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Re: [OM] Film to flange distance in various OM bodies

Subject: Re: [OM] Film to flange distance in various OM bodies
From: clintonr@xxxxxxxxxxxxxxxx
Date: Tue, 18 Jan 2000 00:15:07 -0600
I hate to put a fly into the ointment, but just to confuse matters a
little, I have to point out that, at least on the early OM's, Olympus
had several different pressure plates, each with a slightly different
curvature.  Exactly why was never precisely explained, nor the degree or
orientation of that curvature.  In my files, I've got copies of papers
that illustrate this fact (written in Japanese), and parts pages that
show several different part numbers for the different pressure plates
available.

When you remove the pressure plate of an early OM, you'll see a factory
code, a date code, and often a single digit which identifies which
"shape" plate you had.  If it was necessary to replace it with a new
one, we were told to be sure to use the same type.

While most camera makers build in "adjustments" for the body focus (lens
mount-to-film rail distance), Olympus simply mills the film rails and
attachment points for the body castings very precisely.  Unless an OM
suffers impact, these will never change, so "adjustment" isn't
necessary.

This design concept is reflected in the meter circuitry of the OM-1/1n
and 2/2n, which eschews "variable resistors" to adjust the meter. 
Instead, only fixed resistors and precise quality control are used to
assure accuracy in the meter circuit.


Gary Reese wrote:
> 
> 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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