Re: zeiss spotting scope on digital camera

Dave wrote:
Jerry Avins wrote:
ttdaomd@xxxxxxxxxxx wrote:
Dave wrote:

http://www.birdforum.net/forumdisplay.php?f=284
Oh no. Now I find that I can get other cameras at half the price I
originally paid for my Canon G1 which does not have vignetting and with
more than twice the G1's megapixel count! And I was so happy with my
round image before these posts! Oh, the horror!!! Gotta stop reading,
gotta stop reading... :)))
Keep reading. Vignetting is eliminated when you get the camera's
entrance pupil -- the image of the diaphragm as seen through the lens --
coincide with the telescope's exit pupil -- the circle of light that
seems to hang in space behind the eyepiece. Longitudinal adjustment may
be all you need. Can you select eyepieces?

Where do you find that spec on a camera?

I don't recall ever seeing it as a spec, but it's fairly easily measured by looking. Lay the camera in its back, and arrange two pieces of cardboard above it, one above the other, each obscuring the same half of the lens. Measure the separation of each piece of cardboard from the front plane of the lens so you can reproduce the geometry. Look at the aperture from a few angles to the side, and mark the cardboard where your line of sight intersects the edges.

On a horizontal piece of paper, draw a line representing the front plane of the lens, and two lines parallel to it the distances of the cardboard pieces. Transfer the marks on the cardboard to the paper keeping everything centered, and connect the sight lines. The entrance pupil is where they intersect. It takes longer to explain than to do. I wish pictures were possible here.

The telescopes exit pupil is even easier to find. Point the telescope at a window or light-colored wall, and observe the disk of light that seems to be hanging behind it. Hold a rule (or even a piece of paper with a few marks near the edge for reference) so it bisects the disk and move your head from side to side to observe the parallax. Move the rule toward or away until there is no parallax. The rule is then in the plane of the exit pupil. Most telescopes have retractable eyeguards. You will almost surely want the eyeguard retracted and might need to remove it altogether if that's possible. There's a picture of my camera coupler at http://users.rcn.com/jyavins/photoadapter.htm.

Email me for more details if you need them. I can certainly send you pictures and I might be able to produce some drawings to explain the text above.

Jerry
--
Engineering is the art of making what you want from things you can get.
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Relevant Pages

  • Re: Response to P&S reasons list
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