I'm not an experienced astrophotographer by far, so take this with a grain of salt. The ISO rating of film is determined for 'normal' photographs; so unless you're shooting faster that about 1/30 the ratings don't reflect what's happening on the film. Reciprocity failure starts to creep in for longer exposures. Astrophotography requires film with lower reciprocity failure (with a few exceptions). I've seen websites which measure reciprocity failure for common films and typically the faster films had higher reciprocity failures. If I remember correctly the better films were rated around 200. Also since color films are three layers stacked, with one for each primary color each layer can have a different reciprocity failure resulting in color shifts. The recommended films were Kodak Royal Gold 200, another Kodak professional 400 print film and the 200 Ektachromes. The two print films have been reformulated by Kodak to lower the response to red, which is one of the primary components of a lot of astro photos leaving the Ektachrome 200 films. Both the professional (Ektachrome) and consumer (EliteChrome) have a reasonable reciprocity failure and good responses to reds. For b/w the Agfa 400 c41 process film also has good reciprocity characteristics. As always, YMMV Bill B. On Sunday, October 26, 2003, at 03:36 PM, Joe Bauman wrote:
Dear Friends, I have two questions for the group, which I hope someone can help me with.
1. On the subject of film for long exposures, what's your feelings about 400 vs. 800 films? I heard that you don't really reduce the exposure time with 800. If that is so I'd be better off using 400 as I assume it's not as grainy. True or false?
2. I'm especially interested in M31. Does the darker sky at the Wedge pose any real advantage for photography over results I would get at the gravel pit? I shot two Andromeda photos a while ago at the gravel pit. Overall they weren't as good as I would like because I did not expose one of them long enough and the wind kicked up and caused a sort of double-exposure effect on the other and it also could have used a longer exposure. Still, I was surprised how good the contrast is, with fine gradiations, bright center, dark outer reaches. These results make me wonder if there's anything to be gained by driving to a darker site for photos of something like a relatively bright galaxy, star cluster or nebula.
If anyone has opinions about these queries, I would very much like to get them.
Thanks, Joe
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