RE: [OM] Long Focus vs. Mirror Lens

[Timpe, Jim]

This is the case because these mirror lenses are solid catadioptric designs,
and there's no place 'inside' to put a regular diaphragm.  Most have rear
accessory threads for ND filters.

-----Original Message-----
From: Daniel J. Mitchell 


> Cons are that it almost always has a fixed apeture
> (usually at f8 or so). 

 Why is this the case? I've only ever seen mirror lenses at f8, but what is
it about the design that stops them putting aperture blades in there? 

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