Hi Paul, which lens are you using?
Here is how I work it out using a calculator.
I have a quick and dirty method which I'll post beneath this post.
I'll use my 215mm lens as an example.
The equation I use is
(B/F) squared.
So,
B = Bellows Extension
F = Focal Length
If I measure from the film plane to the front of my lens and that's 320mm, the equation would look like this :
(320÷215)
= 1.4883
Squared (i.e. 1.4883x1.4883) = 2.215
That last number is the Bellows Factor - how much you need to multiply your metered time.
If I metered the scene as f/8 1second, I would multiply 1second by 2.215
My time compensated for Bellows Factor will be f/8 2.215 seconds.
I tend to round up ☺️ so f/8 2.5 seconds.
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