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Author Topic: Why does Kodakchrome receive special treatment in scanning software?  (Read 9510 times)
Ernst Dinkla
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« Reply #20 on: January 15, 2011, 05:57:36 AM »
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Are you saying Kodachrome looks blue because in comparison other slide films are yellowish due to fading? I think that's a stretch!
A scanner profile will preserve the blue cast, not eliminate it. The blue cast is shown in the characteristic curves - would Kodak

publish the characteristic curves of faded film?.
 
In my previous post I attached 2 images. The first one is the unadjusted scan with a Kodak Q60 Kodachrome target (scarse/argyll) profile

applied. The second one is a simulation of the appearance of the first one when projected, using CIECAM02 to model the changes in viewing

conditions. The difference in appearance is due solely to the difference in viewing conditions.

I agree with Jeff and was only adding additional considerations to what had already been said.


I mentioned several causes for possible flaws in Kodachrome scanning, adding to the ones already mentioned and I didn't suggest that your plots show fading.

Kodachrome has a reputation of not fading starting with the version after roughly 1938. Dark fading that occured affected the yellow  dye though. Ektachrome in all its incarnations dark faded faster and mainly on the cyan layer. Kodachrome didn't suffer of yellow staining, Ektachrome does. Both faded in projection on magenta but the Kodachrome much faster. Wilhelm recommended 1 hr maximum exposure for Kodachrome and 2.5 hrs for Ektachrome, Fujichrome 5 hours. Gathered from Wilhelm's book on the permanence etc of color. You will find some of that in the Wiki pages on Kodachrome. http://en.wikipedia.org/wiki/Kodachrome With the change to digital photography the actual scanning of films today will be more focused on film that has been archived/projected than on fresh film. Kodachrome's use even more shrinking over a longer period. So it is certainly not a stretch.

I do not understand why a Kodachrome slide film should get a blue bias to compensate a yellowish projector lamp and no other slide film needs the same compensation. The same observation is made here:
http://www.apug.org/forums/forum40/81778-love-hate-kodachrome-why-all-passion-here.html
Could be corrected on his assumptions of Kodachrome fade resistance though.
What you see as different plots is set in perspective there too, other dyes, other densities, other spectral plots. Kodachrome has a saturated gamut and that will show. More saturation in blue possible which is something else than a blue bias, the last should show in the neutrals. A richer film like Kodachrome will have more deviations on the neutral axis in any display condition as it is harder to control CMY layers, development, lamp aging etc when color contrasts are higher. Profiling in digitalisation can help then but on average not per slide.
There are articles on the Silverfast site on the Kodachrome profiling aspect: http://www.silverfast.com/show/kodachrome-targets/en.html. Blue is certainly a problem when KC is scanned using an EC profile. See also the scanning part of the wiki Kodachrome page: http://en.wikipedia.org/wiki/Kodachrome. And yes, Kodachrome gamut should be available then but a blue cast is a flaw in a scan workflow.

With Jeff's remark on different observers you can add an extra difference occuring in this discussion: the Sprintscan's white cool cathode fluorescent lamp versus the LEDs of the Coolscan 5000. I have seen and measured some scanner lights (Eye One, Epson V750 cold cathode. Mirage II cold cathode) and they differ in spectral output. The two types of linear sensors will have (smaller) differences in RGB filters. Jeff's Imacon has a (halogen) tungsten lamp if I recall it correctly (edit: wrong, it looks like 5000K fuorescents are used on Imacons) and most likely a Kodak linear CCD sensor very similar to the one in the Nikon 8000. With all systems tweaked for the chromogenic dyes of Ektachrome-Fujichrome you can expect deviations with Kodachrome dyes.

A RAW export of a 16 bit file to a RAW processor is possible with Vuescan too. There is something to be said for editing in scanner space and 16 bit. Of course the file itself isn't comparable to a Bayer sensor RAW file.

