Bellows! (Extension Factor)
A few weeks ago, I made a promise to some Twitter followers that I'd answer a very common question that takes place in the world of large format photography. When you're focusing a camera that has bellows, there's a good chance that if you're focusing at portrait distance or closer, you're going to need to compensate for that bellows draw!
The math is, overall, pretty simple.
1. Measure the focal length of your lens.
2. Measure how far the bellows are extended (for this to be relevant, this number should be greater).
3. Divide your extension by the focal length, and then take that number and square it.
4. This will give you the factor by which your bellows are drawn. Since light falls off exponentially, apply your extension factor to Log Base 2 of your number and that's how many stops of extra light you'll need in your exposure.
For an even simpler, more visualized version of this, check out this video: