# Thread: Bellows draw conversion info

1. ## Bellows draw conversion info

Hi All, I'm looking for a way to mathematically estimate how much bellows draw I will need on an 11x14 camera for certain lenses at certain magnifications. Anyone have a kind of "cheat sheet" or an equation they can share with me?

Thanks!

3. ## Re: Bellows draw conversion info

Note that bellows calculations operate on the "Flange Focal Length" of the lens, not its optical focal length.

You must get that number from each lens's datasheet.

For lenses that are significantly longer or shorter FL than "normal", the FFL can be substantially different.

- Leigh

4. ## Re: Bellows draw conversion info

Originally Posted by Leigh
Note that bellows calculations operate on the "Flange Focal Length" of the lens, not its optical focal length.
Just to clarify, this is true for setting the base extension, but for additional extension and magnification and exposure adjustment, the optical focal length is used. As a simple example:

A 200mm optical focal length lens with a flange focal length of 100mm. It needs 100mm of bellows to focus on infinity, but it needs an additional 200mm of extension, not 100mm (300mm total) to focus at 1:1.

5. ## Re: Bellows draw conversion info

Originally Posted by xkaes
Just to clarify, this is true for setting the base extension, but for additional extension and magnification and exposure adjustment, the optical focal length is used.
That's true, however...

The desired answer is the distance of the lensboard from the film, i.e. the length of the bellows.

That is the FFL plus the extension due to close focus.

- Leigh

6. ## Re: Bellows draw conversion info

Thanks all!

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