A year or two ago, I too did not want to mention the 5 different pressure
plates for fear of all the "maybe I have the wrong pressure plate in my
camera" requests. I brought it up anyway, and nothing happened.
John Hermanson
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----- Original Message -----
From: <clintonr@xxxxxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Tuesday, January 18, 2000 1:15 AM
Subject: Re: [OM] Film to flange distance in various OM bodies
| 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
| >
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