Hello OM'ers,
Several readers kindly scratched their heads and then hit their
keyboards in response to my questions about soft images with the 20mm f2
auto macro OM lens on a bellows. Paul Schings, C. H. Ling, Dreammoose,
Brian Swale, Warren Xato, John A. Lind, Chris Barrett, and Frieder Faig
all chipped in with advice. My sincere thanks to all of you for sharing
your expertise and taking the time to try to help me out.
Both Vince Marino at Olympus America and a list member warned me to
check my focus. This was one problem, it turned out. The 1-12 focusing
screen, which has a cross of double cross-hairs and a clear, not matte,
field, needed some fine-tuning with the diopter control.
Frieder Faig and Chris Barrett both nailed it with the question of
aperture--I was stopping down (to f8) too much. The reasons why this
gives lower resolution will take some study. Whatever, I took a series
of shots of the same region on a butterfly wing, using a series of
f-stops: f2, 2.8, 4, 5.6, and 8.
The results at f2 were really pretty good. At f2.8, depth of field
increased slightly, but my subjective evaluation of resolution was that
it did not change. At f4, however, wow! Image quality took a real
nose-dive. This deterioration continued to increase at f5.6 and f8. At
f8, I was back to the results of my first attempts: really soft images,
as though my plane of focus was above or below the subject. But it
wasn't. The sculpturing on the individual scales was easy to compare in
the different prints, and the winner (with no hanging chards or possible
non-payer) was clearly f2.8. This is what Frieder and Chris predicted.
Well, I have a couple acceptable shots out of an experimental
12-exposure roll. I'll try some more soon now that I'm on the right
track.
Many thanks to all who contributed. Your help is really appreciated.
This is certainly a great list.
Dean
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