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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: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Wed, 17 Jan 2001 21:14:48 +0000
At 23:07 1/17/01, Gary Reese wrote:
I'm not sure what unit of measurement the micrometer is calibrated for, but it is a distance measure none the less.

The information I found some time ago gives 46.0mm as the nominal flange to film distance for the OM system.

This distance is of interest to those adapting lenses from other systems and for astrophotography in adapting an OM to a telescope. For the purposes of this experiment, relative measurements are perfectly acceptable. I suspect the "ancient" micrometer measures in inches. To make an absolute measurement of the distance you would have to "zero" it to a dead flat surface . . . or at least record the reading on a dead flat surface and then use that as an offset for the delta.

I agree your results do not support any hypothesis of variation in this distance among the bodies you measured! Indeed, the distance appears to be high precision . . . which is very important for high sharpness at critical focus. I have read that tolerances for flange-to-film on a high-end camera body are usually in the range of a couple ten-thousandth's of an inch. This is supported by the shop manual I have for a ZI Contax IIIa. I was shocked when I first saw the tolerance for it. After some thought about it, I realized why. For 24x36mm film frame to exhibit extreme sharpness of the critical focus region when enlarged to about 10X for a large print, or about 35X for slide projection, this distance must be very precisely set.

All the more reason for us to care for our OM bodies, and protect the flange and back from damage.

Thanks, Gary, for taking the time to conduct your test!

-- John


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