T3D Re: Nimslo aperture design

[john bercovitz <bercov@xxxxxxxxxxx>]

The usual reason that a lens will vignette when it is stopped down 
is that the iris is misplaced along the optic axis.  As an extreme 
example, suppose the iris were put right at the film plane.  As 
soon as you stopped down even a little, you would cut the corners 
off the image.  If you put the iris half way between the film plane 
and the lens, the same thing would happen but it would be softer.  
So you must put the iris where the chief ray crosses the axis.  
That way, the last rays to be extinguished will be rays which cover 
the whole image plane.

If the lens vignettes when you open it up, then it probably has a 
filter holder or lens shade vignetting it.  You can see what's doing 
what to whom if you sight along the corner of the film gate through 
the lens and then adjust the iris.

John B

PS: It's tough to put the iris in the right place in some lenses 
because there is not enough air gap between the lenses in the 
middle or because the center is actually glass.


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