Cliff, thanks for the four curves. I'll play around with Ektachrome and the Hutch method to see what comes of it.
The Ektachrome profile might be the solution to your KC shadow issues. Your Velvia profile might work, too. In the Ektachrome log plot, above, look at how the three channels track nearly perfectly right down to DMAX. Even the wayward red channel has been brought into line. I haven't tried an Ektachrome profile with gamma, yet, since I have only, ever made linear scans with my scanner.
And maybe I should choose Lab clut as my preferred profile, instead of XYZ (see Knockout Rounds, below).
If not your Ektachrome profile. I was using Lab clut only because XYZ does not work as well for linear scans. For gamma >1 scans, I think XYZ is usually better.
For a Gamma 1.0 scan (see Ref 03 in the clouds and snow), 16-bit, raw RGB values are R>28,000 and G&B > 30,000, well above what the D-H curves indicate I should be getting as the brightest scan from Kodachrome. And when the profile is applied, those values increase. I'm not sure anything sensible can be gained by a person of my limited colour knowledge trying to correct a scan by using the Kodachrome D-H curves, because I don't know how those curves relate to an actual scan.
Hmm, my scanner also gives RGB values for DMIN in the same range as yours. I think it's just the scanners giving a little extra exposure. The effect would be to shift the characteristic curves down, so that DMIN is closer to zero density, an equal subtraction of density from the three channels.
All transparency films seem to have a DMIN in the same ballpark. (For that matter, so do negative films.) In that case it would make sense for scanner manufacturers to calibrate so that the maximum RGB numbers are assigned to a density closer to film DMIN, instead of to density = 0 (no film in the scanner).
All that matters is that the highest RGB values in a scan do not exceed the values for DMIN in the Q60 profiling target. If they do exceed DMIN, then the profile can be adjusted by one of the Hutcheson techniques.
If you Convert to Profile using Absolute
Intent, the lightest white on the slide will be L* = 88, which is how DMIN is defined for the Q60 target. It gets darker, not lighter. Then just adjust lightness to a preferable level while editing.
Preliminary Overall Results
Targets tested: Kodachrome, Ektachrome, Fuji (Vevlia, Sensia, Provia, Astia), Agfa. I scanned all my IT8 targets at G1.8 with the Coolscan V ED scanner, then made a second "corrected" copy of each target by averaging 40% of certain GS patches and applying that to the whole patch. For Kodachrome I altered GS15-GS23. Alterations for the other films varied, depending on how much flare from the surrounds was present. All films except Kodachrome showed an increase in density from GS22 to GS23; only Kodachrome showed a slight reversal. Because of this reversal, for the corrected version of Kodachrome I replaced GS23 with the colour value of unexposed Kodachrome (Lab 56, 21, 12); for the uncorrected version, I replaced GS23 with a copy of GS22 (so that the "uncorrected" scan wasn't corrected very much).
Did you see a visible improvement with the corrected Kodachrome?
All S+M profiles when applied to the target showed colour changes in certain colour patches compared to Lab and XYZ (which appeared identical). So S+M was knocked at round 1. For both XYZ and Lab, the difference between "Uncorrected" and "Corrected" was minimal, in most cases undetectable, the only difference being a lightening of the darkest GS patches. Because the "corrected" versions should theoretically give better profiles, and because the differences between "uncorrected" and "corrected" were minimal, the "uncorrected" versions were knocked out in round 2.
That left the "corrected" versions of XYZ and Lab to play off in the final. The difference came down in XYZ's favour because of the way it retained the contrast in certain grainy patches (typically the patches GS17-19) i.e. the XYZ profile kept the grain intact whereas Lab smoothed out the grain. Originally I choose Lab because of this, but after further thought I realised the Lab had the lower contrast in the darkest regions (thus smoothing the grain), so I opted for XYZ as the best.
Matches my results, exactly.
5. Editing will be in the IT8 profile space with 2-4 Curve layers applied. There are several reasons for not converting to a wider gamut space. I have arrived at this tentative decision after a few hundred test edits, but the reasons are not yet final:
(a) Testing seems to indicate that editing in a wider-gamut space makes editing more difficult. I'm not convinced that this is a real phenomenon (i.e. a change in editing procedure might fix the problem), and until I work out why this might be the case, this finding is open to change.
(b) Editing in the IT8 profiled space by applying Curve layers is non destructive. Converting to another profile alters the colour numbers and the process can't be exactly reversed. By staying in the IT8 profile space and editing only by Adjustment Layers, the colour numbers are always only one step removed (the gamma 1.8 step) from what the scanner sees on the slide. This is a significant space saving when archiving, because I won't have to archive the original scan as it is non-destructively incorporated in the edited scan.
(c) All my scans are destined for Rec 709 output (effectively sRGB) on a digital projector. I don't require a wide gamut.
Earlier in the thread I showed that sRGB and Adobe RGB can't contain all of the colors of the Kodachrome Q60 target. But chances are your images don't contain colors as saturated as those on the Q60, so you might not have to worry about clipping colors in those smaller color spaces.
Although you don't necessarily have to convert to a wide-gamut
space, I think it's important to convert to some
space. Otherwise, by not converting at the beginning and editing in the profile space, all your edits to the image are, in effect, modifying the scan before
applying the profile, and the scan will no longer match the conditions under which the profile was made. The edits invalidate the profile. You are also giving up the advantages of a standard working space such as knowing that R=G=B is a neutral color.
That's the theory - but if it's working for you, why not?
1. One of my long-standing photographer mates wants to learn how to scan his Kodachrome slides, so he sent me some slides to play around with and comment on. I asked for "difficult" sides, and he complied. Check out my thoughts at: http://www.mediafire.com/?ymh90cvds5c3w2j
I will check them out!
Try using an Ektachrome profile with your Kodachrome scans and let me know what you think.