR = 8 would probably be errand 12.7 - 3 = 9.7 step. I know it's an oversimplification but in my view not a terrible one. For some reason Dick interpreted my writing as meaning 15.7 stops. So that figure is coming from me, although I stated it to be approximately 9.7 stops.


As Emil Martinec has pointed out in an earlier post, one can calculate the DR for any S:N by interpolating the DXO graphs. The difficult part is interpolation of a log scale which is explained in an article on Wikipedia. Using the DXO SNR plot of the Pentax 645D as an example, to determine the S:N at 0 dB at base ISO one has to determine the gray scale value for 0 dB. It is somewhere between 0.01% and 0.1%. One can do a screen capture of the plot and measure the distances using the ruler in Photoshop and perform the log interpolation.



I get a value of 0.0391% as shown on an Excel spreadsheet. The S:N at 0 dB is 100%/0.0391% or 11.32 stops, which is in agreement with the reported screen DR of 11.37 stops. For S:N of 6 dB, one performs a similar interpolation, and I get a gray scale value of 0.0779%, which gives a DR of 10.3 stops.



Regards,

Bill
« Last Edit: February 26, 2011, 11:44:23 AM by bjanes » Logged
ondebanks
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« Reply #32 on: February 26, 2011, 12:19:05 PM »
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Ray,

The DxO figures are calculated for SNR = 1, which I think is a pretty conventional figure.

Correct to first order, but one nuance is that DxO is based on a total empirical noise (they do not attempt to disentangle the various contributions: read noise, signal poisson noise, dark poisson noise, quantization noise, fixed pattern noise, and pixel response non-uniformity); whereas conventional DR only considers read noise, which is normally the dominant type of noise at extremely low signal levels.


Also, DxO normalizes DR to 8 MPixels assuming a constant print size. A suggestion I made was that a criterion of SNR = 8 could be used instead and DR would be reduced by about three steps. So if the DxO figure for DR would be 12.7 eV a DR value for SNR = 8 would probably be errand 12.7 - 3 = 9.7 step. I know it's an oversimplification but in my view not a terrible one.


That is a slight oversimplification alright (again, because you're thinking only of read noise). But your suggestion made sense to me.


For some reason Dick interpreted my writing as meaning 15.7 stops. So that figure is coming from me, although I stated it to be approximately 9.7 stops.


I still don't see how Dick got 15.7 stops. What he said was this:
So, with DR measured @1:1 SNR=12.7, even my lowly Hasselblad H3D2-50 (which I have upgraded to an H4D-60) had a "real world" Dynamic range of 15.7 stops - thank you for the information.

So he wasn't using your approach - your figures were going in the opposite direction (and yours also passed the "sanity check" that DR can only decrease as the SNR threshold is increased). I've no idea what calculation or measurement he was using, and hope he chips in again to explain.

Ray

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Dick Roadnight
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« Reply #33 on: February 26, 2011, 04:47:54 PM »
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Also, DxO normalizes DR to 8 MPixels assuming a constant print size. A suggestion I made was that a criterion of SNR = 8 could be used instead and DR would be reduced by about three steps. So if the DxO figure for DR would be 12.7 eV a DR value for SNR = 8 would probably be errand 12.7 - 3 = 9.7 step. I know it's an oversimplification but in my view not a terrible one.

That is a slight oversimplification alright (again, because you're thinking only of read noise). But your suggestion made sense to me.

I still don't see how Dick got 15.7 stops. What he said was this:
So, with DR measured @1:1 SNR=12.7, even my lowly Hasselblad H3D2-50 (which I have upgraded to an H4D-60) had a "real world" Dynamic range of 15.7 stops - thank you for the information.

So he wasn't using your approach - your figures were going in the opposite direction (and yours also passed the "sanity check" that DR can only decrease as the SNR threshold is increased). I've no idea what calculation or measurement he was using, and hope he chips in again to explain.

Ray
Thank you for your input.

I was using Eric's approach, but in a inverse way, calculating an increase in DR with a loss in res and an increase in "noise" or interpolation.

