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Extension tube formula
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.
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.
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 !
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 !
