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Old Feb 16, 2004, 8:57 AM   #1
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Default Make a telescope from a lens to use as a non-SLR digiscope?

I hope none of these questions are _too_ stupid. I have dabbled in theroy with telescope building, but it was some time ago. I have used telescopes of the smaller type. I inderstand the relationship between a film placne area and the length of lens needed to provide certain magnification, because I used SLR cameras.

Now suddenly I am using a non SLR digital camera (Oly C750) and I find that the take real tlel shots I do not wory about film plane size and distance, but use the camera as my "eye" and actually take a picture of an eyepiece. I was starting to think about removing the lens of my camera, to use ity as an "SLR" with DIY lens mounts. Do not _ever_ tell my wife <G>

I actually did this with a 50mm lens and a little webcam I have. It gave respecatble magnification, but of course poor shots. The colour was also all wrong, as the webcam's little lens had a red-coloured filter in front of the sensor, I suppose for light balance.

So. I have looked thiough a reasonable pair of Nikon 9 * 25 binoculars (well one half anyway) with the 750 zoomed all the way in. This gave me a small amount of vignetting,with an effective eyepiece dimaeter of about 13mm. It also gave a phenominal magnification! Quality? not sure, as I was hand-holding the entire thing. (No drinkies the previous night, and I could actually see something <G>)

I gather this is 10X (zoom) by 9 X (binocs) to give about the equivalent of a 4500 mm lens. I am therefore looking further <G>

I have a reasonable Tokina 50-250 zoom lens..This has an object aperture of 52 mm and would therefore be quite bright at say 5X as a telescope or telephoto setup for a digicam with a power of 5:1..

Questions

(1) If I buy an eyepiece for this, say of 25mm focal length, will the eyepiece "behave" itself and place itself in focus, assuming the main lens is focussed, if I put the eyepiece where the camera's film plane would normally go?
(2) If so, do I get vignetting, especially if I start stopping the main lens down?
This is where it differs from a normal telescope, where the eyepiece is placed to see as much of the main lens/mirror as is practiable.
(3) If I bought a 25mm _fitting_ diameter eyepiece, what is the usual _lens_ diameter? Am I still going to get vignetting from the camera? I reckon that the camera would stop vignetting at about 19mm width of lens (???) at full zoom, when it's hard up against the eyepiece. I have tried this with a piece of cardboard with a circle cut out. I get shaded, not irredeemable, vignetting at 19 mm at full zoom..

The thing I see here is that if I use a zoom "main" lens, I can overcome the fact that these lenses are so powerful that many people complain they cannot gette all of a bird in to the shot etc. I have a 5:1 zoom to overcome this. I could even use different eyepieces, if I became adventurous, to get a full range.

I would appreaciate any advice here. This is interesting, as I have the room and the chances to take animal and bird shots.
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Old Feb 17, 2004, 6:57 AM   #2
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Quote:
(1) If I buy an eyepiece for this, say of 25mm focal length, will the eyepiece "behave" itself and place itself in focus, assuming the main lens is focussed, if I put the eyepiece where the camera's film plane would normally go?
You can buy eyepiece adaptors for telephoto lenses, so in theory you should be able to do this if you can come up with the mechanics.


Quote:
(2) If so, do I get vignetting, especially if I start stopping the main lens down?
The vignetting, if any, depends on whether you can place the camera within the eye relief of the eyepiece. If you stop the main lens down the image will get darker but I don't think that the vignetting should increase. The exit pupil size will simply be 52mm devided by your effective magnification. If that magnification is 10x (250/25) the you should have a nice wide exit pupil (5.2mm) but not a huge lot of magnification. I have an exit pupil size of 2.4mm with my Digiscoping set-up and that's just ok for my CP4500, but I can get the lens of my CP4500 very close to the eyepiece...

Regards,
Graham.
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Old Feb 17, 2004, 7:42 AM   #3
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Thanks for your reply.

[quote="checklg"]
Quote:
(1) If I buy an eyepiece for this, say of 25mm focal length, will the eyepiece "behave" itself and place itself in focus, assuming the main lens is focussed, if I put the eyepiece where the camera's film plane would normally go?
You can buy eyepiece adaptors for telephoto lenses, so in theory you should be able to do this if you can come up with the mechanics.

