Originally posted by Eruditass 100mm

Although I never quite understood how it works with lenses of the same focal length that have different minimum focus distances and max magnifications - can anyone explain?

I know for a give FL, when it is the FL mm away, will give 1:1. How does this work if the 50 mm lens already goes to 1:1 when compared to a 50mm with a max magnification of 1:4? The focus ring already moves the lens elements 50mm in the 1:1? How far does it move in the 1:4? So increasing it would bring it to what, 2:1?

magnification_change = extension/focal_length

If initial magnification is .25X (1:4) and focal length is 50 then an extension of 50 gives:

mag= .25+50/50=1.25X

If initial mag is 1X (1:1) and extension is 30 and focal length is 50 then:

mag= 1 +30/50=1.6X

If the initial mag is zero and focal length is 50 and the final mag is 0.25X then

0.25=extension/50 so extension = 50*.25= 12.5mm