Re: bellows extension

Ohhhhh, Shannon, do I have the thing for YOU!
I am finishing up teaching a large format class to 24 unsuspecting photo
majors who probably would prefer that photography was 35mm and didn't
include math nor a view camera. SO, to make their lives easier, I made a
little chart of bellows factor.

Bellows factor can be calculated very easily by the formula:

bellows length squared
divided by
focal length squared

which gives you a factor number that you multiply your shutter speed by.

However, students who don't like math weren't thrilled with this formula, so
I took every lens we had in checkout, figured out the inches of each lens,
figured out infinity and then every factor for each inch of bellows. if you
want me to I can send you the chart offlist.

What was so fun is I figured out a down n' dirty method that when the
bellows is extended 1.4x the focal length, you open up 1 stop; 2x is 2
stops, 2.8x is 3 stops, 4x is 4 stops etc. etc! I thought I was soooooo
cool (because it corresponded to fstops), but then when they had their
midterm, NO ONE got those answers right on the test, even though I have this
written in bold at the top of the chart. So even though I I felt the chart
was so useful, I didn't write on it that they needed to read it.

My consolation is that their work, despite the math, is realllly getting
good.

btw you usually measure from film plane to lensboard...
chris

----- Original Message -----
From: "Shannon Stoney" <sstoney@pdq.net>
To: <alt-photo-process-l@usask.ca>
Sent: Friday, December 02, 2005 6:59 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 13:57:03 2005