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Author Topic: Dynamic range and DxO  (Read 23200 times)
ejmartin
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« on: March 09, 2010, 10:54:46 AM »
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I was struck by the following added comment to the recent MFDB article
http://www.luminous-landscape.com/reviews/...ii.shtml#update

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Unfortunately DxO Labs has complicated matters. Their DxO Mark camera tests have become widely read by photographers around the world, and are quoted as gospel by many. Regrettably there are concerns that I, and others have expressed about some aspects of DXO's tests, and I have been in communication with them over this in the past.

The only aspect that is relevant to this discussion is with regard to dynamic range. The standard definition is, as mentioned above, how many F stops above and below middle gray can be recorded while delivering full texture and detail. The DXO definition, according to their web site, is the range between zero signal to noise and full saturation of the sensor.

This approach is not inherently flawed, it's just that it does not take into account the linear nature of sensors. It is therefore not a particularly relevant way of measuring DR to a photographer, as opposed to an engineer.

This is a common complaint about DxO's tests, and is often fodder for uninformed complaints to the effect that "DxO is garbage, my camera doesn't have that much DR".  It is good that Michael points out that what DxO tests for in their DR plots is an engineer's definition of DR

Engineering DR: max recordable signal (light intensity) divided by noise at zero signal; or nearly the same, the range of signal levels over which the signal/noise ratio is greater than one.  (Slight quibble: Michael misstates this definition as "the range between zero signal to noise and full saturation of the sensor." which is incorrect; it is the range between SNR of 0dB and saturation, and 0dB is S/N=1, not zero.)

This engineering definition is different from the photographer's working definition of dynamic range given above by Michael (and which he calls the "standard definition"; not for engineers, of course, but in photography circles, yes).  For the purposes of matching to DxO, let me translate that definition into more technical terms:

Photographer's DR: The range of signal levels over which signal/noise ratio (SNR) exceeds the photographer's minimum acceptable SNR.

DxO's approach is not inherently flawed (and the difference has nothing to do with the linear nature of sensors).  I suspect the reason that DxO doesn't present "photographer's DR" is that its very definition depends on a criterion that varies from person to person; what is minimal acceptable image quality to one may be unacceptable to another.  The difference between these two definitions has been the source of much of the consternation over DxO's DR results.  How then can the photographer then get useful information about "Photographer's DR" from DxO?  

DxO has another graph, for signal/noise ratio (in dB; in more photographer-friendly terms, 6dB=1 stop) as a function of signal (in percentage of saturation).  For instance, for the D3x the graph looks like this:



On this graph, base ISO SNR drops to 0dB (SNR=1) at .015% of saturation, or 12.7 stops down (log(.00015)/log(2)=-12.7).  Indeed DxO quotes 12.8 stops DR at ISO 100 (the "screen" tab of the DR plot).  But suppose you don't think S/N=1 is good enough for acceptable image quality; instead you need S/N of 16 or greater for your standards of IQ.  Then looking at the graph, ISO 100 exceeds S/N=16 (4 stops=24dB) starting at about .6% of saturation, or about 7.4 stops down.  Then for you, the D3x has 7.4 stops of "usable" DR at base ISO.  Note that if we maintain the same criterion uniformly, the DR at ISO 1600 is about three and a half stops (a little less than 10% of saturation).

Looking at the same graphs for the P65+, one sees an ISO 100 engineering DR of a little over 11.5 stops per pixel, and a S/N>16 dynamic range of about 7 stops (lower end at about .8% of saturation).  However this is DR per pixel, and the Phase One has about 3x more pixels than the D3x.  DR scales with the number of pixels in the image, so to compare equitably one should compare DR at a fixed scale in the image by aggregating together 3 pixels worth of DR of the Phase One back; this increases the DR by about the square root of three, adding another 0.6 stop to its DR when matched to the pixel scale of the D3x.  According to the engineering standard the MFDB is still well short of the D3x, however according to a standard more relevant to photography the back comes out slightly better (but less than 1/3 stop), mostly because of the larger sensor area collecting more light over the frame.  The difference is not however the many stops DR advantage that some MFDB proponents claim.  

