Re: (meteorobs) Shooting the skies

John wrote->
>In theory, the camera is most sensitive when the aperture is wide open,
>but often in practise the circle of confusion at that setting is
>unusually large.  Typically closing 1/2 to 1 stop tightens the circle of
>confusion to more than offset the reduction in illumination.  There
>after, the circle of confusion will be continued to be reduced until
>about f/5.6 or f/8, but the reduction in illumination is too great to be
>offset and sensitivity suffers.  After about f/8, diffraction effects
>about the aperture start to increase the circle of confusion and
>sensitivity decreases more disproportionately.

Actually the effects of diffraction are quite dependant on the focal length
used and are not always evident in real tests. I can't place my hands on an
article on the subject which I used to have kicking around. As I recall the
f/stop at which diffraction usually become important is roughly one-fourth
the focal length. (I better be brushing up on some of this because I am
teaching our advanced optics course this coming fall semester.) Thus for a
24 mm lens f/8 is the probable best aperature. But for a 50 mm lens it is a
little past f/ll and for the popular 105 mm lens diffraction is not a
problem even at f/22.

I used to perform resoultion tests on 35mm lenses before I purchased them.
I remember one particular 24 mm lens whose performance at f/22 was better
than at f/11 and as good as it was at f/8dot it had a maximum aperature of f/2
and was reasonably sharp there. I have some nice comet and meteor pictures
I made over the years with that one. I sure hated to see that lens die, but
its mechanical parts falied after 20 years of heavy use.

John's rule of thumb is pretty reliable for lenses you can't test in focal
lenghts of 50 mm or less, but actual practice may not follow the rules, and
it is not true for longer focal lenths where smaller f/stops are permitted
before diffraction becomes a factor.

my two cents,


Terry Richardson
Department of Physics and Astronomy
College of Charleston
Charleston, SC 29424
pager #937-1048
note new area code for the SC Lowcountry!
843 953-8071 phone
843 953-4824 fax
http://www.cofcdot edu/~richardt/

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