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I may make my own for a start. See how it goes and go from there. I hate the idea of $50 for an alu tube <G>
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Quote:
(2) If so, do I get vignetting, especially if I start stopping the main lens down?
The vignetting, if any, depends on whether you can place the camera within the eye relief of the eyepiece. If you stop the main lens down the image will get darker but I don't think that the vignetting should increase.

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That's what I was hoping to hear. I will find a camera store with an eyepiece and see what happens.

Actually, in light of the rest of the post, the eye relief is that distance from the eyepiece through which you cannot see the front of the lens, right? So. I need to be able to get as close as that.

Need to check
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The exit pupil size will simply be 52mm devided by your effective magnification. If that magnification is 10x (250/25) the you should have a nice wide exit pupil (5.2mm) but not a huge lot of magnification.
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Sorry. A what!?<G> I am not sure what an exit pupil is. I _think it's the apparent size of the objective end of the camera lens? You are talking 52 mm and 10X because I mentioned 52 objective lens diam, 250mm lens length and a 25mm eyepiece, right?
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I have an exit pupil size of 2.4mm with my Digiscoping set-up and that's just ok for my CP4500, but I can get the lens of my CP4500 very close to the eyepiece...
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errr...mmmm. Now I am confused again. If what I say above is right, then the "front" end of my tele lens could cause problems, not just the eyepiece. This is like being too far away from the eyepiece when you look through it by eye, I am guessing. You actually see an "image" of the front aperture, containing the field of view, vignetted.

If so, I need to take my camera as well, to see how this all goes.

I have diiscaovered )for sale, or course :-< )a 32mm FL (to give me 8:1, which with a 10:1 camera zoom is quite powerful, and plenty to start on) eyepiece that has an actual 32mm lens, with larger fitting.

If I can't stop eyepiece vignetting with _that_ I will be surpirsed. But the "other end" problem could rear its ugly

Thanks again. I shall report.

One of the greates regerst I had about going difital is the loss of all thjose lovely old lenses. There is hope yet.

You have certainly given me enough to jump in an try it, knowing what to look for.
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Old Feb 17, 2004, 8:47 AM   #4
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Hope this doesn't confuse things further but the exit pupil size is simply the diameter of the beam of light coming out of the back of the eyepiece.

Eye relief is how close your eye needs to be to the eyepiece to see "the full circle".

The larger the exit pupil the better the eye relief, i.e. with good eye relief your eye can be further away from the eyepiece and still see the full circle.

The better the eye relief the better the chance you have of getting a camera lens close enough to the eyepiece to avoid vignetting.

However the amount of vignetting will also be effected by the camera used. Some camera lenses have a wider angle view than others due to the focal length involved. Some cameras like the CP4500 work well in this context, others donot. Your OLY zoom can help you compensate for vignetting but more zoom equals less light.

Best of luck.

Regards,
Graham.
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Old Feb 17, 2004, 6:22 PM   #5
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[quote="checklg"]Hope this doesn't confuse things further but the exit pupil size is simply the diameter of the beam of light coming out of the back of the eyepiece.
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No no! All info gladly recieved!
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Eye relief is how close your eye needs to be to the eyepiece to see "the full circle".
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Haha. The guy in the photo shop mentioned eye relief. I thought he was talking about making it easier to look without eye strain! <G>
Actually that's not fynny....
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The larger the exit pupil the better the eye relief, i.e. with good eye relief your eye can be further away from the eyepiece and still see the full circle.

The better the eye relief the better the chance you have of getting a camera lens close enough to the eyepiece to avoid vignetting.
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Ok. the result is fair enough as you describe it. But a question OOI.

If I am digging too deep, sned me hence to the nearest library (although the Net is usually a far better source these days!)

Is the hignetting you see as you move out of the eye relief area caused by the eyepiece vignette, or the object end vignette? In other words are you seeing the eyepiece , because oif its lens, or the eyepiece "seeing" the circle of the front of the lens?
Hope you know what the _question_ means..
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However the amount of vignetting will also be effected by the camera used. Some camera lenses have a wider angle view than others due to the focal length involved. Some cameras like the CP4500 work well in this context, others donot. Your OLY zoom can help you compensate for vignetting but more zoom equals less light.
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Phew! No questions there. I had that one sussed from liooking through a pair of binocs using the oly. It can get only minimal vignetting with only a 13mm eyepiece. at max zoom.

