I'd like to make sure I have a reasonable understanding of what I have learnt from responses to my previous post (http://www.luminous-landscape.com/forum/index.php?topic=50262
), by condensing into a few statements all the information from respondents. Correct me if I'm wrong:
1. The best single reference for understanding colour is Hunt's Reproduction of Color
, 6th Edition. I hope it's the best, because I've just ordered a copy.
2. Kodachrome has an inherent blue shift caused by the red-sensing layer (and to a lesser extent, the green-sensing layer) having higher densities at any exposure than the blue-sensing layer (p3, http://www.kodak.com/global/en/professional/support/techPubs/e55/e55.pdf
3. To ensure neutrals remain neutral when projected, Kodachrome was purposely designed to have a blue shift so as to counteract the yellowish light of projection lamps (Hunt: 44, 229-30).
4. For reasons not definitely known, most other slide films do not have a built-in blue shift.
5. A slide scanner can be calibrated with an IT8 target (to counter any scanning inaccuracies) using software such as VueScan or SilverKeeper which generate a colour profile that can be applied to a raw scan to correct for inaccuracies in the scanner. Free software to do the same thing include Scarce (http://www.scarse.org/
), Argyll CMS (http://www.argyllcms.com/
), and Lprof (http://lprof.sourceforge.net/help/lprof-help.html
6. Each IT8 target also needs a IT8/CGATS file that contains the colormetric measurements for that target. This text file is required as part of the calibration process. The CGATS file for Kodachrome can be downloaded from FTP.Kodak.com/GASTDS/Q60DATA
and is contained in a folder called K3-Data
. Info about the colour coding of the IT8 targets is in the document TECHINFO.pdf.
7. Calibrating a scanner using an IT8 target cannot remove the inherent blue cast in Kodachrome slides, it can only remove inaccuracies in the scanning process.
8. The blue cast can be reduced by applying Levels correction in Photoshop using the gamma adjustment (middle slider). Approximate settings are Red 1.19 and Green 1.04 (see reply 21, http://www.luminous-landscape.com/forum/index.php?topic=50262.msg415697#msg415697
9. Not only Kodachrome, but all slides intended for projection in a darkened room are designed to have contrast substantially higher than that necessary for optimum reproduction as prints. When slides are projected in darkness, the image appears lighter, and this effect is stronger for darker areas of the image. The result is a loss in perceived contrast, which can be counteracted if the contrast of the slide in increased. To quote from Fairchild (p 10, http://www.cis.rit.edu/fairchild/PDFs/PAP02.pdf
):… when scenes are reproduced on transparency film, to be projected in a darkened room, the physically-measured contrast, expressed in logarithmic coordinates, must be about 1.5 times higher than the original scene in order to create an optimum reproduction. However, printed images viewed in illuminated surroundings are optimal when their physically-measured contrast is equal to that in the original scene. This result is described in detail by Hunt.
10. The increased contrast of a slide (as compared to a print) can be counteracted by the appropriate selection of gamma in Photoshop Levels.
11. CIECAM02 (the most recent CIE colour-appearance model) attempts to simulate the appearance of an image in a different environment (such as a darkened room) by including (among other parameters) luminance information about the background. For example, a scanned slide can be given a similar appearance on screen as it would when being projected in a darkened room. A Photoshop plugin, and sample images, can be downloaded from http://sites.google.com/site/clifframes/ciecam02plugin
I can find no reason among the replies to my original post why Kodachrome should be more difficult to scan than other films, given a good-quality dedicated slide scanner such as a Coolscan 5000. I would have thought the alleged difficulties could be overcome by ensuring that:
• The scanner is calibrated using an IT8 target and the resulting profile applied to the scanned image. This should remove any colour inaccuracies;
• The inherent blue-cast is reduced in Photoshop Levels by a suitable input of gamma to the red and green channels; and
• the high contrast is corrected by altering the overall gamma, again using Levels.
Kodachrome may indeed be more difficult to scan, but why?Additional Comments Based on Replies
23 Jan 2011
12. The gamma adjustments mentioned in point 8 will vary depending on the color space of the image. The stated numbers (red 1.19 and green 1.04) should work while the scan in still in a linear scanner profile space. If the scan is in another space, such as sRGB, most likely the gamma adjustments will be different.
13. When using scanning software provided by the manufacturer of the scanner (e.g. NikonScan for Coolscan devices), and the scanner has not be calibrated with an IT8 target, selecting the Kodachrome setting will likely give better results than a Kodachrome setting in third-party software. This is because the manufacturer of the scanner has probably made an effort to include the RGB sensor characteristics in the Kodachrome setting, whereas third-party software is unlikely to have access to these characteristics, and will assume a generic characteristic.
: If Silverfast provided Kodachrome LUTs in its software to substitute a Kodachrome target profile then I hope it is adapted per individual scanner model (lamp spectrum, sensor RGB dyes). If Hamrick had to do the same for Vuescan it must have been a hell of a job for him.
: … here is what Silverfast says on their own web site about their Kodachrome profiles: “… we have implemented generic Kodachrome ICC-profiles for many supported film scanners …”. (emphasis added)