Jan Steinman wrote:
: In particular, the girth of the 90/2 makes it difficult to
: operate the mount release and the depth-of-field preview. Also, the 50/3.5
: takes cheaper/lighter/smaller 49mm filters, and fits into the slide
: duplicator, and has more depth-of-field.
and:
: wide angle lenses and short extension tubes -- the 21/2 and 7mm of
: extension make you feel you're actually "inside" the macro world, and
: there's much greater depth-of-field, as well.
I don't think this is correct. DOF depends on magnification ratio and
aperture.
With non-macro setups, the focal length and distance determine the
magnification ratio and therefore DOF. With macro setups (macro lenses with
built-in extension tubes like the 50 or 90 or bellows mounted macro lenses),
the extension and focal length determine the magnification ratio and thus
DOF. The larger the extension and the smaller the focal length, the larger
the magnification ratio, and the smaller the DOF.
Since the 50/2, 50/3.5 and 90/2 are give a magnification ratio of 1:2 at
their closest focusing distance, their DOF should be identical. And an 21mm
on an extension tube probably exceeds lifesize magnifications (?) and should
have a very narrow DOF.
Hans
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