Ernst: thanks for the various links. I didn't realise the Coolscan 5000 could multiexpose and thus increase the dynamic range. Nice facility to play around with for single scans, but would cause a significant increase in time and probably be not that useful for me given the large number of slides I want to scan. I'm also a bis sus about multiscans because of the chance of additional blurring caused by the extra passes.
Crames: I had wondered myself, but was very doubtful, whether Kodachrome had been pre-adjusted for a blue-cast to compensate for the yellowish light of projection lamps. Do you know if this idea has been documented in Kodak literature or elsewhere?
The idea of high contrast intrigued me, but I think your explanation is not quite right. Dim lighting allows the shadow detail of a high-contrast projected image to be seen on screen. Solid blacks (implying high contrast) are a very important aspect of digital projectors, and one of the main concerns of picky home-theatre enthusiasts (see "What is all this fuss about black levels" under http://www.projectorreviews.com/epson/home-cinema-8700ub/image.php#black
). But dense blacks are only possible in dim lighting because the white screen will otherwise wash them away. The way I understand it, high contrast in a slide does not compensate "for the low viewing contrast that arises from the dark environment"; rather, the dark environment allows a high-contrast slide to be viewed in all its shadowy glory.Colour Casts
This colour cast "thing" has got me baffled. I'm trying to understand where it comes from, but all I get is conflicting information.
Kodachrome has a Red Cast
Kodak's Density Curve (page 3 of http://www.kodak.com/global/en/professional/support/techPubs/e55/e55.pdf
, similar to the one above in Crame's post) shows the red curve on top. In an extensive search through Google Books, several authors made the comment that Kodachrome has a red cast. One author, in reference to the Density Curve already mentioned, states:The characteristic curve shows both a shoulder and a toe. In Kodachrome 64, the red sensitive layer is consistently more sensitive than the green and the blue sensitive layers which accounts for the Kodachrome typical red cast.
I don't yet fully understand these curves, but it seems to me that such authors may have misinterpreted the graph. At a particular exposure, red is on top, but doesn't that mean red is denser, i.e. darker and therefore less visible, throwing a blue-green cast over the image? I'm hoping someone can explain what a Density graph is conveying.
However, maybe the red cast is due to dark fading
. In the Focal Encyclopedia of Photography, p 383, it says:Dyes in colour films and prints can fade under the effect of light as well as in the darkÖ the terms light fading and dark fading are used to distinguish between them. The dyes present in colour photogrpahs made by the process of chromogenic development are the only dyes know to fade in the dark. There are no other colour documents that behave similarlyÖ
Three variations of chromogenic development processes were developed by manufacturers in the period from the early 1930s to the end of the 1940s. The first commercially available colour slide film was Eastman Kodak's Kodachrome filmsÖ
It was observed early that cyan is the least stable dye in the darkÖ and assumes a reddish overall cast. Under the influence of light, however, cyan dyes are generally the most stable, while magenta is weakest. Photographs faded by exposure to light therefore show a blue-green overall tone, which is caused by the predominance of the surviving cyan plus yellow dyes.
So a red cast could be explained by dark fading
(most slides are kept in the dark after initial viewings), but if exposed to light, they can turn blueish. This leads toÖ
Kodachrome has a Blue Cast
In lots of photographic forums, according to the people who have made comment, Kodachrome supposedly has a blue cast. Maybe it has: in the Kodak PDF already linked to, to the right of the Density graph on page 3 is a graph called Spectral-Dye-Density Curves
, shown below. I don't understand the units on the vertical axis: Diffuse spectral density
. What it seems to show is that cyan has the highest value. Is this the cause of Kodachrome's blue cast?
I am also confident that the Nikon Coolscans themselves throw a blue cast (of various intensity) on all slides scanned without adjustments. I have come to that conclusion after scanning a wide variety of stock, of varying ages and colour casts, using NikonScan, VueScan and Silverfast. Maybe my scanner is at fault. But if NikonScans throw a blue cast, why would that be?
Kodachrome has a Yellow Cast
Very occasionally this is mentioned by people in various forums. A quote from B&H's website about the IT8 calibration target, explains that CCDs see less yellow than our eyes, and this explains the blue cast seen in scans:The human eye will see more of the yellow spectrum contained in an Kodachrome image than the scanner's CCD. Consequently scanning a Kodachrome transparency with an ICC-Profile based on a Fuji or Ektachrome IT8-calibration, will produce bluish scans. Only with a dedicated Kodachrome calibration target can correct and original "Kodachrome colors" be achieved.
Of course that's a sales pitch by SilverFast trying to sell a Kodachrome target. Kodachrome is explicity mentioned, but if CCDs really see less yellow than our eyes, doesn't that say that all scanned slides will have a blue cast? Is it documented anywhere that CCDs have a problem with yellow?
Looking forward to any replies, particularly those with links to respected sources. Finally, can anyone explain what this graph is trying to tell me?