In recently discussing how lens can potentially perform differently on
various camera bodies, I had a hypothesis that Olympus may have changed
their film to flange distance over the years. That distance can and
often does include a built-in focus offset to compensate for average
curvature of field across a manufacturers lens line. In another words,
most lenses, with the important exception of macros, throw an image
which is slightly curved. A focus offset is built into the camera to
actually focus on a plane half-way between where the corners are
focusing and the center is focusing. (Think of a slice through a curve).
Since Olympus progressively built their reputation around their macro
system, I thought that maybe the film to flange distance was tweaked
during model evolution to optimize performance for macros. Or simply to
account for what we all know were improved lens formulas over time.
I tested this hypothesis using a special micrometer that spans the
throat of the mount and advances and retracts a rod towards the pressure
plate. I measured the point at which the rod contacts the plate and
repeated the measurements for an assortment of cameras. I'm not sure
what unit of measurement the micrometer is calibrated for, but it is a
distance measure none the less.
Here is what I came up with:
OM-2000 = 316*
OM-4T = 316*
OM-4 = 316*
OM-2S (#1) = 317*
OM-2S (#2) = 316*
OM-2S (#3) = 317
OM-2S (#4) = 316 (my old, heavily used body)
OM-1n (#1) = 316
OM-1n (#2) = 316
OM-1 (#1) = 316
OM-1 (#2) = 317
OM-1 (#3) = 317
I'd say this rejects the hypothesis that there was tweaking over the
evolution of models, as well as eliminates film to flange distance
differences as a possible source of lens test result variation. Those
bodies marked * were used in lens testing. (Maybe one or two of the
OM-1/1n bodies, also). The practical difference between a 316 and a 317
is nil, since just turning the vernier dial on the micrometer and
judging when there was contact couldn't have been any more precise than
+/- 1 unit.
If Olympus built as much precision into the viewfinder system (incl.
mirror alignment), then any of the above bodies are mechanically capable
of equal focusing accuracy. I'll try and test a OM-2n and a double
digit/letter model and will post a follow-up if I come up with any
variation, which I don't expect.
I'm left with an appreciation of how Olympus tweaked lens performance
over time, as opposed to today's profit driven product evolution from
Minolta, Canon, Nikon and Pentax, which strives to cheapen production
costs by eliminating exotic/expensive optical designs and compromising
on build materials and the tolerances they can attain. At least in the
amateur lenses.
Thanks to John Lind (I believe) for getting me thinking about possible
variation among test bodies. And Bill Rogers of Bill Rogers Camera in
Las Vegas (brcamera on eBay) for his ancient, but perfectly suited
micrometer.
Gary Reese
Las Vegas, NV
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