Less expensive alternative to 400mm camera telephoto

Discussion in 'Digital SLR' started by RichA, Jan 17, 2006.

  1. RichA

    RichA Guest

    RichA, Jan 17, 2006
    #1
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  2. RichA

    eawckyegcy Guest

    Poor close focus, zero autofocus, no aperture control, and no image
    stabilization. What more can we ask for?!

    Nitwit. Rich, even you must be able to think this through: if it were
    possible to obtain EF 500/4 (or similar) results at 1/20 the price,
    don't you think people would already be doing it on a wide scale?
     
    eawckyegcy, Jan 17, 2006
    #2
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  3. RichA

    RichA Guest

    Just because you can't cope with a "manual" lens doesn't mean everyone
    is so crippled.
    As for it not being taken advantage of by others, lets just chalk it up
    to narrow thinking from people like yourself. You probably think
    telephoto has to be white to function
    well. A more refined version of this kind of lens (apochromatic) will
    make
    mince-meat of all camera lenses except for the most expensive apos from
    Canon, Nikon.

    BTW; There is a rudamentary stop down mechanism available for these
    lenses that allows
    them to be stopped down to about f10.
    Close focus is probably limited to around 5ft. How much closer do you
    want with a 400mm
    telephoto??? A rack and pinion focuser on a long lens affords much
    greater focus range
    than internal focusing in a camera lens.
    But it's a cheap solution so anyone who is remotely curious can try it
    for themselves.
     
    RichA, Jan 17, 2006
    #3
  4. RichA

    eawckyegcy Guest

    So sayeth the armchair photo-dingbat. Go ahead and try to use a
    telescope as a lens on a terrestrial camera. Unless one is a complete
    shithead (hey, you!), one rapidly learns the value of one's time.
    Did you know that the Orion ST80 has a _plastic_ tube?

    I bet you didn't.

    HA HA HA HA HA HA HA.
    You appear to have forgotten the ST80 is itself white.
    Yes. A "more refined version". Perhaps you refer to the various
    Takahashi, TeleVue and Williams' offerings? Go ahead and tell us how
    much they cost. Hint: even the Orion 80ED is beginning to hit $500,
    and still no AF, aperture control, and the rest of it.
    Yes, this must be why the fellow you quoted at dpreview's forums needed
    an extender tube to hit 20 feet. Don't you even read the articles you
    reference?
    .... and discover why everyone else wants a has a telephoto lens that
    fits the camera. Why waste a good $200+ for nothing?
     
    eawckyegcy, Jan 18, 2006
    #4
  5. RichA

    Rich Guest

    The Orion is a bargain at $500.00.
    The Tak Sky 90 is about $1900.00. You get what you pay for,
    but with the Taks you are paying for mechanical excellence (no
    plastic) and optical accuracy.
    But for prime focus, you'd be hard pressed to tell the difference
    between the cheap Orion achro and the apos, except for some
    minor colour error. If you use an eyepiece to do projection
    (higher focal length) photography, that is where the apo will
    come into it's own. Consider a camera lens with a 25x-50x
    tele-converter on it. How would it perform? One guess.
    Those scopes are asked to do this when they image planets.

    But the reason these designs perform so well is that they are
    relatively "pure" optical systems. They don't cram 6 or more elements
    into one tube like camera lenses do. Some zooms have 17 elements,
    up to 30 individual surfaces, that's why they are so bad when it comes
    to being compared to primes or any other decent lenses.
    A cemented 80mm achromat simply does not lose light or
    contrast because it only has 2 multicoated lens surfaces. It's as
    pure an optical system as they get.
    Then Orion has changed the focus tube draw length. Sky Instruments
    (who are the importer of these) had them and the minium focus distance
    was considerably shorter.
    Robots need not apply.
    -Rich
     
    Rich, Jan 18, 2006
    #5
  6. RichA

    Ray Fischer Guest

    By that logic a single convex lens must have the best quality possible.
     
    Ray Fischer, Jan 18, 2006
    #6
  7. RichA

    Rich Guest

    In fact, for monochromatic light, it can be. Which is why some solar
    telescopes have used it. But not for polychromatic light, obviously.
    But if you are a DSLR user, an inexpensive telescope with a few
    eyepieces and a projection adapter can be a very flexible way to
    obtain different long focal lengths to experiment with.
    -Rich
     
    Rich, Jan 19, 2006
    #7
  8. Canon's 70-200 f/2.8 IS L compares badly to primes and 'any other decent
    lenses'? Your religion there *is* my belly laugh.

    Oh, did I mention that it has 23 lenses in 18 elements?

    -Wolfgang
     
    Wolfgang Weisselberg, Jan 19, 2006
    #8
  9. RichA

    Rich Guest

    It's not really a telephoto, is it? 200mm has such a low manification
    (about 4x) that the extra elements don't matter as much as they would
    if it was a 400mm or longer telephoto. The higher the native
    magnification of the lens, the better it has to be. 18 elements = 36
    surfaces. Reflection scatter and loss at each surface means you lose
    50% of the light from the image. The rest is lost or suffused over
    the surface of the image, resulting in lower contrast, resolution and
    colour saturation. You cannot avoid this.
    -Rich
     
    Rich, Jan 19, 2006
    #9
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