On 4/19/2010 6:54 PM, Chuck Norcutt wrote:
> Ah, yes. I forgot about that. But is it only zooms or any lens that works
> via internal focusing?
>
Undoubtedly someone has gone to the trouble of designing an internal
focusing prime with close to constant effective focal length, but it's
got to be tricky and expensive to do. OK, maybe nobody has built one; I
just want to cover my bases.
In any case, most of them do change focal length as focused closer. That
was one factor in my choice of the Tamron 90/2.8 macro, which uses a
conventional long helix to focus. The Canon 100/2.8 macro is IF. So as
one focuses closer, the working distance gets even shorter than would be
expected with conventional design.
With any lens at 1:1, the the focal plane to subject distance is 4x the
focal length. The Canon 100 mm, instead of 400 mm, has a close focus of
1:1 @ 310mm, for an effective FL of 77 mm.
Moose
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