A final caveat: The useful DR to a photographer can be limited by more than indicated by the S/N figure of merit; for instance Canon DSLR's have a base ISO plagued by a lot of pattern noise in shadows, which can be visually much more objectionable than the random grain of unpatterned noise while not showing up in the noise standard deviation.  On the other hand, pattern noise seems very well controlled on the D3x.  The pattern noise can limit the useful DR -- how much one is willing to push shadows -- more than might be indicated by the S/N graphs.
« Last Edit: March 09, 2010, 01:41:29 PM by ejmartin » Logged

emil
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« Reply #1 on: March 09, 2010, 11:40:18 AM »
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Quote from: ejmartin
...

Thank you for droppin' some science on the issue.

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ErikKaffehr
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« Reply #2 on: March 09, 2010, 11:46:50 AM »
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Emil,

Thank's for your explanation. We had seen a little to much DxO-bashing recently, in my view. I hope that the effort Michael Reichmann and Mark Dubovoy plan will shed some more light on the subject.

A good effort of explaining the differences between DSLRs and MFDBs would be helpful for everyone considering to acquire an MFDB and may also shed some light on the more mythical advantages MFDBs may have.

To be meaningful it's my strong opinion that the test should be accompanied with a set of downloadable RAW images. For those considering the investment a reasonable fee for downloading the mages would most certainly be a good investment.

I actually considered renting an MF-digital equipment just to satisfy my curiosity, but the cost for just a couple of days rental would be similar to the price of my FF DSLR (Sony Alpha 900).

Best regards
Erik


Quote from: ejmartin
I was struck by the following added comment to the recent MFDB article
http://www.luminous-landscape.com/reviews/...ii.shtml#update



This is a common complaint about DxO's tests, and is often fodder for uninformed complaints to the effect that "DxO is garbage, my camera doesn't have that much DR".  It is good that Michael points out that what DxO tests for in their DR plots is an engineer's definition of DR

Engineering DR: max recordable signal (light intensity) divided by noise at zero signal; or nearly the same, the range of signal levels over which the signal/noise ratio is greater than one.  (Slight quibble: Michael misstates this definition as "the range between zero signal to noise and full saturation of the sensor." which is incorrect; it is the range between SNR of 0dB and saturation, and 0dB is S/N=1, not zero.)

This engineering definition is different from the photographer's working definition of dynamic range given above by Michael (and which he calls the "standard definition"; not for engineers, of course, but in photography circles, yes).  For the purposes of matching to DxO, let me translate that definition into more technical terms:

Photographer's DR: The range of signal levels over which signal/noise ratio (SNR) exceeds the photographer's minimum acceptable SNR.

DxO's approach is not inherently flawed (and the difference has nothing to do with the linear nature of sensors).  I suspect the reason that DxO doesn't present "photographer's DR" is that its very definition depends on a criterion that varies from person to person; what is minimal acceptable image quality to one may be unacceptable to another.  The difference between these two definitions has been the source of much of the consternation over DxO's DR results.  How then can the photographer then get useful information about "Photographer's DR" from DxO?  

DxO has another graph, for signal/noise ratio (in dB; in more photographer-friendly terms, 6dB=1 stop) as a function of signal (in percentage of saturation).  For instance, for the D3x the graph looks like this:



On this graph, base ISO SNR drops to 0dB (SNR=1) at .015% of saturation, or 12.7 stops down (log(.00015)/log(2)=-12.7).  Indeed DxO quotes 12.8 stops DR at ISO 100 (the "screen" tab of the DR plot).  But suppose you don't think S/N=1 is good enough for acceptable image quality; instead you need S/N of 16 or greater for your standards of IQ.  Then looking at the graph, ISO 100 exceeds S/N=16 (4 stops=24dB) starting at about .6% of saturation, or about 7.4 stops down.  Then for you, the D3x has 7.4 stops of "usable" DR at base ISO.  Note that if we maintain the same criterion uniformly, the DR at ISO 1600 is about three and a half stops (a little less than 10% of saturation).

