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mihag
23-Apr-2014, 23:12
The lens plane would be 2x focal length from the subject (and the film plane 2x focal length from the lens plane) at 1:1. So, when at 2:1, is it 1x & 4x respectively?

jose angel
24-Apr-2014, 07:47
I have a mnemonic rule: if the film plane is equal to one focal length, the focus is at infinity. If it is two times the FL, the magnification is 1x. If three times the FL, the mag is 2x. If four times, the mag is 3x. So "magnification is focal length times minus one".

About the subject to lens plane: Another tip; at 1:1, the distance is the same as the bellows draw (two times the focal length). But if you go to higher magnifications, that distance is getting gradually shorter, somewhere in between two times (1:1) and one time the focal length.
(Maths are not my strongest point, but I think mathematically this distance cannot be one time the focal length, but it gets gradually closer... hope Leonard Evens is not reading this! ;))

mihag
24-Apr-2014, 07:58
Thanks Jose, I was pretty close then. The reason for asking is that I need to know if the rail of my camera is long enough to go to 2:1 with a 120 macro lens.

jose angel
24-Apr-2014, 08:03
Easy; for 2:1 (2x), you need three times the focal length (3x120=360mm).

Dan Fromm
24-Apr-2014, 17:08