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Extension tube formula


Graywolf 13 1.0k United Kingdom
30 Dec 2007 10:08PM
I have a formula to calculate the magnification when using an extension fing of a given size with a focal length.

Does anyone have a formula which would calculate the change in focus distance for a given extension size? Specifically 12mm and 25mm canon extension rings.
JohnHorne 16 1.0k
31 Dec 2007 10:01AM
As a rough-and-ready calculation, you can calculate the film-to-subject distance, D (in millimetres), using the formula D = F x (M+1)^2 / M
where F is the focal length of the lens in millimetres and M is the magnification.

So if you know the magnification when using an extension thing and the magnification without, you can subtract one from the other to calculate the change in focus distance. (The Canon website gives maginification ratios for each lens with and without extension tubes.)

Probably quicker to determine the change empirically though !
Graywolf 13 1.0k United Kingdom
31 Dec 2007 12:20PM
Top Banana.

Thanks a lot.

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