Re: bellows extension

Hi Shannon,

The focal length for lenses is measured with the lens focused to
infinity. First, to find the place "where you stop" on the front
standard (where the lens is), focus on something distant (mountain,
moon, you get the picture, so to speak) and then measure from the film
plane a distant equivalent to your focal length. If your lens is a 200mm
for instance, you'd measure 200mm from the film plane to find the point
on the front standard from which all your bellows compensation reading
should be measured. You might want to mark this point with a Sharpie or
pencil.

Now about Bellow Compensaton (BC) in general. I always found it easier
to use inches instead of mm when figuring BC. This way you can treat the
measurements like f-stops and intuitively calculate your BC. First you
convert your focal length into inches. In the case of the 200mm lens,
that's 8 inches. Then, whenever you focus on something closer that makes
you rack out your bellows, just measure the extension in inches. Say
you're shooting a product shot of your computer mouse and you want it
life-size on 4x5 film. You focus and find that you have 16 inches of
bellows draw. The difference between 8 (your focal length) and 16 is TWO
STOPS. That is, the difference between f8 and f16 is two stops. It's
that simple.

I'm sure others will have even simpler methods to calculate BC but this
one always worked for me.

On the same thread, another good reason to always carry a camera is
because your aperture ring is also a pizza calculator. A pizza that's 16
inches in diameter is four times as much pizza as an 8 inch pizza. Cool!

Hope this helps!

Dan

Shannon Stoney wrote on 12/2/05, 7:59 AM:

> 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
>

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