Dalantech wrote in post #18549361
Does anyone have a good explanation of why macro lenses with internal focusing lose focal length at minimum focus? I know it has something to do with how the lens elements are shifting, and possibly even the aperture assembly moving. Can't find any resource on the web and my "Goolge Fu" is usually pretty good.
I have never seen an explanation published. I think it is perhaps good to simply think of a macro lens as 'variable focal length', like a zoom. The difference is that with zoom the photographer can choose FL at will, and the optics move to accomodate that FL. With macro, the photographer has zero direct control of FL, but indirectly affects FL via the focus control...changed FL at closes focus distance.
I don't think I have seen that a zoom like the 100mm Canon has a 'intermediate FL' setting for the optics, when the magnification achieved is 1:2 vs. the 77mm FL at 1:1.
For anyone who owns the Canon 100mm macro lens, that test is not difficult to accomplish...simply get a 1:2 image on focal plane and measure subject-to-focal plane distance! A distance different from 2*(4FL), anything other than 2*(4*77) or 2*(4*100) to accomplish 1:2 scale, is indicative of an intermediate FL setting (not 77mm, not 100mm) for the lens.
For a manual focus optic of say 8 elements in 4 groups, when you focus closer -- including macro distances -- all 8 elements in 4 groups move AS A UNIT farther from the focal plane to focus. Always moving the optics AS A SINGLE UNIT.
Because of the long 'throw' needed to make that movement away from the focal plane for macro distances, that would tax the battery unnecessarily and also make focus a much slower response. So the solution for AF lens designers was to put in a means to alter the grouping/spacing of the optics to shorten the FL so that AF throw is shortened, so that they make (4*FL) a smaller number and less taxing to the batteries!