Maybe I can add some info on this thread.
Aperture: most lens perform better stopped down, and in your case with a 50
mm you can obtain better results also with star trails maybe a F2.8, but in
any case I wouldn't stop at more than F4, because in that case the trails
will be fainter and you will lose a lot of stars.
Second: as for the film, you will benefit from using a film with a low
reciprocity defect. This defect will lower the ISO of the film for long
exposures. For a choice of film see
http://www.robertreeves.com/filmtest.htm
Third: humidity. With a long focal lens (135 mm and more) that have a
cylindrical hood I used with good results a piece of thick paper placed
outside the hood, longer than the hood. This will catch the humidity, and
even not a perfect method, you will use the lens for a longer time before
the humidity will condense on the lens. With a 50 mm lens this method is
more complicated, because it is difficult to place the paper. With a wide
angle lens this method isn't good, so you can try the already suggested
method (heater) or a mini fan placed laterally to the lens.
Ralf Loi
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