There are two separate and interesting things in this post.
* * * (1) SLR camera lenses * * *
John Denker said, "Note that the optical center of the lens on an SLR is
waaay inside the lens structure."
In fact, the optical center is easy to spot because lens designers
locate the aperture blades at that point.
Take an SLR camera. Set the "depth of field preview" to on/preview so
that the aperture blades actually close/open as you rotate the aperture
ring, Look through the viewfinder as you open/close the aperture and
notice that closing the aperture does not give you "tunnel vision."
That is, the aperture blades are located in the correct position so that
they reduce the light without reducing the field of view.
Now try the same thing by making a variable aperture with your thumb and
index finger, and hold this aperture in front of the lens as you look
through the viewfinder. Your thumb&finger aperture will reduce the
field of view as you close it down because it's not at the optical
center of the lens.
* * * (2) A famous astronomy half lens * * *
When I first saw the subject line of this thread, I immediately thought
of the first successful measurement of stellar parallax by Friedrich
Bessel in 1838. Bessel used an impeccable heliometer made by Joseph von
Fraunhofer to measure angular separations between stars to sufficient
precision that parallax could be observed and measured. (Heliometers
were first invented to measure changes in the apparent diameter of the
sun.)
Imagine Fraunhofer going through all the labor of grinding, correcting,
assembling a nearly perfect 6.2" achromatic telescope lens, only to saw
it in half across its diameter. Having ground several telescope
mirrors, but never attempting a lens let alone an achromatic lens... I
cringe at the thought of sawing an outstanding 6" lens in half.
After the lens was sawed in half, it was reassembled on a telescope with
micrometer screws that could slide the two halves past each other. The
way this fits into the current dialogue is that each half lens produces
a complete field of view. That is, you do not see half the view from
one half the lens, and the other half of the view from the other half of
the lens. Rather, you see the full view from each half. The two views
superimpose if the two half lenses are in alignment. If the two halves
are out of alignment you essentially have "double vision" over the
entire field of view.
If you have the lens halves in alignment (so all stars are aligned with
themselves), then measure the micrometer adjustment needed to make one
star align with a different star, you have effectively measured the
angular separation of these two stars quite precisely. Now do that
again about 6 months later, and if the separation is different you have
measured a parallax shift.
I personally was unaware this is how Bessel measured parallax until I
read the book "Parallax: The Race to Measure the Cosmos" by Alan W.
Hirshfeld. If you have not read this book, go get it and read it. It
is outstanding. You may wish to add it to your library as I did, and
you can even get the hardcover for about $5 these days.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu