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[Phys-l] half lens



If you cut a plastic lens in half (through the diameter), you can still
form real images. Compared to the original (full) lens, the images are the
same size (same focal length), but appear half as bright.

So I asked my students a follow-up lab question: If you put black tape
over half a camera lens (the old SLR type), how would the images be
affected? Obviously, they would be intact but half as bright, no matter
which half of the lens was covered. But a few students, who are also in a
photography class, said you would only see half the object/scene. They do
this as a project using a "photo divider," which attaches to the front of
the lens and can rotate. So one can do double exposures of half a scene at
a time. Wow!

I had never heard of this before. And it seems to contradict the half lens
demo I did in class. Can someone tell me how these camera attachments
work? Do the objects have to be "very close" to the camera?