Flatbeds crept into this discussion for other reasons, the Nikon Coolscans, the SprintScan, are of another class.


met vriendelijke groeten, Ernst Dinkla

New: Spectral plots of +220 inkjet papers:
http://www.pigment-print.com/spectralplots/spectrumviz_1.htm
« Last Edit: January 19, 2011, 03:12:51 AM by Ernst Dinkla » Logged
crames
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« Reply #21 on: January 15, 2011, 09:31:26 AM »
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I mentioned several causes for possible flaws in Kodachrome scanning, adding to the ones already mentioned and I didn't suggest that your spectral plots show fading.

Well, you said "One may wonder whether fresh Kodachrome<>Ektachrome slides are much closer in their color representation if suitable scanner profiling is used for both." From this I understand you to say that the blue cast could be the result of using film that was not fresh, ie faded. If that were the case, then since the Kodak curves also show the blue cast, they would have to be based on non-fresh or faded film, too, otherwise the Kodak curves would not show the cast.

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Kodachrome has a reputation of not fading starting with the version after roughly 1938. Dark fading that occured affected the yellow  dye though. Ektachrome in all its incarnations dark faded faster and mainly on the cyan layer. Kodachrome didn't suffer of yellow staining, Ektachrome does. Both faded in projection on magenta but the Kodachrome much faster. Wilhelm recommended 1 hr maximum exposure for Kodachrome and 2.5 hrs for Ektachrome, Fujichrome 5 hours. Gathered from Wilhelm's book on the permanence etc of color. You will find some of that in the Wiki pages on Kodachrome. http://en.wikipedia.org/wiki/Kodachrome With the change to digital photography the actual scanning of films today will be more focused on film that has been archived/projected than on fresh film. Kodachrome's use even more shrinking over a longer period. So it is certainly not a stretch.

Thee blue cast of Kodachrome is inherent to fresh Kodachrome. It has nothing to do with the fading properties of other emulsions. I don't have my copy of Wilhelm handy, but if what you say is correct about magenta fading with light exposure, that would create a green cast, would it not? But let's limit the discussion to properly developed and stored Kodachrome scanned using ICC profiles.

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I do not understand why a Kodachrome slide film should get a blue bias to compensate a yellowish projector lamp and no other slide film needs the same compensation. The same observation is made here: http://www.apug.org/forums/forum40/81778-love-hate-kodachrome-why-all-passion-here.html

I don't know why either. Perhaps because it makes printing and scanning more difficult?

Quote
With Jeff's remark on different observers you can add an extra difference occuring in this discussion: the Sprintscan's white cool cathode fluorescent lamp versus the LEDs of the Coolscan 5000. I have seen and measured some scanner lights (Eye One, Epson V750 cold cathode. Mirage II cold cathode) and they differ in spectral output. The two types of linear sensors will have (smaller) differences in RGB filters. Jeff's Imacon has a (halogen) tungsten lamp if I recall it correctly and most likely a Kodak linear CCD sensor very similar to the one in the Nikon 8000. With all systems tweaked for the chromogenic dyes of Ektachrome-Fujichrome you can expect deviations with Kodachrome dyes.

But all of these deviations are compensated with a proper ICC profile.

Here is an experiment to test the theory that the blue cast is due to the curves. The first graph shows the Kodachrome 25 curves digitized and graphed in Excel.
The second graph shows the same K25 curves after the Red density is scaled by 0.84 and the Green density is scaled by 0.96. These scaling serve to align the three cuves.

Next is a scan with Kodachrome ICC profile applied and converted to sRGB without any adjustments. The last image has been adjusted (in the profile space) in levels with Red midtone level (gamma) set to 1.19 (1/0.84) and the Green midtone level set to 1.04 (1/0.96) - in other words, the same adjustments that aligned the curves in the Excel model. The last image has also been reduced in contrast with an RGB midtone level adjustment of 1.25. No other adjustments have been made. The adjusted image is warmer and the excess blue in the whitewashed barn wall is removed along with excess blue in the shingled roof of the shed on the right. Certainly the curve-aligned image is without the blue cast and is a better starting point for additional editing - and a better representation of the image as it would appear projected.
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Ernst Dinkla
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« Reply #22 on: January 15, 2011, 10:41:18 AM »
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There is a good explanation at that link why both Kodachrome and Ektachrome come close to one another in projection despite different curves for RGB dyes. Call it a metameric match in projection light. The curves you see are curves for different dyes, it isn't a different dose of the same dyes creating the curve displacement.