In the real world we do not always achieve a Disc Of Confusion (DOC) or resolution of 10 microns all the time in all parts on the image, (particularly if using small apertures and/or hand-holding) and if we use a SNR of 1/8, using his logic, we add 3 instead of subtracting 3, get 15.7 stops of DR,,, even if it only gives us a DOC of 80 microns in deep shadows.

Good interpolation and smoothing can make the shadows less noisy, whatever the DR count.


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« Reply #34 on: February 26, 2011, 07:43:44 PM »
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Other problem is that at some point lens flare, blooming, reflections across the CCD etc. become limiting.  You can have 16 bits of SNR from the camera electronics package, but that is measured using localized detector illumination generated by an ideal optic.  In actual practice, the intrascene DR is limited by optical flare from brighter regions or electronic contamination from detector areas that contain bright data.  In my experience, the electronic contamination is much more pronounced with CMOS and the like than with CCDs.


Peter,
I've heard of this DR limitation due to lens flare mentioned before but I can no find no specifics or examples or comparisons.

We're all familiar with the obvious examples of lens flare when the camera is pointed in the direction of the sun or some very bright reflection. However, the proposition that lens flare, even when the sun is behind the camera and when a lens hood is in place, will still limit dynamic range, needs investigating.

The questions that spring to my mind are:

(1) What is the DR limit of lens flare, in terms of EV or F/stops, in lenses considered to have the best flare protection?

(2) How does such a limit vary amongst different models of lenses?

(3) Is there any benefit, in terms of shadow quality, to be gained when the DR of the sensor exceeds the DR limit of the lens used?

Before I bought the D7000, I attempted to find out just how significant in practice were the DXO claims of such exceptional DR for the D7000.

I came across a lot of negative comments along the lines of, "Roger Clarke has demonstrated that shot noise is the predominant noise in DSLRs at extemely low 'pixel saturation' and therefore claims of 13 stops of DR are meaningless in practice."

Yet, oddly enough, the standard studio scenes used by Dpreview in their reviews of cameras demonstrated clearly, cleaner shadows in the D7000 images compared with, for example, the Canon 60D, so this claim of  'shot noise' limitation on DR seemed bogus to me.

Shortly after taking delivery of my D7000, I retrieved from my archives a Dynamic Range Test Chart created by Jonathan Wienke for the purpose of assessing the subjective significance of DR limitations.

But such a method completely bypasses the problem of lens flare since it involves progressively reducing exposure of a fixed target under constant lighting conditions, then examining the quality of the image which has been underexposed by a specific number of EVs.

Below are two exposures of this test chart which differ by 13EV, the first one is a reasonable ETTR at 4 seconds' exposure, and the second exposure at 1/2000th of a second demonstrates the extreme degree of image degradation in the 14th stop.
Needless to say, image quality 3 stops up from the 14th stop, the 11th stop, is much, much better, and quite acceptable for shadows.

Can anyone comment on flaws in such methodology?

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« Reply #35 on: February 26, 2011, 09:24:25 PM »
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You can't reliably measure dynamic range with something printed on a sheet of paper. It seems to me you are just moving a 6 or 7 stops target over the range of a +/- 13 stops capture device. See that as two rules sliding side by side. Of course, when the light coming from the target is insufficient, the bottom 3 stops (for example) become irrelevant. Stretching the 4 remaining stops to match the appearance of the 7 stops properly recorded will, of course, lead to problems. Look at your histogram - its a very coarse sampling of the signal.

These links show how it can be done. Arri's method (2nd link) is said to correlate well with standards and I found interesting how they defined the lower treshold for signal validity

"a signal is valid when it is able to transport the spatial content"

That definition takes into account, to some extent (one can always argue that some setups would be better for some frequencies, etc, ad infinitum), the lens flare, blooming, reflections across the CCD, MTF etc... issues.

http://www.dxomark.com/index.php/Learn-more/DxOMark-database/DxOMark-testing-protocols/Noise-dynamic-range

http://www.arri.com/fileadmin/media/arri.com/downloads/Camera/Camera_Technologies/2009_09-08_DRTC_Brochure.pdf

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« Reply #36 on: February 26, 2011, 11:54:36 PM »
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You can't reliably measure dynamic range with something printed on a sheet of paper. It seems to me you are just moving a 6 or 7 stops target over the range of a +/- 13 stops capture device. See that as two rules sliding side by side.http://www.arri.com/fileadmin/media/arri.com/downloads/Camera/Camera_Technologies/2009_09-08_DRTC_Brochure.pdf

Pierre,
That printed sheet of paper is a real-world object. I could have photographed something more beautiful, such as a vase of flowers, or the head of David. The reason I didn't is because subtle differences in  detail, with increasing underexposure, would not have been so readily identifiable.

