In your message dated: Wed, 29 Apr 1998 19:00:02 EDT you write:
>Hi all!
>
>I was shooting stars recently on a mountain top, with a temperature
>of around -20c. And then I wondered...
>
>Does low temperature have any important effect on film sensitivity?
>How can I compensate accurately for that?
Yes, it makes it more sensitive, but the amount varies with emulsion.
As I recall, however, the increase in sensitivity is just a negation of
reciprocity failure, so it's only important for long exposures.
>
>I was also wondering... (this might be a stupid question)
>Since stars are quite close to true point light source,
>could we use them to evaluate the resolution of a lense?
Of course. There is a whole book called "Star Testing Astronomical
Telescopes". The lessons in that book could just as easily be applied
to camera lenses, since all a camera lens is is a fancy telescope. The
only thing to be careful of is atmospheric seeing (the "twinkling" of
stars). With a point source in the lab, you don't have to worry about
seeing, but you also don't get thousands of point sources for free :)
Tests I suggest:
Take a short (~2 sec) exposure with 400-800 ASA film
(interesting side note: there is no such thing as "ISO 100 film". The
ISO specification is just a combination of the ASA and DIN values, so
the proper ISO designation is "ISO 100/21") with your favorite lens of a
star field that includes at least some 1-2nd mag stars scattered across
the frame. If this lens has a longer focal length than about 400mm,
you'll want to put it on a tracking mount. For longer lenses,
you'll also want to point to the Milky Way. Do this on slide film, and
then examine your results with a *good* loupe or project them with a
*very good* projector lens. Now look at the star images. Images at the
edges should be free of coma (makes the stars look like little comets
with the tails all pointing radially out from the center of the frame)
and color fringes. For mirror lenses, take pictures of a very bright
star, centered in the frame, both slightly in and out of focus, and
look at the centering of the central obstruction (secondary mirror).
You usually can't adjust it, but at least you can see how good the
collimation is.
Cheers,
--Lee
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