bellows length for specific focal lengths

Hi all,
I've been shooting LF for a few years, but it's been a while since I posted, and I think some of my terminology is off. Sorry for any mistakes.

I have a convertible lens that is, I think, 210/420 (am at work, lens is at home). I can't use without the front element and at the longer focal length because the bellows won't extend far enough.

Is there an easy site out there that will help me figure out how long the bellows does need to be? Is there a calculation?

Hopefully an easy question with a quick answer...

thanks.

You don't need a site. Most of the lenses used in LF have rear nodal points near the diaphragm. For them, diaphragm-to-film distance at infinity is close to focal length. Diaphragm-to-film distance at 1:1 is close to 2 focal lengths. Both imperfect approximations but good enough. Linear interpolation isn't quite right but will be close enough.

The rear half of a convertible lens has, IIRC, its rear node somewhat in front of the diaphragm so for infinity the diaphragm-to-film distance will be a little short. But its a starting point.

Okay cool. Thanks for the quick response. So that's a pretty long bellows...at least compared to what I have!

1 / F = 1 / Df + 1/Ds

where
F = focal length,
Df = distance from film to lens rear node,
Ds = distance from subject to lens front node

For a subject at infinity, the lens to film distance = the focal length F, so 1/F + 1/infinity = 1/F
For a 1:1 (full-size) image, the lens to film distance and lens to subject both = 2*F, so 1/2*F + 1/2*F = 1/F

If the nodal points within the lens are not known, you can use the front of the lens board with minimal error for most lenses.

- Leigh

Right and as Leigh said, for rough work just assume that the bellows extension at infinity is approximately the same as the focal length and at 1:1 it will be twice as long. The old convertible and triple convertible Symmars have tables available online. Even something like 400mm is going to require a monorail view and in my limited play I have found that practical "coverage" gets narrower. If your 210 covers 4x5 and a bit more your "400" will only cover 4x5 and a bit more as well.