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E200 Ektachrome is a reversal film (it's used for slides because it "reverses" the normal negative image). It is a good film film for astrophotography because it does not suffer from reciprocity failure (where the effective film speed is reduced when making very long exposures). It can also be pushed, a process of overdeveloping the film to boost dim images as if the film was more sensitive than its normal ISO-200 rating. Often when pushing film, it becomes grainy and contrasty. Normally I ask to have the film "push processed by two f-stops". The film lab knows what this means and how to do it, even if I don't. I have been quite satisfied with the results (all of my Milky Way Panoramas were made this way). Recently, I neglected to tell them this detail, and the lab correctly developed it as if normally exposed. After I logged this into my list of "One Thousand Astrophoto Blunders," I decided to salvage some information from it. Here was a sample of E200 that was not pushed to compare with my previous pictures. To compare how much the grain is affected when pushing E200 by two f-stops, I made digital high resolution scans of two similar frames. Both are of twilight colors after sunset or before sunrise. Unfortunately the exact exposures were unrecorded, but were on the order of 1 second at f/5.6 and yielded approximately the same film density. They were scanned on a Scanview ScanMate 4000 drum scanner at 4000 pixels per inch. No post-scanner image processing was done except for a slight gamma adjustment to allow better visual comparison of the samples.
A side-by-side scan shows the relative exposure behavior of push-processing. |
