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Old 26th July 2021, 01:38 PM
Molli Molli is offline
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Join Date: Oct 2014
Location: Victoria, Australia
Posts: 149
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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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