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OT Zoom lens

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  • OT Zoom lens

    I have two zoom lenses that appear to be decent ones, a Fujinon, and a Canon. Both made for video cameras. It sure would be nice if I could adapt one to my digital camera. Making a mount arrangement is not a problem, but there would have to be an optical adaption, since I'm not willing to remove the optics from the camera. I'm sure it would work if I did remove the lenses and carefully spaced the zoom lens from the ccd, but again, I won't be doing that. (and I know that's not all there is to it).If it's reasonably possible to do this, I'd be getting an optical zoom of about 20-1, and I'd be getting some use out of a perfectly good lens. And I'd have a reason for another shop project, a tripod. I have some knowledge of focal distances, convergence and divergence, pincusioning, etc, so I should be able to understand an explanation of what would be required to do this. I have been playing with a few simple lenses out front of the camera, and I'm getting nearly 2x zoom without affecting the camera's focusing, it's own zoom, or distorting the image or color fringing. The image is even right side up, though it wouldn't matter to me if it wasn't.
    Now I'm probably making you photographic and astronomical types cringe, but I'd like to hear your thoughts on this.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    One way to interface other optics is to set your digital camera to infinity if it will let you. The zoom lens is designed to create a real image on the surface of the imaging device. What you want to do is grab that real image, treat it as a virtual image and use a decent achromat to convert that virtual image to one at infinity. The "flatter" side of the achromat would face the virtual image between the achromat and the back end of the zoom lens. The curved end faces infinity, which in this case is what the camera has been set to accept. A small piece of drafting film or other diffuse surface can help you find where the image from the zoom lens sits. That info, together with the achromat focal length, should help with your final configuration.

    Mostly thinking out loud here as I don't know what physical constraints you will run up against.

    Also, you can probably get a decent image but since we're talking a pretty simple interface (no custom lens groups), various forms of distortion and color fringing can occur, especially in the corners.

    Alternate approach: set the camera to infinity and see if you can position the zoom lens and focus it to match the infinity requirement. The zoom lens will probably not behave in a parfocal manner. What you're asking the zoom lens to do here is to place the image at infinity instead at its original image plane.



    • #3
      Nothing wrong with that idea. It's called Afocal Coupling. It's done all the time for astrophotography. You need to couple the camera as close as possible to the lense without interefering with the focusing or zoom of the camera. You may well experience vignetting. If so, then zooming in with the camera will usually reduce or eliminate it. How well it works will vary greatly.

      See here:
      Free software for calculating bolt circles and similar: Click Here


      • #4
        I don't understand why you don't do as you said you would not; just remove the existing lenses on your digital camera & mount the zoom lenses at the proper focal distance. The solutions discussed will get an image, but your likely turning an expensive, high quality zoom len into a condensing quality system. You might as a prelim, compare the video camera image size, with the new image size on the digital camera, to see how feasable your refit is. Best of luck.


        • #5
          Trying to remove the lense on most digital cameras, especially zoom lenses, is a sure way to expensive junk.
          Free software for calculating bolt circles and similar: Click Here


          • #6
            Well, crossthreaded, this is a new camera, for one. I don't want to void the warranty. I don't want to lose the macro, or the compact size. I wouldn't be able to swap from original to the zoom lens without taking the camera apart to do it. Most often I won't be carrying the zoom anyway, since weight and bulk usually are factors when I'm out on the trail.
            I did make up a mount for what's called a tele conversion lens I had laying around. That gives me a fixed zoom factor of about 1.5 or so. The image looks good without distortion, though there appears to be a slight loss of contrast, or a slight increased brightness. I can live with that, and probably can make some setting changes to compensate a bit. That piece is small and light, so it's going in the backpack. When the camera is on, this piece slips over the extended lens tube, and I just hold it there, it's easy and fast. It will stay there if I let go, but if I point downwards, it will fall off. I also made up a tube to hold an eyepiece I have, and mounted that on the Canon zoom. Some careful adjusting, and by eye, I get a nice sharp image. Can't seem to get the camera to see anything through it, though.
            Den, your achromat idea sounds good. I checked the position of the real image, and it's about 5/8 inch behind the zoom's exit lens, and about 1/3 inch or so wide, nice and sharp as seen on a piece of paper. That corresponds to where the ccd is in the video camera. I tried some lenses I have, which are convex, and of different focal lengths, but without getting a recognizable image into the camera. I'm wondering now, what kind of simple lens would work best here, and what focal length might be suitable? I did try your second idea, the camera can be set to infinity, but I couldn't get anywhere near a focused image. I did experience vignetting, which disappeared with the camera zoomed past 2x.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #7

              Since I like to play I tried out a scenario using a 15mm diameter x 25mm FL achromat. I have some C-mount hardware and positioned the lens 45mm from the shoulder of the C-mount on my zoom lens.

              The camera was set at infinity and the 45mm mentioned was arrived at by a few iterations to try and get parfocal behavior from the zoom.

              End result ... nice image (camera was zoomed in to eliminate vignetting) but INVERTED

              I forgot about the fact that you would need an erecting system to keep things upright. For astronomy you may not care (or if you do you fix it) but for camera use, not too cool unless the camera flips over.

              After zooming in and critical focus, the image remained focused over the lens zoom range.

              This site has a number of scope and camera adapters but most of his products work with a monocular or other terrestrial scope which already has a set of inverting prisms. I got my Nikon adapter from him when he was just starting out. Good stuff and cheap.


              Added: I forgot we're talking digital here and inversion doesn't matter all that much as long as you can compose your image while it's upside down


              [This message has been edited by nheng (edited 02-21-2004).]


              • #8
                Well, I have this antique movie camera lens about 18 inches long with a zoom stroke of 4 inches.. Hmm... With my nikon 4300 I should be able to find new galaxies..



                • #9
                  Played with it some more, got an image into the camera, but not able to get good focus, even over a small part of the screen. I can get a sharp focus by eye, and also get the zoom's focus distance to be accurate according to it's markings. I'm confused as to why the camera won't see a sharp image when my eye gets it right on. Isn't this almost the same as a telescope or binocular once it's adjusted for best image? I'm not worried about distortions or color fringing at this point, I just think I should be able to get the camera to see at least part of the image in focus. The single lens I'm using does work better than the eyepiece as far as what the camera sees, but the eyepiece gives a proper image, undistorted and free from aberations, albeit upside down. Well, off to my makeshift optical bench now for some more experimenting.
                  Update, I did find the spot where the achromat needs to be, now to fine tune so the pic is good over the zoom's range. I need to try the eyepiece again. Part of the problem has been that it's hard to see the image from the camera properly unless I'm right in line with the lcd. I had the camera out in the sun, and I almost can't see the screen. I guess I'll be the guy wearing the black hood and holding the (shop made) shutter release. Thanks for the help.

                  [This message has been edited by darryl (edited 02-22-2004).]
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-