>>Hi-- I just focused a light bulb with a magnifying glass on
>>a white sheet of paper and slipped a cardboard under it (in
>>contact) with a 1/4" or so hole cut in it. Moving the "aperture"
>>around under the magnifying glass made no difference I could detect
>>in the image position. What you found with the lens and filters,
>>I can't explain, but I am guessing it is what Vivitar uses in
>>their "3-D" zoom. David
>
>This is a beautiful experiment, thanks for sharing it. Here is what
>I see. Turning on a table lamp in an otherwise dark room, I position
>a simple plano-convex lens under it to focus an image on a piece
>of white paper. The bulb is in focus, other parts of the lamp are
>more-or-less out of focus but recognizable. Now hold a piece of
>cardboard with a one-quarter inch cutout about halfway between the
>lens and the image. You can see the 'circle of confusion' that
>represents the bulb on the cardboard. The image below is darkened and
>somewhat better focused.
>
>Now move the cardboard around, keeping the cutout within the bright
>cirle which is the unfocused bulb, and watch the focused image below.
>The bulb will seem to rotate a bit and other lamp parts that are
>out of focus will move around. A larger diameter lens gives a greater
>effect, however moving everything across the room from the lamp
>reduces the effect.
>
>The bulb seems to rotate because each point on the lens 'sees' it from
>a different perspective, which is how single-lens 3D works (Vivtar
>used to sell a lens called Q-Dos based on this principle). The out-
>of-focus parts move around because the light rays creating them
>have not yet converged or are already diverging, and the cutout
>aperture selects only a small portion of them.
>
>I guess the equivalent experiment for me would be to use a pair of
>cutouts and place red and blue filters over them. Then, by alternating
>red/blue filters over one eye (the other closed) I should see the image
>jump and rotate.
>
>Anyway, regarding the original question, at large lens-to-object distances
>there will be little effect caused by off-centered apertures, but
>at closer ones there should be some effect, which may or may not
>be noticeable.
>
>Nice talking with you.
>
>Paul Kline
>pk6811s@acad.drake.edu

Hi-- Good job! If the aperture is at the lens optical center, moving
it will not move the image, but, as you point out, moving the aperture
away from the lens center (either in front or behind), changes things.
I have taken many photographs based on this idea. Try aiming a short
to medium tele at sun reflections on wind rippled water, then, using
a very wide aperture, defocus the sun spots until they are a bunch of
disks. Then, put some complicated plant subjects in the way, with at
least part of it out of focus. Fairly sharp repeated images of the
plant will appear in the sun spots.
David Ruether