A quote from that page:

If you compare the first figure to the third figure, you can compare the Ektachrome with Kodachrome. You can immediately see that the curves of the 3 Kodachrome dyes are NOT matched, and that the Cyan is considerably higher than the other two. As a result, a visual neutral may look cyanish. To see why I say “may” look at the 4th figure which is a unit neutral of the Kodachrome dye set. You can immediately see that the cyan dye requires more density to give a visual neutral of 1.0 due to the fact that the cyan dye is very narrow in absorption. It is also low in unwanted densities and this introduces a big dip or gap in the neutral at about 600 nm. The lower unwanted absorption of the cyan in the green and blue region of the spectrum are responsible in part, for the improved overall color saturation.

The result of all of this though is that Kodachrome K-14 products have slightly more illuminant sensitivity than the E6 product family, but since they are all intended to be projected, who cares. It also means that even under identical illumination, some people may view Kodachrome slides differently than others, due to the slight, but normal differences in the color receptor pigment balance in individual eyes. On the other hand, Kodachrome colors have a vibrant saturation with less impurities for this same reason, but in some cases, that advantage is scene dependant.<<

end of quote

Call the blue shift in scanning a metameric failure due to changed scanner lighting and another observer (scanner sensors) and I can agree. The blue shift isn't Kodachrome color and isn't there for projection light compensattion.

Yellow fading in dark storage will result in a blue cast. Cyan fading of Ektachrome will result in an orange/red cast combined with yellow staining more orange. In short the two drift apart, one to the blue the other to orange. Any projection will cause magenta loss and an additionally shift, Kodachrome to bluegreen and Ektachrome to a dirty orange.

With the Kodachrome shift to blue caused by the scanner hardware (though that can vary per scanner as I already mentioned) any fading of both Kodachrome (blue-bluegreen) Ektachrome (red/orange-dirty orange) will at least increase the shift to blue already created by the scanner hardware. A fresh Kodachrome and Ektachrome IT8 calibration target may help with fresh slides to get a better match, faded slides will not match as nice and set both film types more apart. The other variable is of course fading of the targets but we are professionals so we have fresh targets :-)


met vriendelijke groeten, Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/
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crames
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« Reply #23 on: January 15, 2011, 05:28:21 PM »
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Call the blue shift in scanning a metameric failure due to changed scanner lighting and another observer (scanner sensors) and I can agree. The blue shift isn't Kodachrome color and isn't there for projection light compensattion.

Ernst, you're a hard person to convince.Smiley

The blue shift in scanning is due to the fact that most scanners don't use tungsten illuminants at 3200K, and when using profiles, full adaptation is made to D50.

Quote
With the Kodachrome shift to blue caused by the scanner hardware (though that can vary per scanner as I already mentioned) any fading of both Kodachrome (blue-bluegreen) Ektachrome (red/orange-dirty orange) will at least increase the shift to blue already created by the scanner hardware. A fresh Kodachrome and Ektachrome IT8 calibration target may help with fresh slides to get a better match, faded slides will not match as nice and set both film types more apart. The other variable is of course fading of the targets but we are professionals so we have fresh targets :-)

I say again, any blue shifts caused by the hardware will be corrected by the ICC profile. Yet, a blue cast remains even with a profile, because the profile ensures that the inherent blue cast is accurately reproduced.

