The extention is measured from the film plane to the nodal point of the
lens, usually the middle of the assembly, yes to No. 1.
Bellows extention factor is a calculation for light 'drop off' that is
calculated at four times per stop, so for longer extention that the usual
focal length (for close ups) calculate one click on the iris ring for every
two times longer the extention.
S.
----- Original Message -----
From: "Shannon Stoney" <sstoney@pdq.net>
To: <alt-photo-process-l@usask.ca>
Sent: Friday, December 02, 2005 5:59 AM
Subject: 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 13:06:47 2005
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