Looking at the same graphs for the P65+, one sees an ISO 100 engineering DR of a little over 11.5 stops per pixel, and a S/N>16 dynamic range of about 7 stops (lower end at about .8% of saturation).  However this is DR per pixel, and the Phase One has about 3x more pixels than the D3x.  DR scales with the number of pixels in the image, so to compare equitably one should compare DR at a fixed scale in the image by aggregating together 3 pixels worth of DR of the Phase One back; this increases the DR by about the square root of three, adding another .6dB to its DR when matched to the pixel scale of the D3x.  According to the engineering standard the MFDB is still well short of the D3x, however according to a standard more relevant to photography the back comes out slightly better (but less than 1/3 stop), mostly because of the larger sensor area collecting more light over the frame.  The difference is not however the many stops DR advantage that some MFDB proponents claim.  

A final caveat: The useful DR to a photographer can be limited by more than indicated by the S/N figure of merit; for instance Canon DSLR's have a base ISO plagued by a lot of pattern noise in shadows, which can be visually much more objectionable than the random grain of unpatterned noise while not showing up in the noise standard deviation.  On the other hand, pattern noise seems very well controlled on the D3x.  The pattern noise can limit the useful DR -- how much one is willing to push shadows -- more than might be indicated by the S/N graphs.
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LKaven
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« Reply #3 on: March 09, 2010, 12:36:52 PM »
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Thank god, the cavalry finally rode in on this one...
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MarkoRepse
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« Reply #4 on: March 09, 2010, 02:28:50 PM »
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Quote from: LKaven
Thank god, the cavalry finally rode in on this one...
Agreed.

This explanation makes perfect sense to me as well. Sensor size shouldn't be able to benefit DR as profoundly as it was stated in the original article. 6-7 stops vs 12-13 is a very great difference. Stops are an exponential scale! The reason MF backs (vs 35mm) produce better images is simply because they contain better quality electronics. The P40+ has the same pixel size as the P65+ and both should produce identical results, ignoring the crop factor. Shrink/crop the sensor even further to 35mm specs and none of the quality will disappear; with a loss in resolution, of course.
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bjanes
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« Reply #5 on: March 09, 2010, 04:08:23 PM »
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Quote from: ejmartin
On this graph, base ISO SNR drops to 0dB (SNR=1) at .015% of saturation, or 12.7 stops down (log(.00015)/log(2)=-12.7).  Indeed DxO quotes 12.8 stops DR at ISO 100 (the "screen" tab of the DR plot).  But suppose you don't think S/N=1 is good enough for acceptable image quality; instead you need S/N of 16 or greater for your standards of IQ.  Then looking at the graph, ISO 100 exceeds S/N=16 (4 stops=24dB) starting at about .6% of saturation, or about 7.4 stops down.  Then for you, the D3x has 7.4 stops of "usable" DR at base ISO.  Note that if we maintain the same criterion uniformly, the DR at ISO 1600 is about three and a half stops (a little less than 10% of saturation).

Looking at the same graphs for the P65+, one sees an ISO 100 engineering DR of a little over 11.5 stops per pixel, and a S/N>16 dynamic range of about 7 stops (lower end at about .8% of saturation).  However this is DR per pixel, and the Phase One has about 3x more pixels than the D3x.  DR scales with the number of pixels in the image, so to compare equitably one should compare DR at a fixed scale in the image by aggregating together 3 pixels worth of DR of the Phase One back; this increases the DR by about the square root of three, adding another 0.6 stop to its DR when matched to the pixel scale of the D3x.  According to the engineering standard the MFDB is still well short of the D3x, however according to a standard more relevant to photography the back comes out slightly better (but less than 1/3 stop), mostly because of the larger sensor area collecting more light over the frame.  The difference is not however the many stops DR advantage that some MFDB proponents claim.
Emil,

