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  • Lens boffins required.

    Just wondering if I'm being a little too over simplistic, but here goes.
    Seeing how the legacy lens on a 4/3 camera give a two times magnification, due to using the center of the glass, would it be practical to reverse the elements of a teleconvertor to regain the effective focal length of lens used?
    My fear is that focus issues may be the problem down to the angle light is coming in to the sensor. Maybe I should just have a bash and see, but would like to know what problems to expect.
    E500 - E600 - E620 - HLD5 x2 - 17.5-45mm - 14-42mm - 14-45mm - Sigma 30mm f1.4 - 25mm f2.8 - 40-150mmMI (3.5) - Sirius 60-300mm f4-5.6 - Chinon 55mm f1.4 - Chinon 200mm f3.5 - FL50 - FL36 - Raynox DCR2020PRO

    http://www.flickr.com/photos/photonutter/

  • #2
    Re: Lens boffins required.

    Let me have a go at this:
    Lens designers have been making things called "telephoto" design for years. This is a simple trick where you place a negative (concave) element at the back of a medium focal length design. This spreads the image all over the inside of the camera and the film just gets the middle bit. This makes the lens work as if its a longer lens.. That's why they called it a "telephoto" design. If it didn't have the extra rear concave element then it would just be a plain "long focal length" lens - and it would be -er - physically much longer :-)

    If you prepeat this trick with an add on item with another concave element, you have invented the 2x teleconverter...

    OK, so they have also being doing another trick all these years to make a medium focal length lens seem shorter. This is called a "retrofocus" design (so I believe). This involves placing a negative (concave) element at the front of the lens design. Its like the reverse of the telephoto ides. This makes a "short lens" physically longer (just look how big the 11-22 lens is - yet its supposed to be as short as 11 mm !! The big deal is that it leaves loads of space at the back of the lens for a mirror and other camera like things...

    Your problem is how to make a lens look shorter by putting something different behind it. I think this would have to be a positive element (convex). Hmm. Possible, but I suspect it would be a disappointing experiment

    Pete

    (I'll get my coat, I'm off down the pub now)
    Look, I'm an old man. I shouldn't be expected to put up with this.


    Pete's photoblog Misleading the public since 2010.

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    • #3
      Re: Lens boffins required.

      Originally posted by photonutter View Post
      lens on a 4/3 camera give a two times magnification,
      This happens on all lenes fitted to the 4/3 sensor even the digital lenes, all lens manufacturers quote what would be the 35mm standard as agreed many years ago. I think that answers one part of your question.

      Using older lenses, as you said only use the centre of the glass due to the smaller size of the sensor, I have been using old lenses for some time mainly Zukio OM or Tamron OM fit as I have a collection going back to 1979 and they perform as well now on my digital bodies as they ever did on my OM bodies. the only modification you need is a 4/3 addaptor for the you are using, these are readily available on "ebay" for reasonable prices you may also want to consider an "AF" confirm chip like the dandilion which you can programe with the apature setting for use in "A" priority mode and it will give you an indication when in focus.

      I hope I have been of help

      Regards Barrie
      Barrie Norman

      http://bwpn.zenfolio.com/

      http://naturalhistorycambs.blogspot.com/

      Digital Equipment: Olympus E1 - E30 - E-500 | EC-14 & EC-20 Teleconverter | Zukio 14-45mm, 40-150mm, 150mm F/2, 50-200mm SWD, 70-300mm | Sigma 10-20mm F/4-5.6 SLD, 18-50 f/2.8 EX DC MACRO, 150 F2.8 Macro & 50-500mm (Bigma) |Tamron 135mm f/2.5 Macro 200mm f/3.5 | Optecka 800mm f/8 Prime | Wallimax 500mm F/6.3 Mirror Lens | Metz Mecablitz 36 AF-4 O Digital Flash | Manfrotto 055XB Tripod, Opticron Carbon Fibre "Traveller" Tripod, Sidewinder Gimbal Head, Manfratto 676B monopod, Manfrotto 322RC2 Head Lowpro Nature Trekker AWII, Lowpro Vertex 100 AW and Lowpro Toploader Pro 75 AW.

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      • #4
        Re: Lens boffins required.

        Originally posted by photonutter View Post
        Just wondering if I'm being a little too over simplistic, but here goes.
        Seeing how the legacy lens on a 4/3 camera give a two times magnification, due to using the center of the glass, would it be practical to reverse the elements of a teleconvertor to regain the effective focal length of lens used?
        My fear is that focus issues may be the problem down to the angle light is coming in to the sensor. Maybe I should just have a bash and see, but would like to know what problems to expect.
        No.The teleconverter has roughly the same effect whichever way round it is mounted (ignoring distortion etc) and the original lens focal length will be doubled.

        As it happens I have been playing with the x2 and some macro shots so just for fun I tried it.

        Pic 1 with x2 teleconverter
        Pic 2 with reversed teleconverter. Note the lenses were held together by hand and there is a lot of light leakage. Also focussing is poor, but you can see the overall effect is the same.



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        • #5
          Re: Lens boffins required.

          As suspected, had a play with both a concave and convex elements from an old project lens. Both alter the focusing distance to more of a macro and super macro, but without changing viewing angle. This may also be down to the distance from the film plane being increased too. Might have a play with distances to see if I can improve this, though to be honest, think I'm loosing the plot.
          E500 - E600 - E620 - HLD5 x2 - 17.5-45mm - 14-42mm - 14-45mm - Sigma 30mm f1.4 - 25mm f2.8 - 40-150mmMI (3.5) - Sirius 60-300mm f4-5.6 - Chinon 55mm f1.4 - Chinon 200mm f3.5 - FL50 - FL36 - Raynox DCR2020PRO

          http://www.flickr.com/photos/photonutter/

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