* The IR filter of digital cameras almost completely blocks the light of hydrogen alpha at 656 nm, which gives the lovely red colour to photos of nebulae. Film, however, is quite sensitive to this, and a shot on film of, say, Orion shows the Orion Nebula glowing red. On the 10D, however, it is an insignificant grey blob -- much less extensive and attractive. No amount of fiddling with the red channel seems to be able to bring up the red. You can even now buy a doctored 10D with the IR filter removed just for astro work.
* Star colours are much less strong on digital. The array of four colour pixels gives lower colour resolution than film, as we discussed on Prodig a year or so ago. This is really obvious when comparing faint stars -- on film, even the faintest stars usually have colour, whereas on digital they all appear white. (A star's colour depends on its temperature, so stars cooler than the Sun appear yellow or orange, and hotter stars appear blue. Sunlike stars are white.)
* Major drawbacks of film for astro work are of course reciprocity failure and the low contrast at the toe of the characteristic curve. But these can be an advantage as well, which I didn't appreciate until attempting constellation photos with the 10D. Any light pollution around in even a comparatively dark site (in the UK) will rapidly build up into an unattractive brown background, whereas film, particularly slow film such as K64, is relatively insensitive to it and will give a darker background to the stars.
Of course there are numerous benefits of digital in astro work, but I wouldn't like to see the end of the yellow box just yet.
Robin Scagell Galaxy Picture Library
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