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Author Topic: Do some vendors give up colour separation for high ISO?  (Read 4984 times)

ErikKaffehr

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Do some vendors give up colour separation for high ISO?
« on: February 29, 2016, 05:00:00 pm »

Hi,

There is a lot of mentions about DSLR vendors giving up on colour fidelity/accuracy/separation for high ISO capability. I think this is a lot of folklore and little facts.

In part I would think that some of that information is coming from this article: http://www.dxomark.com/Reviews/Canon-500D-T1i-vs.-Nikon-D5000/Color-blindness-sensor-quality.

I am a bit in doubt about the correctness of the DxO-mark article. The colour science is probably OK but I have some doubts about the conclusions. There are a lot tricks that can be done to increase ISO, like median filtering of channels. My guess is that Canon believes their choice is best for colour reproduction.

DALSA and Kodak (now OnSemi) publish some Spectral Response Curves, other than that, accurate data is rare. There are some independent measurements. Anders Torger has a link to some here: http://www.ludd.ltu.se/~torger/dcamprof.html#ssf_links

Tim Parkin is an english landscape photographer who has done some testing with different cameras. Both he and his friend Joe Cornish feel that the Kodak based CCD backs give bad reproduction of chlorophyl greens. Joe Cornish has now upgraded to a DALSA based Phase One back from his original P45+ and he is quite happy with the colours from the new back.

Tim Parkin found that there is an excellent correlation between his perception of correct colour and DxO-s SMI (Sensivity Metamerism Index) for different cameras. This article is worth reading, but OnLandscape is a pay site.
https://www.onlandscape.co.uk/2012/02/the-myth-of-universal-colour/

Tim suggested me to write an article for OnLandscape, but my effort run out in sand for some reason, a draft of the article is here:
http://echophoto.dnsalias.net/ekr/Articles/OLS_OnColor/OnColor.pdf

My opinion is that colour reproduction depends much more on colour profiles than on camera sensors.

One indication for this is that it is possible to use say an IQ-250 profile in on a D800 file in Capture One and it is said to yield better colour than the D800 profile.

Anders Torger also made the point although IQ-250 and Credo 50 use the same sensor, Capture One yields typical "Leaf colour" for Credo and "Phase One Colour" for the IQ-250. Anders also says that Hasselblad's Phocus yields similar colours for all their backs.

Just to demonstrate the effects of profiles, check the images below:
LR6Capture One
P45+
Sony A99 SLT

The correct bluish purple is this (measured with a spectrometer):

Green varies a lot more:


I found very little spectral data to plot, but I have found some interesting data at the sites Anders Torger referred to.

I attach two plots, the first one compares spectral sensitivity for Canon's 5DIII with the very old D2X from Nikon that used a Sony CCD and with a modern low end NEX series camera from Sony using a CMOS sensor. As you can seen the D2X and Sony NEX5N curves are almost identical with the NEX5N balanced for higher color temperatures. So, Sony uses the same dies on two cameras many generations apart.

Canon's red channel has a less steep gradient has more sensitivity for the red channel between 510 - 560 nm. I guess that it may gives less colour separation but possibly more accurate colour.

The second one compares "Phase One", NEX5N and H2D. The data doesn't say which models, but I would guess that P1 is using DALSA and H2D is using Kodak. Red sensitivity is very different between the P1 and H2D backs with the Sony sensor in the middle. Sony's sensor has a bit extended sensitivity for the green channel into the blues.

Best regards
Erik
« Last Edit: February 29, 2016, 05:58:15 pm by ErikKaffehr »
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Erik Kaffehr
 

AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #1 on: February 29, 2016, 06:18:55 pm »

off topic, but ...

Iliah Borg suggested elsewhere the following to read = http://www.aphesa.com/downloads/download2.php?id=1  which illustrates that "smooth" SSF/CMF curves you can find (mostly, not all) around are not exactly what is happening (and rather illustrate some sloppy work on behalf of measurebators) - here is his measurement of the unknown sensor (not normalized between channels)

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ErikKaffehr

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Re: Do some vendors give up colour separation for high ISO?
« Reply #2 on: March 01, 2016, 12:27:11 am »

Hi,

The image you linked to is not visible. Could you fix that link, if possible?

Thanks for the link to that paper, I have been aware of the ripple effects for a long time but they are not obvious in the data I plotted. Possibly because the measurements were at 10 nm intervals and the monochromator used probably not resolving higher. That paper is an interesting read!


Best regards
Erik


off topic, but ...

Iliah Borg suggested elsewhere the following to read = http://www.aphesa.com/downloads/download2.php?id=1  which illustrates that "smooth" SSF/CMF curves you can find (mostly, not all) around are not exactly what is happening (and rather illustrate some sloppy work on behalf of measurebators) - here is his measurement of the unknown sensor (not normalized between channels)


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Erik Kaffehr
 

AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #3 on: March 01, 2016, 02:06:28 pm »

The image you linked to is not visible. Could you fix that link, if possible?

not sure why, in my browser (FFox) it is OK = http://s26.postimg.org/ga85dvmg7/Sensor_Sawtooth_copy.jpg
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ErikKaffehr

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Re: Do some vendors give up colour separation for high ISO?
« Reply #4 on: March 01, 2016, 02:45:58 pm »

Hi,

Thanks, I can see it!

Yes indeed, the spectral plot shows the resonances the article talks about. That is not really evident in the data sets I have plotted and that my be dependent on the channel width used.

My intention with plots was to show similarities and differences between sensors and for that purpose I think they are good enough.

Best regards
Erik

not sure why, in my browser (FFox) it is OK = http://s26.postimg.org/ga85dvmg7/Sensor_Sawtooth_copy.jpg
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AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #5 on: March 01, 2016, 05:33:08 pm »

My intention with plots was to show similarities and differences between sensors and for that purpose I think they are good enough.
the devil is in the details... I suggested, rather made a guess, (elsewhere where it was posted) that as we are dealing with integrating anyways then more precise sawtooth does not add any "noticeable" precision vs smooth graphs (interpolated from 10nm steps), but then he replied then methameric errors will be very noticeable... so even very similar graphs for the same camera (one measured casually and one with efforts to uncover this effect) can really be noticeably different, no ?
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Torbjörn Tapani

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Re: Do some vendors give up colour separation for high ISO?
« Reply #6 on: March 01, 2016, 06:08:57 pm »

I can't say if it affects color separation but I found this tidbit on the new Nikon D5 "Nikon claims the D5 to offer leading high ISO performance due to color filter array tweaks that allow the sensor to capture more light than before."
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torger

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Re: Do some vendors give up colour separation for high ISO?
« Reply #7 on: March 02, 2016, 06:51:10 am »

To get good high ISO you want to keep as much light as possible, that is ideally the camera would be monochrome.

Color separation is a complex area though. A good basis for good color separation is a low noise sensor, then you can separate colors even if the channels differ very little. That is you can have wide overlaps and let through lot of light in all filters and still be able to separate colors.

With a noisy sensor it's better to have more difference between color filters so the channels differ more (you get more saturated colors natively). In extreme cases you don't have overlap at all which I think was the case for some older prismatic TV cameras in the analog days. This indeed gives very sharp separation, but due to the lack of overlap there are colors that will be registered to be the same that would be seen as different with overlapping filters.

All(?) modern sensors today have wide overlaps and and quite low saturation colors natively. This allows for registering lots of colors, and improves ISO as more photons are captured, but also put requirement on the sensor to be low noise so we can through the color profile add back saturation and visibly separate colors.

When working with DCamProf I noted that many cameras, specifically Sony, had a high sensitivity on the blue channel. This made it very complicated to make realistic blue tones in daylight (daylight has high blue content in itself). The solution is to render blues lighter than realistic, which indeed is a popular "look" which many color profiles implement in way way or another, regardless if the sensor requires it or not. I felt that Canon's color filters where better balanced to produce realistic colors though.

The DxOmark SMI index is a measure how well the ColorChecker 24 color can be matched with a matrix-only profile. A disadvantage of that test is that it doesn't say anything on how the cameras react to "extreme" colors (very high saturation colors), so the Sonys fair very well (much thanks to their low noise) although I personally prefer the response I've seen in Canon cameras as their easier to manage in the extreme range.

Are there tradeoffs ISO vs color separation? Maybe/probably, but I don't really know how they are done. A guess is that the high blue channel sensitivity in the Sony is a high ISO thing, plus that they realized that most subjectively like lighter-than-realistic deep blues anyway. That doesn't really hurt color separation though, just the possibility to make realistic colors in cool light. You could make the color filters be more similar in the overlaps to let through more light, and increase the similarity more than what is suitable for an ideal color separation tradeoff.

With DCamProf I made a color separation diagram which can be used to make comparisons how well a camera can separate colors compared to the eye: http://www.ludd.ltu.se/~torger/dcamprof.html#ssf_csep
However one needs to factor in the sensor noise to make really good sense of that.

If we talk about the "medium format magic colors" or any other camera's supposedly magic color I'd say that it's almost only about well-designed subjective color profiles that fits the taste of their users. Any sensor today of reasonable size in good lighting condition have sufficient color separation to design almost any type of look you want, at least that's the indications I've got from my work, so for any ISO sacrifice made it doesn't seem to be large. Going from the camera's raw colors to a robust color profile is not an easy thing though. One may think it's only about shooting a chart and matching, which in the case of reproduction work it may be, but for all-around photography you need to apply contrast, a "film curve", and that will modulate colors as a side effect (as our brain's color perception is tightly linked to contrast). That's why matrix profiles designed for a linear curve works so bad with contrast applied, oh well bad if you ask me, some like the result anyway and it's much about taste.

A well designed color profile is designed to match the film curve it's intended to be used with. There are really no established color science models to do this so it's up to the profile designer to invent their own models, and the manufacturers have done so, and so have I with DCamProf. We all end up with different results of course, as we have different design targets and tastes.
« Last Edit: March 02, 2016, 06:59:18 am by torger »
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torger

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Re: Do some vendors give up colour separation for high ISO?
« Reply #8 on: March 02, 2016, 07:34:20 am »

Oh, a quick comment on the low SMI score of the P45+ shown in Tim's article. Kodak liked to make subjective colors and their sensors have quite saturated native colors, and indeed a matrix-only profile can't match the CC24 that well -- hence a poor score. Another reason to a relatively low score is that the Kodak sensor is noisy compared to a modern Sony sensor.

A non-linear profile (LUT) can make a quite good match with the CC24 though if you want to, but here's the interesting part -- color is subjective and if you decide to work with the Kodak subjective colors you can make a clean robust profile with a subjective look which you actually may like. So color is not necessarily bad due to the low score, in this case it's a strong indication that the color is subjective and not well-suited to make reproduction style color, but may produce a pleasing look.

How well does SMI correlate to "color separation" or "color accuracy"? Uhmm... not so much I'd say. You must factor in the color profile. It's true that a high SMI score is a good basis to build a color profile with any look you want both subjective and neutral, while a low score locks you in with some subjectivity. If you pick any default color profile in Capture One or Lightroom or whatever there's a lot of subjectivity built-in already, which is shown in the initial post. With Lightroom there's also very different profile designs for different cameras so it's hard to compare different cameras even within Lightroom. That may be the case with Capture One too, I don't know. In any case I haven't seen any indications that you would be able to "see through" the color profile and make conclusions about how the native raw colors are underneath.

Photographers that worry about color should really make their own profiles to take control over the situation. Unfortunately there's not so much software out there to do that. That was the reason I wrote DCamProf but I can't say it's a software "for the masses", it's a bit too complicated to use for that.
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hjulenissen

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Re: Do some vendors give up colour separation for high ISO?
« Reply #9 on: March 02, 2016, 04:19:45 pm »

For many of us, colors are highly subjective anyways, and using various profiles and manual tweaking, we get what we want.

For those who really want subjectively accurate color reproduction (coca cola commercials? Art reproduction?), as long as true multi spectral cameras is out of the question. What about using a set of color filters in front of the camera, calibrated as a set? Using, say 3 different filters/exposures on a camera that itself has 3 different color channels should give you 9 degrees of freedom. Chosen wisely (ie color sharpening that suppress light in the transition zones, increasing color resolution in critical skin tones, sky tones...) ought to make a general purpose camera a lot more color accurate at the cost of more exposure work and some software package.

See:
http://www.smithsonianmag.com/innovation/scientist-accidentally-developed-sunglasses-that-could-correct-color-blindness-180954456/
https://en.m.wikipedia.org/wiki/Quantum_dot_display
« Last Edit: March 02, 2016, 04:23:01 pm by hjulenissen »
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AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #10 on: March 02, 2016, 04:53:05 pm »

coca cola commercials?

shall be better served by computer graphics or just painting the logo colors in from the numbers...
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Guillermo Luijk

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Re: Do some vendors give up colour separation for high ISO?
« Reply #11 on: March 02, 2016, 07:22:30 pm »

My opinion is that colour reproduction depends much more on colour profiles than on camera sensors.

My intuitive guess, with zero testing nor scientific data to demonstrate it, has always been the same. From the first day I saw a before/after comparison in a profiled Canon camera I thought that was the main variable in colour rendering. So as white balance is the variable over any cast produced by the lens (leaving aside yellowed legacy lenses and so).

Regards!

ErikKaffehr

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Re: Do some vendors give up colour separation for high ISO?
« Reply #12 on: March 03, 2016, 12:51:20 am »

Hi Anders,

That is probably a good explanation. Thank you very much!

From what I have seen, I was mostly happy with the colours from P45+ using a home made DCP profile in LR. I never got happy with C1, so I cannot comment on that.

Best regards
Erik



Oh, a quick comment on the low SMI score of the P45+ shown in Tim's article. Kodak liked to make subjective colors and their sensors have quite saturated native colors, and indeed a matrix-only profile can't match the CC24 that well -- hence a poor score. Another reason to a relatively low score is that the Kodak sensor is noisy compared to a modern Sony sensor.

A non-linear profile (LUT) can make a quite good match with the CC24 though if you want to, but here's the interesting part -- color is subjective and if you decide to work with the Kodak subjective colors you can make a clean robust profile with a subjective look which you actually may like. So color is not necessarily bad due to the low score, in this case it's a strong indication that the color is subjective and not well-suited to make reproduction style color, but may produce a pleasing look.

How well does SMI correlate to "color separation" or "color accuracy"? Uhmm... not so much I'd say. You must factor in the color profile. It's true that a high SMI score is a good basis to build a color profile with any look you want both subjective and neutral, while a low score locks you in with some subjectivity. If you pick any default color profile in Capture One or Lightroom or whatever there's a lot of subjectivity built-in already, which is shown in the initial post. With Lightroom there's also very different profile designs for different cameras so it's hard to compare different cameras even within Lightroom. That may be the case with Capture One too, I don't know. In any case I haven't seen any indications that you would be able to "see through" the color profile and make conclusions about how the native raw colors are underneath.

Photographers that worry about color should really make their own profiles to take control over the situation. Unfortunately there's not so much software out there to do that. That was the reason I wrote DCamProf but I can't say it's a software "for the masses", it's a bit too complicated to use for that.
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ErikKaffehr

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Re: Do some vendors give up colour separation for high ISO?
« Reply #13 on: March 03, 2016, 01:02:39 am »

Hi Anders,

Thanks for sharing your experience!

It is interesting to hear that you find the Canon approach a good one, although I don't feel it is a great surprise.

With the blue sensitivity on the Sony sensors, that has not been obvious to me. Can you also see it on Phase One (IQ-250) and Nikon that also use Sony sensors?

My guess from looking at the spectral plots was that some sensors are more balanced for incandescent light, so they have high sensitivity for blues. It makes a lot of sense for average shooting that is a mix of different colour temperatures.

Best regards
Erik



To get good high ISO you want to keep as much light as possible, that is ideally the camera would be monochrome.

Color separation is a complex area though. A good basis for good color separation is a low noise sensor, then you can separate colors even if the channels differ very little. That is you can have wide overlaps and let through lot of light in all filters and still be able to separate colors.

With a noisy sensor it's better to have more difference between color filters so the channels differ more (you get more saturated colors natively). In extreme cases you don't have overlap at all which I think was the case for some older prismatic TV cameras in the analog days. This indeed gives very sharp separation, but due to the lack of overlap there are colors that will be registered to be the same that would be seen as different with overlapping filters.

All(?) modern sensors today have wide overlaps and and quite low saturation colors natively. This allows for registering lots of colors, and improves ISO as more photons are captured, but also put requirement on the sensor to be low noise so we can through the color profile add back saturation and visibly separate colors.

When working with DCamProf I noted that many cameras, specifically Sony, had a high sensitivity on the blue channel. This made it very complicated to make realistic blue tones in daylight (daylight has high blue content in itself). The solution is to render blues lighter than realistic, which indeed is a popular "look" which many color profiles implement in way way or another, regardless if the sensor requires it or not. I felt that Canon's color filters where better balanced to produce realistic colors though.

The DxOmark SMI index is a measure how well the ColorChecker 24 color can be matched with a matrix-only profile. A disadvantage of that test is that it doesn't say anything on how the cameras react to "extreme" colors (very high saturation colors), so the Sonys fair very well (much thanks to their low noise) although I personally prefer the response I've seen in Canon cameras as their easier to manage in the extreme range.

Are there tradeoffs ISO vs color separation? Maybe/probably, but I don't really know how they are done. A guess is that the high blue channel sensitivity in the Sony is a high ISO thing, plus that they realized that most subjectively like lighter-than-realistic deep blues anyway. That doesn't really hurt color separation though, just the possibility to make realistic colors in cool light. You could make the color filters be more similar in the overlaps to let through more light, and increase the similarity more than what is suitable for an ideal color separation tradeoff.

With DCamProf I made a color separation diagram which can be used to make comparisons how well a camera can separate colors compared to the eye: http://www.ludd.ltu.se/~torger/dcamprof.html#ssf_csep
However one needs to factor in the sensor noise to make really good sense of that.

If we talk about the "medium format magic colors" or any other camera's supposedly magic color I'd say that it's almost only about well-designed subjective color profiles that fits the taste of their users. Any sensor today of reasonable size in good lighting condition have sufficient color separation to design almost any type of look you want, at least that's the indications I've got from my work, so for any ISO sacrifice made it doesn't seem to be large. Going from the camera's raw colors to a robust color profile is not an easy thing though. One may think it's only about shooting a chart and matching, which in the case of reproduction work it may be, but for all-around photography you need to apply contrast, a "film curve", and that will modulate colors as a side effect (as our brain's color perception is tightly linked to contrast). That's why matrix profiles designed for a linear curve works so bad with contrast applied, oh well bad if you ask me, some like the result anyway and it's much about taste.

A well designed color profile is designed to match the film curve it's intended to be used with. There are really no established color science models to do this so it's up to the profile designer to invent their own models, and the manufacturers have done so, and so have I with DCamProf. We all end up with different results of course, as we have different design targets and tastes.
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Erik Kaffehr
 

torger

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Re: Do some vendors give up colour separation for high ISO?
« Reply #14 on: March 03, 2016, 04:21:08 am »

I haven't worked too much with the IQ250 or the Nikons so I don't know, but the Pentax 645z certainly still has it. This could be controlled by the camera manufacturer quite easily I suppose even if the CFA would be exactly the same as you apply a custom IR filter in front which also could dampen the blues if you want to. That would hurt ISO performance a bit too though so I'm not sure why people would do it.

If you look at the deep blue patch on a CC24 in real life and then look at it when a generic standard profile is applied you will most likely see that the blue patch is rendered too light. Our human eyes are that sensitive to those deep blues so they become really dark. By lightening them we can see more shades so it's a popular subjective look in profiles. If you do want to make realistic deep blues which are very dark you need to on the mentioned Sonys subtract a lot of blue, which means that the profile becomes unstable and can clip to black if you get some strange narrow band color which can happen for artificial lights.

It's fairly easy to make a camera profile that behaves well in say the CC24 range of colors, but if you want a profile that doesn't break even in strange lighting conditions and super-saturated colors it becomes quite difficult. Matrix-only profiles generally perform poorly in these conditions as robustness require non-linear adjustments. Almost all general-purpose camera profiles today have some sort of gamut compression built in, and some also change the hues a fair bit to make it fit easier in known gamuts. This exaggerates differences between profiles when you look at saturated colors, like pictures of flowers.

We're still dealing with digital cameras like they were film cameras, and the camera profile corresponds to the "film roll", with a lot of subjectivity built in. A more modern design would be that the camera profile only provides a translation into a pure colorimetric representation, and then film curves and look profiles and gamut compression etc would be applied on top inside the raw converter. This is not how raw converters work today though. From a color science perspective it's poor design, but it's indeed quite practical and it gives a lot of power to the profile designer to control the look. So if you don't like the colors of Lightroom or Capture One or whatever it's possible to fix just by making an own color profile.

Yes indeed, a high sensitivity blue is good for low color temperatures (where blue becomes weak) so the sensor designer needs to make a tradeoff.

Another interesting note about robustness is that you can see that Adobe's own profiles for both the A7r-II and the Pentax 645z are under-saturated, that is it has lower saturation than realistic. With overlapping filters the native raw colors get less saturated, and increasing saturation means separating the channels more through subtractions. This process makes the profile less robust as you can always get into strange light conditions when you have a strong response on one channel and weak in others so you subtract into negative numbers.

With a LUT you can easily avoid this, but not without risking to hurt gradients (by shifting from strong subtraction to no subtraction over a narrow range). With the those two cameras I simply found it to be impossible to make a profile which both had realistic saturation in the normal range and at the same time 100% robustness. Adobe solved it by making the profiles somewhat desaturated. However I think it's then better to have two profiles, one with good color which works in 90% of the cases, and then switch to a low saturation profile to handle those strange light conditions (that can happen in say night scapes with color artificial lights). It's less user-friendly though and Adobe values both user-friendliness and robustness very highly, so they instead chose to compromise quality, at least as I see it.

Hi Anders,

Thanks for sharing your experience!

It is interesting to hear that you find the Canon approach a good one, although I don't feel it is a great surprise.

With the blue sensitivity on the Sony sensors, that has not been obvious to me. Can you also see it on Phase One (IQ-250) and Nikon that also use Sony sensors?

My guess from looking at the spectral plots was that some sensors are more balanced for incandescent light, so they have high sensitivity for blues. It makes a lot of sense for average shooting that is a mix of different colour temperatures.

Best regards
Erik
« Last Edit: March 03, 2016, 04:29:46 am by torger »
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Jack Hogan

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Re: Do some vendors give up colour separation for high ISO?
« Reply #15 on: March 03, 2016, 05:17:04 am »

My opinion is that colour reproduction depends much more on colour profiles than on camera sensors.

I agree Erik, based on my improving but still quite limited understanding of the subject.  Torger makes some interesting points on Sony's blues, but that seems to be more for subjective pleasing purposes as opposed to related to improving noise or discrimination.

Jack
« Last Edit: March 03, 2016, 05:26:56 am by Jack Hogan »
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Jack Hogan

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Re: Do some vendors give up colour separation for high ISO?
« Reply #16 on: March 03, 2016, 05:27:49 am »

Iliah Borg suggested elsewhere the following to read = http://www.aphesa.com/downloads/download2.php?id=1  which illustrates that "smooth" SSF/CMF curves you can find (mostly, not all) around are not exactly what is happening (and rather illustrate some sloppy work on behalf of measurebators) - here is his measurement of the unknown sensor (not normalized between channels)

Interesting paper. I don't think the linked image is representative of current CFA sensors though.  Such large oscillations would clearly be seen in SSF data collected at 10nm intervals (for instance in the references Erik linked) or even in home tests (virtually every one of those bumps in the home test is accounted for by the spectral power distribution of the sun at that wavelength).

Jack
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Jack Hogan

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Re: Do some vendors give up colour separation for high ISO?
« Reply #17 on: March 03, 2016, 10:01:48 am »

The correct bluish purple is this (measured with a spectrometer):...
Green varies a lot more: ...

Erik, that's interesting.  May I ask how you produced the 'correct' colors?

Jack
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AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #18 on: March 03, 2016, 10:06:36 am »

Such large oscillations would clearly be seen in SSF data
and he provided his measurements with such sawtooth, granted he has more elaborate setup that allows him to sample finer than 10nm even with monochromator set for 10nm (as you can also measure the light going into monochromator with finer than 10nm steps with spectrophotometer)...
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AlterEgo

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Re: Do some vendors give up colour separation for high ISO?
« Reply #19 on: March 03, 2016, 10:11:57 am »

for instance in the references Erik linked
if this was about RIT graphs those were quick proof of concept measurements w/o any attempt to calculate finer than 10nm data (and they had spectroradiometer better than some consumer level spectrophotomer like i1Pro-something) and you can  see that they even keep the lens on the camera, instead of mounting camera to monochromator and measuring the transmission lens separately.

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