A penetrating analysis that the subjectivists may have difficulty refuting, but then they feel that their experience trumps scientific measurement. Your conclusions are similar to what I posted with an Imatest analysis for my Nikon D3. The Imatest total DR is in the range of the engineering DR calculated from Roger Clark's data and is close to the screen DR reported by DXO. The photographic DR reported by Imatest for various noise floors is of course less than the engineering DR, depending on the quality desired for the shadows.

A signal:noise value might not convey the full visual impact of noise in an image, and the noise spectrum and other unevaluated factors may come into play. I think most would agree that visual inspection of the image and correlation between perceived quality and the S:N would be appropriate. One should verify that the variable being measured does correlate with perception. For example, MTF correlates much better with perceived sharpness than does the resolution of a USAF bar chart.
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Josh-H
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« Reply #6 on: March 09, 2010, 05:17:16 PM »
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Quote from: LKaven
Thank god, the cavalry finally rode in on this one...

Well said.

Thankyou to the OP for clearing this up.
« Last Edit: March 09, 2010, 05:20:46 PM by Josh-H » Logged

BernardLanguillier
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« Reply #7 on: March 09, 2010, 05:50:53 PM »
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Quote from: ejmartin
Then looking at the graph, ISO 100 exceeds S/N=16 (4 stops=24dB) starting at about .6% of saturation, or about 7.4 stops down.  Then for you, the D3x has 7.4 stops of "usable" DR at base ISO.  

Looking at the same graphs for the P65+ ... and a S/N>16 dynamic range of about 7 stops (lower end at about .8% of saturation).  

However this is DR per pixel, and the Phase One has about 3x more pixels than the D3x.  this increases the DR by about the square root of three, adding another 0.6 stop to its DR when matched to the pixel scale of the D3x.  

A final caveat: The useful DR to a photographer can be limited by more than indicated by the S/N figure of merit; for instance Canon DSLR's have a base ISO plagued by a lot of pattern noise in shadows, which can be visually much more objectionable than the random grain of unpatterned noise while not showing up in the noise standard deviation.  On the other hand, pattern noise seems very well controlled on the D3x.

So in essence, the P65+ has 0.2 stop more DR than the D3x at a given print size.

Cheers,
Bernard

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ejmartin
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« Reply #8 on: March 09, 2010, 06:50:29 PM »
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Quote from: BernardLanguillier
So in essence, the P65+ has 0.2 stop more DR than the D3x at a given print size.

Cheers,
Bernard

For this particular criterion, yes.  For a looser standard, the advantage starts going to the D3x, since by the time we get down to S/N>1, the D3x has an advantage of more than half a stop of range.  For a tighter standard, the difference in sensor area kicks in and the gap widens in favor of the P65+; for purely shot noise limited photography, twice the sensor area provides an extra stop of DR for a given minimum S/N.  At least if we ignore issues such as the relative quantum efficiency of the sensors.
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« Reply #9 on: March 09, 2010, 07:24:27 PM »
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Quote from: ejmartin
For this particular criterion, yes.  For a looser standard, the advantage starts going to the D3x, since by the time we get down to S/N>1, the D3x has an advantage of more than half a stop of range.  For a tighter standard, the difference in sensor area kicks in and the gap widens in favor of the P65+; for purely shot noise limited photography, twice the sensor area provides an extra stop of DR for a given minimum S/N.  At least if we ignore issues such as the relative quantum efficiency of the sensors.

Can we think of other aspects potentially impacting real world DR besides those you already mentioned above (sensor shadow noise, resolution, size,...)?

Cheers,
Bernard
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ejmartin
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« Reply #10 on: March 09, 2010, 08:35:35 PM »
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Quote from: BernardLanguillier
Can we think of other aspects potentially impacting real world DR besides those you already mentioned above (sensor shadow noise, resolution, size,...)?

Cheers,
Bernard

Two things that come to mind are relative strength of color filters in the CFA, and spectral characteristics (eg color temperature) of ambient light.  

A mismatch in white balance in the raw data means that one or more color channels will have to be amplified, and so one will be limited by the top end of one channel as compared to the (lifted) bottom end of a different channel, squeezing the available DR.  This can be mitigated by the use of color filters, either on the camera, or in studio situations on the light source.

There is also the effect that if the color properties of CFA filters are not close to those of RGB primaries, large coefficients appear in the color profile that transforms between the two, which serves to amplify chroma noise.  This effect was explained nicely in a DxO article:
http://www.dxomark.com/index.php/eng/Insig...-sensor-quality
« Last Edit: March 09, 2010, 09:33:59 PM by ejmartin » Logged

emil
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« Reply #11 on: March 09, 2010, 11:04:59 PM »
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Hi,

What about lenses, flare level and so on? DSLR lenses used to be complex often zooms. A simpler lens with fewer air/glass surfaces would have less flare. Lenses with moving internal lens groups probably cannot be as efficient in repressing internal reflections as lenses with all lens groups fixed.

Best regards
Erik




Quote from: ejmartin
Two things that come to mind are relative strength of color filters in the CFA, and spectral characteristics (eg color temperature) of ambient light.  

A mismatch in white balance in the raw data means that one or more color channels will have to be amplified, and so one will be limited by the top end of one channel as compared to the (lifted) bottom end of a different channel, squeezing the available DR.  This can be mitigated by the use of color filters, either on the camera, or in studio situations on the light source.

There is also the effect that if the color properties of CFA filters are not close to those of RGB primaries, large coefficients appear in the color profile that transforms between the two, which serves to amplify chroma noise.  This effect was explained nicely in a DxO article:
http://www.dxomark.com/index.php/eng/Insig...-sensor-quality
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ErikKaffehr
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« Reply #12 on: March 09, 2010, 11:17:02 PM »
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Hi,

I have not done a detailed analysis of the charts like Emil, but I would also expect about one stop of advantage for MFDBs. But I have another question, why are MFDBs relatively poor performers at high ISO? Whatever advantage they have they also be able to keep it at higher ISO.

The factors I see are:

Most MFDBs are said not to have variable pre-amps, so the ISO settings above "base ISO" are just "fake ISO". This seems not be the case with then new P65 plus, however.

MFDBs normally don't have microlenses. Microlenses don't play well with lens shifts to my understanding.

The CGA may use narrower filters. This could result in more saturated colors, but not necessarily better colors.

Best regards
Erik


Quote from: ejmartin
I was struck by the following added comment to the recent MFDB article
http://www.luminous-landscape.com/reviews/...ii.shtml#update



This is a common complaint about DxO's tests, and is often fodder for uninformed complaints to the effect that "DxO is garbage, my camera doesn't have that much DR".  It is good that Michael points out that what DxO tests for in their DR plots is an engineer's definition of DR

Engineering DR: max recordable signal (light intensity) divided by noise at zero signal; or nearly the same, the range of signal levels over which the signal/noise ratio is greater than one.  (Slight quibble: Michael misstates this definition as "the range between zero signal to noise and full saturation of the sensor." which is incorrect; it is the range between SNR of 0dB and saturation, and 0dB is S/N=1, not zero.)

This engineering definition is different from the photographer's working definition of dynamic range given above by Michael (and which he calls the "standard definition"; not for engineers, of course, but in photography circles, yes).  For the purposes of matching to DxO, let me translate that definition into more technical terms:

Photographer's DR: The range of signal levels over which signal/noise ratio (SNR) exceeds the photographer's minimum acceptable SNR.

