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  • OT OpTics

    Two questions- I have an older Fujinon telephoto lens made for a tv camera. Is there any reasonable way it can be adapted to a digital camera

    Secondly, how would I go about bringing more light into my camera for low light shots? Can I mate a large objective lens with other suitable lenses to do this?

    While on holiday, I took a couple pictures through one side of a binocular, and it worked out ok. In other words, I got a focussed image and with minimal distortions, though with some vignetting ( I think that's the term). I couldn't reduce the magnification factor (of course) but I did get a useable result. I'm wondering if I can assemble a monocular with minimal magnification, but able to bring in more light. I'm not worried at this point about edge distortions, etc, just whether this can be done, and if it's reasonable to do.

    One of the better cameras out now will give a better performance in this regard, but at the better part of a grand, that's still a ways off for me at this point. I have a gazillion (well maybe only 20-30 odd) lenses that I could possibly put to use somehow, including a projection lens from an older big screen tv- (what else can I use this one for?).
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Pete Ganzel is the man to ask. His website describes dozens of lens mount conversion projects. http://www.pbase.com/pganzel/root&view=tree
    Allan Ostling

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    • #3
      Can I mate a large objective lens with other suitable lenses to do this?
      Certainly. That is how astrophotography with a telescope works. There are several ways to produce a focused image on the focal plane. With a digital camera you are limited to just one of those ways unless you have a digital SLR. Otherwise the only way is what is called "afocal coupling" of the two optical systems. Any optical system that produces a focused image produces it as what is called a "virtual image". That virtual image appears to exist at some distance in front of the viewer.

      Your eye or the lens of the camera can both focus upon that image. Holding the camera to the eyepiece to obtain the image is afocal coupling of the optics. The objective of the camera should be as close as possible to the eyepiece as is practical to prevent vingetting. It may still occur depending on the focal ratio of the camera lens and can be avoided by using some optical zoom to narrow the field of view.

      Of course, there is no free lunch. By using a larger objective you are changing the Focal ratio. The larger the ratio the greater the magnification and the smaller the field of view. This has one very important implication. The brightness of an extended object (the blue sky or an illuminated scene) is proportional to the size of the objective lens and inversely proportional to the field of view of the object or scene. The net result is that it makes no difference what the size of the objective is to the surface brightness of any scene or object that fills the entire field of view. Only objects that are smaller than the field of view appear brighter with a larger objective, in particular objects that are much smaller such as point sources like stars. For an extended object the larger objective collects more light but because of the reduced field it is from a proportionally smaller area. No free lunch.
      Last edited by Evan; 10-09-2007, 05:01 AM.
      Free software for calculating bolt circles and similar: Click Here

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      • #4


        If this worked, you should be seeing a pic of my lens assembly. From the left there's a magnifyer for the viewscreen, with cover on and a cutout allowing me to see the controls on the camera. Next is a different camera set in place to show where the camera fits. Between that and the Canon zoom lens is an eyepiece, inside the grey tube. Where the green pvc gets thicker- those are clamp arms which when pulled out allow the zoom lens assembly to swing downwards and out of the way of the camera. Those arms will clamp together to hold it that way as well as the way you see it here. Holding it all up is a pair of nimh cells, which is proabably the best use for those particular cells. Though they have the same 2100mah capacity as the original cells, they only run the camera for about a third of the time.

        At any rate, this whole setup is over a foot long, and is just a tad cumbersome. The wonders of technology have shrunk this to the size of the Panasonic FX18, which is the camera I'm currently looking at to buy.

        Thanks Evan and Allan for the explanations and the link. I guess it's kind of futile for me to fiddle with these telephoto lenses I have, when all I have to do is throw some money at the problem.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Evan has described the problem well, the technical term is the optical invariant, a less technical term is the optical blivit theorem, you can't fit ten pounds of photons through a five pound lens. The ultimate F number of whatever combination you put in front of your camera lens will never be lower than that of the lens on the camera. And, as Evan says, the brightness of the image is dependent on the F number (as well as lost light due to absorption and reflections, to a much smaller extent).

          Joe

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          • #6
            Originally posted by Evan
            Any optical system that produces a focused image produces it as what is called a "virtual image".
            Is this right? A negative lens, such as a plano-concave, forms a virtual image where the diverging rays from a point source appear to be coming from. A positive lens, such as a bi-convex, forms only a real image where the converging rays actually meet.

            I think you may be confusing "virtual image" and "aerial image."
            Last edited by aostling; 10-09-2007, 12:06 AM.
            Allan Ostling

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            • #7
              Nope, no confusion here. See this explanation.

              http://hyperphysics.phy-astr.gsu.edu...pt/image2.html
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Originally posted by Evan
                Nope, no confusion here. See this explanation.

                http://hyperphysics.phy-astr.gsu.edu...pt/image2.html
                That's a good explanation. Now neither of us is confused (at least about this). I'm going to have to explore this Hyperphysics link some more.
                Allan Ostling

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