> Hi Dean,
> I usually use this site as a reference. I am bothered by your measured
> 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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