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Old Mar 6, 2004, 9:02 PM   #1
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Default Different eyepiec sizes, same vignetting?

I have an Oly C750 non-SLR style digicam. I am resigned to
the fact that these cameras are limited for this, because of
vignetting.

BUT.

I have experimented with a pair of 9X25 Nikon binocs in front of the
digicam's lens. The binocs have an eyepiece diameter of about 13mm. At
full zoom on the Oly I get vignetting to the point where about 1/2 -
2/3 of the top and bottom of the shot are OK. The sides are worse,
naturally.

My problem is that with standalone eyepieces, with lens diameters of
maybe 25mm, or even larger, I seem to get the same vignetting. I set
the eyepiece up right against the camera lens, then use a 50-250 zoom
camera lens as the "scope". The vignetting seems to be from the
eyepiece, not the SLR lens.

Can anyone explain why the larger diameter eyepieces are causing just
about the same vignetting?

The lens I am using is an f4(5.6 at zoom) - f22, 50-250mm, with a Pentax K mount (45mm internal diam or so), and with the Object lens being 52mm diam. Looking through the lens, using the eyepieces that I tried, produced no vignetting at all, even with the iris stopped right down. The vignetting seems to be caused entirely by the eyepiece anbd the camera in all cases.

Any help appreciated.
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Old Mar 7, 2004, 10:17 PM   #2
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I will take a shot at explaining this (within the limits of my understanding). Think of the eyepiece like a projector lens. Now some projectors will fill a 10 foot diagonal screen in a small room. Others are designed for a 12 diagonal foot screen in a relatively large room. The eyepieces are designed to project an image on the back of your eye. Most common eyepieces are designed for a short distance and small projection. Eyepieces that project a Wide image are usually noted by the terms: WIDE, ULTRA WIDE, WIDE FIELD (widefield also applies to fast focal ratio telescopes). The other thing is the distance. An eyepiece that projects a long distance is said to have a long eye relief. People who wear glasses look for long eye relief because they need more room between the eyepiece and their eye.

To make a long story short: most common eyepieces do not project an image that can be used by digital cameras. Some "long eye relief" - "wide field" eyepieces may work. There are also some special design eyepieces such as those by Scopetronics or Williams Optical that address the problem.

This is a URL where you can find out a little about the design of eyepieces:

http://www.hypermaths.org/quadibloc/science/optint.htm


LewTwo
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Old Mar 7, 2004, 10:53 PM   #3
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---------------------------------
Thanks for your reply.
----------------------------------

[quote="LewTwo"]I will take a shot at explaining this (within the limits of my understanding). Think of the eyepiece like a projector lens.
snip
Most common eyepieces are designed for a short distance and small projection.
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OK so far.
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snip

To make a long story short: most common eyepieces do not project an image that can be used by digital cameras.
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When you say "cannot be used", what do you mean. I was getting qite good images. It was just the vignetting.

I still can't understand how an eyepiece that is 25mm across can give me the same vignetting as one that is 13mm across. What am I losing in a $200 pair of binocs with 13mm eyepiece lens, that I get in a $150 eyepiee with a 25mm lens diam, if I get the same apparent image width?

I almost understand with your projector analogy. But the scope eyepieces I looked at were "long eye relief".

If I go to 2" eyepieces, will I _still_ have problems?
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Some "long eye relief" - "wide field" eyepieces may work. There are also some special design eyepieces such as those by Scopetronics or Williams Optical that address the problem.
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I looked at the Scopetronics site. Unfortunately their "camera special"is a 40mm focal length, and that would mean a great loss of power for my work, with a 50-250mm lens. I would have to start looking at a Barlow, with all the added problems, glass etc that would add. I will look at Williams.
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Old Mar 7, 2004, 11:18 PM   #4
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>>When you say "cannot be used", what do you mean.

I mean without vigeting problems ...


>> I still can't understand how an eyepiece that is 25mm across can give me the same vignetting as one that is 13mm across.

The diameter of the glass on the end of the eyepiece is ONLY an "indicator" of its ability to project a WIDE field. A lot more is dependent on the internal design of the eyepiece.

If I completely understood it then I would design and build one myself... but I don't. I think the problem is that one needs to design an eyepiece for a digital camera and ignore the human eye. Of course that would mean one would need at least two eyepieces (hopefully of the same focal length).

Another solution might be a relay lens system between the camera and the eyepiece ... but that means more knowledge and more glass as well.
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Old Mar 8, 2004, 1:03 AM   #5
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OK. Thanks for that. Now I wonder whther a 2" EP _would_ work!
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