At 03:50 PM 6/7/2005, Moose wrote:
>The 50/3.5 has its best performance at f8-11 and declines below that. F22
>is available, but shouldn't be used for critical work. If you look at all
>the 50mm tests, you will notice a distinct tendency for f16 ratings to be
>lower than wider apertures. The 135/4.5 macro offers apertures down to
>f45, but the performance slide starts at f22 and by f45 is really iffy,
>worse than the 50/3.5 at f22.
Both those lenses stop down that far because they're macro's. I have just
done a series of them pushing to 1:1 which is life-size on film and more
magnification than I had used in the past. Depth of field can be extremely
shallow when simply making a "close-up" between 1:8 and 1:4
magnification. Crank it up to 1:1 magnification and it's extremely shallow
making the photography of anything but a relatively flat, and keeping the
desired subject material within the depth of field nearly impossible.
The recent photograph of the dwarf Iris took several hours to
accomplish. About 2 hours was spent getting lighting direction and its
quality of diffusion on the Iris how I wanted them. About an hour was
spent making various photographs, where critical focus was placed on it,
and making minor tweaks with the axis of lens related to the axis of the
Iris blossom so that areas of it were almost exactly the same distance from
the film plane . . . and that was stopped down to f/11. I could have
gotten to f/16 if I'd had sufficient light power . . . but at 1:1 using
lens extensions, there's also a 2-stop light loss and could only get to
f/22 with the flash meter at full power with the light as close to it as I
could without causing a number of other problems. The depth of field in
the Iris photograph appears to be about 1/8th inch or less (for about an
11x14 print). How I would have loved to stop down to f/16 (max for the
85/2) but couldn't.
The 50/3.5 stops down to f/22 and the 135/4.5 stops down to f/45 to allow
the greater depth of field when it's desired at high magnification . . .
the trade-off being some softening from aperture diffraction to gain
additional depth of field. With high magnification macros, something is
either in or out of the depth of field; there's nearly zero "boundary" zone
before it's unmistakably out of focus.
-- John Lind
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