>You mention this is a DSLR so this trick might not
>work.  But, for a film SLR you can make an infinity
>focus rig from an empty GLASS slide mount, some
>frosted scotch-tape and a second camera with a known
>good infinity focusing lens ...

This is essentially how the Zeiss system does it. A suitable target is
projected down the lens under test, and reflected off a piece of film in the
film plane, back out the lens, onto a mirror and out the eyepiece. I think
the eyepiece uses a semi-silvered mirror to direct the image out, as the
projection is done straight down.

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