This chart has been designed so that smallest variation in the degree of meaningful detail is readily apparent with changes in DR, expressed as changes in exposure.

Quote
Of course, when the light coming from the target is insufficient, the bottom 3 stops (for example) become irrelevant. Stretching the 4 remaining stops to match the appearance of the 7 stops properly recorded will, of course, lead to problems. Look at your histogram - its a very coarse sampling of the signal

Don't be fooled by the histogram. Look at the image. Below is a screen grab of a 50D image of the same real-world object, at the same exposure representing the 14th stop, under the same lighting conditions.

The histogram of the 50D image looks much more beautiful than the D7000 histogram which is much coarser by comparison.
What I see is a coarse D7000 histogram of a degraded image with some meaningful detail, as opposed to a less coarse 50D histogram of a totally degraded image with virtualy no detail at all.

I include also the 50D, with histogram, at 4 secs exposure to demonstrate that the initial 50D ETTR shot was not underexposed. In fact, it looks more exposed than the initial D7000 ETTR shot to me.
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« Reply #37 on: February 27, 2011, 05:02:17 AM »
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Peter,

Below are two exposures of this test chart which differ by 13EV, the first one is a reasonable ETTR at 4 seconds' exposure, and the second exposure at 1/2000th of a second demonstrates the extreme degree of image degradation in the 14th stop.
What is of interest to me is that the colour is almost completely lost in the 14th stop as well as the detail... so there is little or no useful data there at any res.
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« Reply #38 on: February 27, 2011, 09:17:55 AM »
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Peter,
I've heard of this DR limitation due to lens flare mentioned before but I can no find no specifics or examples or comparisons.


Ray, I wish I could provide useful answers.  I can do that for my discipline (low light imaging) but not for my hobby (photography).  The discipline is just that - a highly optimized and standardized procedure for sensitive and precise (different things) detection of targets at the very limits of the envelope.  Biology, physics, astronomy - all similar.  We can start with a true 16 bit detector, for example, and apply techniques that bring optical noise down to a small enough proportion of total flux that we achieve adequate precision from the entire system.  That won't be 16 bits but might be pretty close.  Doing this creates some absolutely beautiful images but that is not usually the point.  The point is that we can relate a given intesity level in the image to a target characteristic in the real world.

See what I mean?  Very little to do with photography, which is an interpretative process.  Photography is inherently confounded with all sorts of known and unknown signal and noise factors.  An example of an unknown noise factor is internal flare within your favorite lens.  Sometimes you actually like that lens because of its flare characteristics so what benefit arises from speficying a system SNR with it in place?  Even if you could, manufacturing variability from lens to lens would swamp the precision of any high-bit system.

I agree that manufacturers need to specify the SNR of pro-grade cameras using standardized testing.  That gives us an idea of how well the hardware is implemented.  However, the SNR value of any of the top cameras is poorly predicitive of how well the camera will generate pleasing photographs.  It will have much more to do with how well the camera handles narrow parts of the dynamic range - shadows, for example - in which electronic noise contributions can be very evident.  At that point we start to get into the relative benefits of amplification (high iso) vs integration (longer exposures) and I really don't want to go there.

No worries.  Make pictures with equipment that gives you results that you like. For me, it's MFD and film.

Peter





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ErikKaffehr
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« Reply #39 on: February 27, 2011, 09:32:36 AM »
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Hi,

I have the impression that lens flare is difficult to measure. It's definitively an issue in certain situations, like shooting portraits with an evenly illuminated window as background.

The reason it's hard to measure is that the appearance of flare depends much on how light is falling on lens.

I enclose a sample where lens flare caused serious problems but the image could be saved with some work with graduated filters in Lightroom.

Best regards
Erik



« Last Edit: February 27, 2011, 09:42:08 AM by ErikKaffehr » Logged

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