DxO's approach is not inherently flawed (and the difference has nothing to do with the linear nature of sensors).  I suspect the reason that DxO doesn't present "photographer's DR" is that its very definition depends on a criterion that varies from person to person; what is minimal acceptable image quality to one may be unacceptable to another.  The difference between these two definitions has been the source of much of the consternation over DxO's DR results.  How then can the photographer then get useful information about "Photographer's DR" from DxO?  

DxO has another graph, for signal/noise ratio (in dB; in more photographer-friendly terms, 6dB=1 stop) as a function of signal (in percentage of saturation).  For instance, for the D3x the graph looks like this:



On this graph, base ISO SNR drops to 0dB (SNR=1) at .015% of saturation, or 12.7 stops down (log(.00015)/log(2)=-12.7).  Indeed DxO quotes 12.8 stops DR at ISO 100 (the "screen" tab of the DR plot).  But suppose you don't think S/N=1 is good enough for acceptable image quality; instead you need S/N of 16 or greater for your standards of IQ.  Then looking at the graph, ISO 100 exceeds S/N=16 (4 stops=24dB) starting at about .6% of saturation, or about 7.4 stops down.  Then for you, the D3x has 7.4 stops of "usable" DR at base ISO.  Note that if we maintain the same criterion uniformly, the DR at ISO 1600 is about three and a half stops (a little less than 10% of saturation).

Looking at the same graphs for the P65+, one sees an ISO 100 engineering DR of a little over 11.5 stops per pixel, and a S/N>16 dynamic range of about 7 stops (lower end at about .8% of saturation).  However this is DR per pixel, and the Phase One has about 3x more pixels than the D3x.  DR scales with the number of pixels in the image, so to compare equitably one should compare DR at a fixed scale in the image by aggregating together 3 pixels worth of DR of the Phase One back; this increases the DR by about the square root of three, adding another 0.6 stop to its DR when matched to the pixel scale of the D3x.  According to the engineering standard the MFDB is still well short of the D3x, however according to a standard more relevant to photography the back comes out slightly better (but less than 1/3 stop), mostly because of the larger sensor area collecting more light over the frame.  The difference is not however the many stops DR advantage that some MFDB proponents claim.  

A final caveat: The useful DR to a photographer can be limited by more than indicated by the S/N figure of merit; for instance Canon DSLR's have a base ISO plagued by a lot of pattern noise in shadows, which can be visually much more objectionable than the random grain of unpatterned noise while not showing up in the noise standard deviation.  On the other hand, pattern noise seems very well controlled on the D3x.  The pattern noise can limit the useful DR -- how much one is willing to push shadows -- more than might be indicated by the S/N graphs.
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« Reply #13 on: March 09, 2010, 11:49:25 PM »
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Emil, now that the Pentax 645D is announced, some are criticizing it because it has "only" 14-bit color instead of the customary 16 advertised for MFDB.  But I would bet that there isn't a single digital camera on the market that delivers more than 14 good bits, even when allowing for a "good dither" bit.  I don't see any camera that delivers more than 14 stops of even "engineering DR."  Am I missing something?  Are the MFDBs really delivering 16 bits?
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ejmartin
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« Reply #14 on: March 10, 2010, 12:32:44 AM »
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Quote from: LKaven
Emil, now that the Pentax 645D is announced, some are criticizing it because it has "only" 14-bit color instead of the customary 16 advertised for MFDB.  But I would bet that there isn't a single digital camera on the market that delivers more than 14 good bits, even when allowing for a "good dither" bit.  I don't see any camera that delivers more than 14 stops of even "engineering DR."  Am I missing something?  Are the MFDBs really delivering 16 bits?

What seems to be the sensor spec sheet from Kodak (from this post on the 645D)
http://www.kodak.com/global/plugins/acroba...ductSummary.pdf
lists a saturation capacity of 42,000 electrons and a read noise of 13 electrons, which translates to a DR a little under 11.7 stops; it also quotes a DR of 70.2dB which is again about 11.7 stops.  Thus, I don't see how they could achieve a pixel DR that needs more than 12 bits of encoding.  One might derive a very slight benefit by oversampling at 13 or 14 bits, but I would think 14 is plenty.  Time to face up to the fact that 16 bits were never necessary in MFDB's.

