Re: bellows extension

----- 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
>
   The initial position of the bellows is when the lens is
focused for infinity. Since you know the focal length of the
lens, at least approximately, the reference position is one
focal length from the film to the lens. This works even for
telephoto or retrofocus lenses. Then measure the additional
distance when you are focused close. The correction is equal
to the ratio of the _total_ distance to the reference
distance in stops. So, when you are focused for 1:1 object
to iamge size the extension is equal to two times the focal
length and the correction is two stops.
   The reason for the correction is that the familiar
inverse square law of light applies to the light from the
lens to the film. Since the light from the lens is also
proportional to the square of the f/stop the relation
between bellows extension and f/stops becomes linear.
   Now, to confuse things just a little. The _effective_
stop remains the same and other calculations such as depth
of field and resolution loss due to refraction should be
calculated for the _effective_ stop, not the marked stop.
So, given the 1:1 condition, if the exposure meter indicates
a stop of f22 you must open the lens to f/11 but depth of
field, etc., are still calculated for f/22.
   Its difficult to choose a ratio where correction becomes
necessary but 8X is probably OK.
   If there is no really distant object to use one can focus
a lens exactly at infintity by autocollimation. Its a simple
process, at least for sheet film cameras. For a rough focus,
close enough for this purpose, put a flat mirror over the
front of the lens, a shaving or makeup mirror will do but
make sure its flat and not magnifying. Hold a pen light
against the ground glass near, but exactly at, the center.
Focus the reflected image of the penlight sharply. The
camera is now focused exactly at infinity.
   The true focal length of lenses usually varies a little
from the marked focal length but for this purpose the
difference is not significant. Measuring one focal length
from film to lens when the lens is at infinity focus will
give you the position of the rear principle point (by
definition) and this is your reference point for measuring
the extension since it is the point where the image
_appears_ to the film to be coming from.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com