T3D Re: Two apetures in a single lens? (TECH-3D digest 321)

["Bob Howard" <bobh@xxxxxxxxxxxxxxxxxx>]

RE: John B. et al discussing closeup methods including two apertures 
in a single lens. This reminded me that a common trick for enlargers 
was to put a lens cap on that had been cut so that a narrow opaque 
strip was centered on the lens. The projected negative then had a 
rangefinder effect (eg two images that merged when in focus). This 
comes about because any portion of a lens can act like a whole lens.
e.g.Trying to block half a lens with your hand just dims the image 
but does not give half a field, unless you are some distance in front 
of lens.
 BobH


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