The engineering DR determines the needed bit depth; that is one place where it comes into play -- the bit depth need not substantially exceed the engineering DR; if it does, one is effectively oversampling the camera output (which may have slight advantages, but usually is well past the point of diminishing returns).  But to repeat, engineering DR is a good bit larger than most photographer's working DR.  In fact, as the S/N plot shows, the engineering DR provides an upper bound on the value of any other, more restrictive, photographer's DR.  So for instance, the photographer's DR (per pixel) on the new Pentax (and, BTW all MFDBs I've looked at in DxO's data) is bounded by an engineering DR of less than 12 stops; and if your minimum S/N criterion is more restrictive than S/N=1, it only goes down from there.
« Last Edit: March 10, 2010, 06:44:28 AM by ejmartin » Logged

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« Reply #15 on: March 10, 2010, 02:50:52 AM »
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In case anyone hasn't seen it, the following was added to the review yesterday...

Paragraph Removed Editor

The struck-out paragraph below was part of the original article. It caused quite a bit of controversy on this site's forum and elsewhere. I initially added a comment to the bottom of the article, but that wasn't sufficient. Too many people's favourite ox had already been gored.

Please consider the paragraph below to be removed. There's no point in actually removing it, because once on the net, things last forever, and undoubtedly some zealots would consider it a coverup if we did so.



The reason that Mark and I have decided to remove it is not because we don't agree with its basic sentiment, but because it is serving as a distraction for the main point of the review, which is a comparison between two different medium format backs.

There is wide agreement among photographers that use or have used both formats that MF has an advantage of several F/stops in DR versus the small cameras. The precise number will vary depending on the specific camera and back being compared as well as the comparison methodology, but the difference is quite noticeable in actual images. From 30 feet away? Maybe not. Let's just chalk that phrase up as a bit of editorial hyperbole rather than something intended to be taken literally.

The issue of the differences between medium format and 35mm is a fascinating one though, and Mark and I intend on pursuing it in greater depth in the days ahead, and with more rigour than with a throw-away line or two.
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« Reply #16 on: March 10, 2010, 03:27:16 AM »
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Quote from: michael
In case anyone hasn't seen it, the following was added to the review yesterday...

Paragraph Removed Editor

The struck-out paragraph below was part of the original article. It caused quite a bit of controversy on this site's forum and elsewhere. I initially added a comment to the bottom of the article, but that wasn't sufficient. Too many people's favourite ox had already been gored.

Please consider the paragraph below to be removed. There's no point in actually removing it, because once on the net, things last forever, and undoubtedly some zealots would consider it a coverup if we did so.



The reason that Mark and I have decided to remove it is not because we don't agree with its basic sentiment, but because it is serving as a distraction for the main point of the review, which is a comparison between two different medium format backs.

There is wide agreement among photographers that use or have used both formats that MF has an advantage of several F/stops in DR versus the small cameras. The precise number will vary depending on the specific camera and back being compared as well as the comparison methodology, but the difference is quite noticeable in actual images. From 30 feet away? Maybe not. Let's just chalk that phrase up as a bit of editorial hyperbole rather than something intended to be taken literally.

The issue of the differences between medium format and 35mm is a fascinating one though, and Mark and I intend on pursuing it in greater depth in the days ahead, and with more rigour than with a throw-away line or two.

Sound just like in the audio world with a lot of subjectivism, but also real unexplainable differences      

Have a Nice Day.

Thierry

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NikoJorj
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« Reply #17 on: March 10, 2010, 04:02:31 AM »
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First, thanks to Emil for his detailed insights on the subject, and thanks also to Michael and Mark for the clarification and mostly for the amount of insights they share too.


