Increasing optical zoom

Discussion in 'Photography' started by Steve Irwin, Jun 8, 2005.

  1. Steve Irwin

    Steve Irwin Guest

    Hi all,

    I'm looking at increasing the optical zoom of my Canon Powershot A70 and
    wondering if anyone has had any experience/comments.

    At the moment I've made up a mounting plate so that I can attach a 8
    times magnification monocular (basically half a binocular) to the front
    of the camera. It works quite well (given the price of the monocular),
    but the biggest problem is that sometimes 8 times magnification is too much.

    What I was thinking is to use some of my old SLR lenses as the objective
    lens of my own telescope. If I could get a -25mm lens (or something
    similar) and mount that between the SLR lens and the camera then that
    should form a telescope, with magnification F/25 (where F is the focal
    length of the SLR lens). Since I've got SLR lenses ranging from 28-400mm
    then that should give a nice range of magnifications. Obviously it would
    require some mounting brackets and a little bit of playing around to get
    the right spacing between the lenses, but that should all be fairly easy.

    I've had a go at the basic idea by using a 50mm and 28mm SLR lens
    back-to-back and it seems to work okay, giving about double
    magnification, but obviously the image is inverted because that is with
    2 positive lens. Also, with 2 positive lenses the distance between them
    is greater and so the field of view is reduced. If I can get a negative
    focal length as the "eyepiece" lens then that should avoid the image
    inversion and improve field of view.

    So...anyone know where to get a strong (~25mm) negative lens cheaply, or
    has anyone else got any ideas?

    Steve Irwin, Jun 8, 2005
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  2. I did get the TC52 - a teleconverter with 2,4x power. Optically
    it's not much difference from the 3x zoom, but it 'steals' a lot of
    light (2 stops). You also got to get the corresponding lens-adapter
    (which also takes 52mm filters!).
    Rolf Egil Sølvik, Jun 8, 2005
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  3. Steve Irwin

    Steve Irwin Guest

    Thanks for the suggestion.

    Yeah, I've seen those teleconverters - they cost almost as much as the
    camera. Normally I don't mind spending a bit on lenses, but it didn't
    look much quality for the price. Since it's just for a compact camera I
    was hoping for a cheaper alternative.

    I've had a chat with someone who once tried a diverging lens (he
    borrowed from a lab at work) with an old SLR lens. He only had access to
    a 1/2" diameter lens, and said that it worked okay, but the field of
    view was reduced. I reckon that if I can get access to a larger -25mm
    lens it should work. The main problem is finding one that isn't going to
    cost an arm and a leg. I'm in Australia, and although I can buy such a
    lens fairly cheaply (~US$20) on the net, they wanted to charge me almost
    US$200 in shipping...bugger that. If I can find something common that I
    can get a lens from I'll give it a go and let you know how it works out.

    Steve Irwin, Jun 9, 2005
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