Iris placement

[P3D john bercovitz <bercov@xxxxxxxxxxx>]

There was some discussion of placing the iris before, after, or 
in the lens.  If you don't want vignetting when you stop down, 
you put the iris where the chief ray crosses the optic axis.  Of 
course if you have a very narrow field of view and a large-aperture 
piece of glass (telescope), it hardly matters where the aperture is 
because any ray passing through it will hit some glass and get sent 
to the right place if the lens is free of zonal aberrations.  (And 
yeah, the glass I'm talking about could be aluminized. 8-)  Placement 
of the iris has been used in the past to partially cure aberrations 
of certain poorer lens designs.  Same arguments apply mutatis mutandis 
to an iris-type shutter.  A mechanical shutter needs to be right next 
to the iris or it needs to be a flying slit shutter (e.g. focal plane 
shutter) if you don't want a trace of vignetting.  Sometimes it is
nearly impossble to put both the shutter and the iris in the same 
location because of lack of space between lens elements.

John B


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