OK! Enough good news to head for the shop I reckon (I live in the "sticks" so a trip to the shop is not to undertaken lightly.)

Thank you very much for all so far.
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Old Feb 18, 2004, 6:19 AM   #6
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Quote:
Is the hignetting you see as you move out of the eye relief area caused by the eyepiece vignette, or the object end vignette? In other words are you seeing the eyepiece , because oif its lens, or the eyepiece "seeing" the circle of the front of the lens?
Hope you know what the _question_ means..
The vignetting is caused by the eyepiece.

Regards,
Graham.
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Old Feb 18, 2004, 11:53 PM   #7
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So far, I have been into town and checked out a 32mm Plossel eyepiece. The results were quite interesting. I could use the camera to see a resultantmagnification of around 80:1, which was quite spectacular. This was with theloss of only a couple of F-stops as well, as the 35mm-based lens is quite large, compared to a digicam's. I could also zoom out to about 6X from the full 10X, without vignetting. The only problem was that the Plossel's ocular is buried deeply in the tube away from the eyecup. So even folding the eyecup down did not allow me to get the camera close to the eyepiece lens.

Shttubg the aperture down did _not_ result in any vignetting. Thsi all seems interesting.
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Old Feb 27, 2004, 7:32 AM   #8
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I am going to "spam" this a bit

OK. Has anyone used an Olympus C750 to work with spotting scopes or eyepiecs on telescopes to take "digiscoping" shots?

I have now been to five shops in my village that calls itself a "Capital Spitty" and seen only two eyepieces that came close to suiting me.

This included a visit to a shop that I rang and _asked about 25-30mm eyepieces_, explained what I wanted, said that I had a setup that allowed me to LOOK at eyepieces, could I come in and have a look. "Yeah sure". I drove 60Km to this shop. "Oh. No. We don't have any 25 or 30 mm eyepieces. This was a _teleescope shop_!

None of these guys sell or _know anything_, as far as they will admit, about anything except one make each, and one range within that make, and if you are very very lucky they may have one on the shelf in some sizes......in a shop whose capital is tied up in dozens of multi-thousand dollar telescopes!

aaargh! They _deserve_ my Internet shopping, all of them!

Sorry. I repeat my question. Has anyone used an Olympus C750 to work with spotting scopes or eyepiecs on telescopes to take "digiscoping" shots? I would like to discuss things with them.
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Old Mar 2, 2004, 1:40 AM   #9
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I have looked at a couple of old SLR zoom lens I have around and had similar results. The eyepiece needs to physically inserted in the back of the lens.

I made a scope from a old 500mm mirror F/8 zoom last weekend. I was stuborn enough to want a 45 degree erecting diagonal. I removed the back of the mirror lens and turn the origonal "T" mount off. I then took a 1-1/4 erect image diagonal and shortened the nose piece to about 3/32 and glued to the lens back. Next I removed the focusing stop screw. I now have a 500mm scope that is about that focuses from 3 feet to infinity. Problem is that it needs a LOT of light. I have not yet tried the digital camera on it.
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Old Mar 2, 2004, 3:18 AM   #10
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Quote:
Originally Posted by LewTwo
I have looked at a couple of old SLR zoom lens I have around and had similar results. The eyepiece needs to physically inserted in the back of the lens.

I made a scope from a old 500mm mirror F/8 zoom last weekend. I was stuborn enough to want a 45 degree erecting diagonal. I removed the back of the mirror lens and turn the origonal "T" mount off. I then took a 1-1/4 erect image diagonal and shortened the nose piece to about 3/32 and glued to the lens back. Next I removed the focusing stop screw. I now have a 500mm scope that is about that focuses from 3 feet to infinity. Problem is that it needs a LOT of light. I have not yet tried the digital camera on it.
Thanks for coming back

What diameter is the Object end of the lens you used....what FNumber is its best? What FL is the eyepiece? If you need a lot of light, sounds like you have a very powerful scope. I am working with a 50-250 F4 zoom lens from an SLR, and with a 25mm eyepiece, it's blinding! Only 10:1, but still a wonderful picture.

My problem is preventing vignetting. My Oly 750 is a known problem here.
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