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[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 15:12:33 -0700
Leandro DUTRA wrote:
> 2008/4/19, Daniel Sepke <daniel.sepke@xxxxxxxxx>:
>   
>>  OM flange to film plane is 46mm.
>>     
> Very interesting, it seems to be inversely proportional to the
> compactness of the OM bodies.  
Be precise, now! I assume you mean to say "Olympus bodies", not "OM 
bodies". All OM bodies are 46 mm.

The register distance of SLRs, from 4x5 inch to 4/3" is always a design 
trade-off between the need for the mirror to flip without hitting the 
lens and the desire for the shortest practical distance. The desire for 
a short distance, aside from reducing camera bulk, is because of its big 
impact on lens design.

A relatively simple, symmetrical lens design for a (D)SLR is only 
possible down to a focal length where the lens barely lets the mirror 
flip. On 35 mm film and FF sensor SLRs, this usually works out to be 
about 40 mm. Hence the 40/2 Zuiko 'pancake' lens and similar lenses from 
others.

Starting with 35mm, WA lenses for FF (D)SLRs are retrofocus designs 
(rear node behind the las element), which are larger, heavier, more 
complex and harder to make to the same IQ. That's why the 35/2, and even 
the 35/2.8, are longer and heavier than the 40/2.
> While the Four Thirds, Pen F, 
These cameras have smaller 'sensors', thus smaller mirrors and shorter 
register distances.
> Leica M and screw mounts flange focal distance is indeed small
The situation for non-SLR designs is different. The lens may physically 
reach very close to the film, as all it has to clear is a focal plane 
shutter. Choice of register distance has more to do with simplicity of 
design. I'm sure the Leica rangefinder distance was chosen simply to 
keep the front of the camera flat on a camera made as thin as possible 
around the film roll size.

Moose

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