Mark, thanks for sharing.
I've never heard about this particular application.
But in essence, this is what is known to general public as "bifocal
lenses" (in glasses).
Igor
On 6/29/2015 7:06 PM, Mark Roberts wrote:
Ever used a split diopter or, as they are sometimes called, half
diopter? It's essentially half a close-up diopter that you screw onto
the end of your lens; they cut the glass in half and leave the area in
the filter ring empty.
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