bellows extension

Hi, I am using a new camera that I borrowed, a Canham. One thing I
like about this camera is the little marks on the side that show the
extension of the bellows, in millimeters. (My tachihara has this
too, but for some reason they are numbered in the wrong direction!)
I am confused though about two things: how to measure bellows
extension, and when to adjust exposure for it.

1) I assume that you start measuring at the film plane, but then
where do you stop? One book said at the center of the lens, where
its narrow part is, I guess approximately at the front standard.

2) The same book said that you use bellows extension when you are
focusing on something that is away from the lens by a distance of
less than eight times the focal length of the lens. So, for a 90mm
lens, that would be 72 cm. With anything closer than that, you need
bellows extension factor, and anything further away than that, you
don't. Is that correct?

--shannon
Received on Fri Dec 2 07:57:01 2005