Cliff, thanks for the four curves. I'll play around with Ektachrome and the Hutch method to see what comes of it. And maybe I should choose Lab clut
as my preferred profile, instead of XYZ (see Knockout Rounds
You shouldn't get lighter than dmin if you Convert-to-Profile from the scanner profile using Absolute. Are you using the same exposure that you used when scanning the profile target?
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.
Guy, are you still with me?
Yes, still here playing around, though I can see an end in sight. At some stage I have to move away from testing and start actual scanning. I've been generating profiles for Agfa, Velvia and Ektachrome and testing them based on the Kodachrome results. A well-exposed Velvia RVP50 slide is so easy to scan and edit compared to Kodachrome. In some cases (see Gil 06
, one of several Velvia slides I've borrowed from a photographer mate), the unedited profiled scan is superior to my attempt at editing it. I've never come across that when editing Kodachrome, which seems to always need 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).Knockout Rounds
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.Summary
The best profile from the Nikon Coolscan was obtained by:
1. Using a gamma of 1.8 while scanning at 4000 dpi;
2. Resampling in PS to 2000 to reduce file size for archiving;
3. Correcting certain GS patches to reduce the problem of flare, and in the case of Kodachrome alone, to remove the density reversal between GS22 and 23.
4. Resampling to 1000 for input into Coca for profile generation. I did not alter the Box Shrink parameter from the default 3.5 to my theoretical best of 7.5, because the patch correction of step 3 would have minimized the effect of flare.
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.Additional Tests
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