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View Full Version : Bellows extension question.

J. Lee
12-Jun-2006, 17:14
How do you determine the amount of extension you need for various lens sizes? When I change a lens I unlock and move the front on the rails at something at infinity and lock it in place once I've found my infinity focus. So, if you can't find that with a longer lens, then that means the extension isn't long enough, correct?

Thanks much.

Ron Marshall
12-Jun-2006, 17:19
That's correct. What focal length and camera are you working with?

J. Lee
12-Jun-2006, 17:31
I've got a Rodenstock 480mm that I occasionally use on my Century Universal 8x10. It works, but I pretty much have to extend the lens to the front with all the beds extended. Do you know the camera, and, if so, does that seem correct? I think at full extension that particular camera is 30-something inches. So, how do you do the math?

Thanks much.

Ralph Barker
12-Jun-2006, 17:42
A 480mm lens should require something close to 19" (480 / 25.4) of bellows extension. So, something sounds amiss if you are extending to 30" for infinity focus.

J. Lee
12-Jun-2006, 17:45
I've only used it on that camera a couple of times, so I'll have to do more exploring. Now, my question is, how did you come up with that lens needing 19" of extension; what's the math?

Thanks much.

Ron Marshall
12-Jun-2006, 17:47
Ralph showed you, there are 25.4 mm per inch.

J. Lee
12-Jun-2006, 17:47
I've only used that LENS a couple of times, not the camera, is what I meant to write.

J. Lee
12-Jun-2006, 17:48
thanks. i'm tired. bed time. Sorry for the confusion, and thanks for the help.

tim atherton
12-Jun-2006, 17:49

or

http://tinyurl.com/zwrf2

Ernest Purdum
12-Jun-2006, 17:49
Something is weird here. You are right in thinking that your Century Universal has something over 30 inches extension. This should be way more than is required to focus a 480mm lens at infinity. 480mm is only about 19'. The distance from lensboard to groundglass should be something fairly close to that figure. I suspect that your lens has had a cell swapped with some other lens, or that you have only one cell of a convertible lens.

J. Lee
12-Jun-2006, 17:52
So, the 19" is where infinity focus would be then? Again, sorry. Jet-lagged in Paris at the moment.

Thanks much.

Ron Marshall
12-Jun-2006, 17:55
Yes, infinity focus should be about 19".

J. Lee
12-Jun-2006, 17:57
Thanks much.