One more quote from Hunt's definitive reference, The Reproduction of Colour, pgs. 229-230. If this doesn't convince you, I don't know what else to say:

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In figure 14.9 are shown typical sets of characteristic curves of red, green, and blue integral density plotted against log exposure for a reversal (a) and two print(b) and (c) materials, when the colour of the exposing light is such that most exposure levels are reproduced so as to appear grey in the viewing conditions typical of those used in practice for each material. When the viewing conditions consist of projection by tungsten light in a darkened room, the light from the projector appears yellowish (Hunt, 1965), and therefore to obtain results that appear grey the picture has to be slightly bluish (see Section 5.7 [quoted previously]); this is why the curves of Fig. 14.9(a), which relate to materials for tungsten-light projection, are not even approximately coincident, the blue densities being lower than, and the red densities higher than, the green densities, in order to produce the bluish result required...

Figure 14.9(a) is attached below. The caption reads:

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Fig. 14.9. Typical characteristic curves of integral density plotted against log exposure when the colour of the exposing light is such that most exposure levels are reproduced grey. (a) Reversal colour film (such as Kodachrome film) for making camera pictures for projection by tungsten light...

Not only does Hunt state that it is the case for Kodachrome, the example curves are essentially the same as the K25 curves posted previously.
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guyburns
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« Reply #24 on: January 17, 2011, 07:04:31 AM »
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For anyone coming to this point who wants a summary, I have started a new thread here:

http://www.luminous-landscape.com/forum/index.php?topic=50387.0
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dmerger
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« Reply #25 on: January 18, 2011, 10:35:59 AM »
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This thread seemed odd to me, and now I think I may know why.  Silverfast recently launched its promotion of its “improved Kadachrome workflow.”  Now, all of a sudden we have a guy who registers on this web site on January 11, 2011, and the very next day posts a question about scanning Kodachrome.  Coincidence?  Then, of course, we have the usual promoters of Silverfast extolling the wondrous virtues of Silverfast with posts that read like something straight out of a marketing campaign.


Of course, we’ll probably never know for sure if any of these guys are shills, but the coincidence, if that is what it is, is extraordinary.
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Dean Erger
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« Reply #26 on: January 18, 2011, 10:35:36 PM »
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Guy, your explanation has convinced me that my suspicion that you may be a shill was unfounded.  I apologize. 
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Dean Erger
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« Reply #27 on: January 31, 2011, 03:30:04 PM »
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When printing with the Dye Transfer Process, we used Principle Masks to lower the contrast range of the slide from around 2.9 down to around 2.1, but in the process we also removed large, known distortions in the original slide dyes. For example, an Ektachrome cyan dye absorbs 100% of red light, just as it should do, but it also absorbs 30% of green light. This means that the cyan dye also contains 30% magenta that does not belong there. If we shot a principle mask through a red filter and develop it to 30% of the contrast or gamma that is needed for a separation, then put that on top of the raw film when the separation shot through the G61 green filter is made (the one used to expose the magenta matrix), then everywhere cyan and magenta both appear in the image 30% of the exposure will be blocked in the magenta matrix. This way the 30% false magenta in the original cyan dye in the slide can be eliminated. A cyan absorption in the Magenta dye was treated the same way.

I regularly made two sets of red separations, one with an R29 filter and one with an R25 (most people used one R25 separation). By changing the percentages of which separation was given more time on the split exposures, I could make the balance in the greens in the print warmer or cooler, as this varied the curve of the cyan. There was probably also a difference in the exposure of the principle masks used when printing Kodachrome vs Ektachrome and other E6 films, but I do not remember exactly. I could dig for it in the attic if anyone is interested.

Here is a simple account followed by an explanation in more detail about how Dye Transfer was done, and why it produced superior colour rendition over all other print processes at the time:
http://avalon.unomaha.edu/afghan/techinfo/TECHINDX.HTM
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crames
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« Reply #28 on: February 01, 2011, 10:36:31 AM »
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There was probably also a difference in the exposure of the principle masks used when printing Kodachrome vs Ektachrome and other E6 films, but I do not remember exactly. I could dig for it in the attic if anyone is interested.
Yes, that would be interesting. I'm wondering if there is also a difference in development to alter the contrast of the masks?
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Cliff
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