At 17:24 16.12.01, John A. Lind wrote:
Some of the details on his site state they're tested at infinity. I doubt
he's using film, but that the tests are aerial. Two reasons for this:
(1) To test at infinity focus on film in a camera body, a lens extension
would have to be inserted between the lens and the body. While possible,
I'm not certain it's that practical.
It's not possible to test performance at infinity by using an extension
between lens and body. The focusing scale would say "infinity", but that
scale is only correct when the lens is used without extension. The testing
done would actually be for some other reproduction scale. Still somewhat
relevant for our old lenses, but nearly useless for lenses with internal
focusing.
There are two ways I can think of that would make testing at infinity possible:
a) Parallell incoming rays, checking point size in focal plane.
b) Point source in focal plane, checking the spreading of the light bundle
coming through the lens.
For an amateur the most practical would be a) by using the stars, point
sources at infinity for all practical purposes. It is very revealing, as
everyone who's done astrophotography will know.
Regards,
Thomas Bryhn
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