Coma is an optical aberration that shows as the bending or bluring of
straight lines or the smudging of point sources of light. Stars are perfect
point sources of light, so a star test is "simply" taking a picture of
stars. I say "simply" cuz it ain't so simple <g>. However, once you get
the mount and guiding part right, and a good dark place with no clouds
during new moon and not windy you set up the camera, set the lens wide open
and open the shutter for 5-10 minutes or so, depending on film speed.
Progressively stop the lens down, adjusting exposure time (accounting for
reciprocity) then evaluate the results.
Typically, a lens that performs very well in terrestrial photography falls
on its face in a star test. The Zuiko 50 and 90 macros are notable
exceptions to this. The problem is aberrations that are always present in
any lens design do not show as prominently when the image is a continuous
tone image. But in the case of a black background speckled with point
sources of light, there is no way to mask even the smallest distortion.
Stars appear oval shaped, elongated or even gull-wing shaped. It leaps from
the image and is usually worst at the edges of the frame. Coma can be
controlled, even eliminated, but at the expense of something else - size,
weight, speed, cost. You can't have everything.
John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far".
Acer (aka whippersnapper, aka grasshopper <g>)Victoria
<siddim01@xxxxxxxxxxxxxxx>
> What's "coma"? And by star-test, I assume one shoots into the sun with a
> small aperture to get a pointy starburst effect on the sun?
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