For what it's worth, about the definition of dynamic range...
Yesterday I re-read "the negative". Sorry to take the antiques out of the dust, but at chapter 4, discussing zone system and tonal scale, there are 3 scales defined :
- full scale from black to white (O to X),
- dynamic range of the first useful values above the latters (I to IX)
- and then textural range of zones which convey definite qualities of texture and the recognition of substance (II to VIII).

What I'd like to stress is that a definition of DR stopping at the darkest step where detail can be almost-perfectly rendered around middle gray on a print is not that photographic.
In a print, the extremities of the scale are very often rendered in barely readable tones, first because they need to be almost white or black, second because I feel that I need some tone compression at the end of the scale to avoid some unnatural cutoff from detail to no detail.
Edit, rethinking to it : do I try only to mimic film-like appearance (though I didn't nearly as much printing as I do know at thet time), or to deal with the limitations of my system (DRebel : APSC sensor dating back of 2003, more plausible bias), doing so? That's possible.

So, some detail loss is surely acceptable... the problem being to know how much, back to start.

From what I see (and feel  ) of the differences between my raw-enabled P&S (SD800+CHDK) and my goodol'Rebel, one other thing to watch when doing DR assessments might be color rendition in the shadows.
To the extreme, I've seen some very weird color casts showing when trying to lighten the shadows too much in sigma DP1 (foveon - cool, another old war restarted in the same thread   ) raws, but in the case of my P&S, I often feel that shadows are undersaturated and therefore lack of detail (people may call that "muddy" shadows?).
« Last Edit: March 10, 2010, 04:06:38 AM by NikoJorj » Logged

Nicolas from Grenoble
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« Reply #18 on: March 10, 2010, 06:59:33 AM »
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Quote from: NikoJorj
From what I see (and feel  ) of the differences between my raw-enabled P&S (SD800+CHDK) and my goodol'Rebel, one other thing to watch when doing DR assessments might be color rendition in the shadows.
[snip] in the case of my P&S, I often feel that shadows are undersaturated and therefore lack detail (people may call that "muddy" shadows?).

Chroma noise is affected by many things, including S/N in the raw data, the amount of boosting that needs to be done in white balance, color filter array properties, and also very much on the converter's demosaic algorithm.  Since it appears more unnatural than luma noise (photon shot noise is a physical phenomenon, but it results in fluctuations in brightness, not color), it should be dealt with, and often this entails a degree of desaturation of the image (again, depending on how it's dealt with).  How much one is willing to tolerate that enters into one's subjective assessment of how far into shadows one is willing to go.
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emil
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« Reply #19 on: March 10, 2010, 07:46:40 AM »
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Quote from: ejmartin
Two things that come to mind are relative strength of color filters in the CFA, and spectral characteristics (eg color temperature) of ambient light.  

A mismatch in white balance in the raw data means that one or more color channels will have to be amplified, and so one will be limited by the top end of one channel as compared to the (lifted) bottom end of a different channel, squeezing the available DR.  This can be mitigated by the use of color filters, either on the camera, or in studio situations on the light source.

There is also the effect that if the color properties of CFA filters are not close to those of RGB primaries, large coefficients appear in the color profile that transforms between the two, which serves to amplify chroma noise.  This effect was explained nicely in a DxO article:
http://www.dxomark.com/index.php/eng/Insig...-sensor-quality
Along the same lines, it is instructive to compare the color properties of the P65+ and the D3x under D50 illumination, which are shown here under the fair use doctrine and refer back to the DXO discussion comparing the Canon 500D and the Nikon D5000. The red channel of the P65+ sensor has poor differentiation between red and green (relative sensitivity), similar to the 500D, and the D3x does better in this channel. However, the P65+ has white balance coefficients closer to unity, which should be an advantage. Nonetheless, the D3x has smaller matrix conversion coefficients, especially in for the green channel and this should reduce chroma noise.

[attachment=20813:DXO_P65.PNG][attachment=20814:DXO_D3x.png]
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