From: Carl Weese (cweese@earthlink.net)
Date: 12/18/02-11:43:02 AM Z
>
> I guess to really understand how a telephoto lens is different from a lens
> that just has a long focal point for your format, you would have to take one
> apart and look at it.
Nope, just focus it on a distant subject. If (assuming a view camera) the
lensboard-to-film distance is the same as the focal length of the lens, it's
just a lens with relatively long focal length. If the lens-to-film distance
is shorter than the focal length, the lens is a telephoto design. The
purpose of this is, for example, to use 14" focal length on a camera with
only 11" of bellows extension.
By the way, when you focus on infinity and the distance from film plane to
optical center of the lens is *longer* than the focal length, you've got a
retrofocus lens. The usual purpose of this of course is to clear the mirror
of an SLR when a standard short lens would get whacked by the mirror.
---Carl
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