[OM] Re: OM lens mount to film plane distance

Subject: [OM] Re: OM lens mount to film plane distance
From: Moose <olymoose@xxxxxxxxx>
Date: Sat, 19 Apr 2008 14:48:43 -0700
USHER99@xxxxxxx wrote:
> Hi Dean,
> I usually use this site as a reference.  I am bothered by your  measured 
> discrepancy.
> I trust the 46 is correct as it is what is typically quoted.
Oh lordy, here we go, the International "measure your register distance" 
project. Before we start, may I offer some perspective?

1. Film is not flat. Take a look at a strip of negs or a slide. Koduck 
even made projection lenses with intentionally non-flat fields to give 
better center to corner sharpness than a flat field lens.

Think the pressure plate makes it flat? Put a junk roll in your camera, 
open the shutter in B and look at the reflection from a penlight. 
Flatter, yes, but not flat. Try the same thing with a DLSR. Waaaaay 
flatter than film, but still, if you measure precisely enough, not quite 

2. Film has a non-trivial thickness.Not all of the substrates that carry 
the emulsions are the same thickness. Not all emulsions, even B&W, are 
the same thickness. I seem to recall that some of the finest resolution 
films have very thin emulsions, thus little silver and slow speed. 
Faster films get that way mostly by having more light sensitive silver 
compound, at least in part through greater thickness of the light 
sensitive layer(s). Color emulsions have three or four separate light 
sensitive layers. So the target sensing device is not 2D, but 3D.

3. Precision in manufacturing tolerances costs money. Manufacturing to 
main precision costs money and/or weight. So designers don't specify 
more precision for any one part of the system than required is for 
overall system accuracy.

4. A ruler and eyeball don't comprise a very precise measuring tool.

So for fun, I grabbed an OM-1 and a digital micrometer. The distance 
from the front of the mount to the inner film rail varied slightly from 
point to point, with a maximum of 46.08 mm and minimum of 45.89 mm. To 
further emphasize the fact that such numbers aren't exact,  The 
micrometer spec is resolution and repeatability of .01 mm, but absolute 
accuracy of .02 mm. The micrometer is quite new, so should be in spec.

Adjusting the measurements for that, the range of register distances on 
this one OM-1 may be as wide as 45.86 to 46.11 mm.

My suggestion? Just use the nominal spec. of 46 mm and don't worry about it.


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