Digital Rebel: Quick & Easy Way to Focus to Infinity?

Discussion in 'Digital Photography' started by S. Miller, Oct 2, 2003.

  1. S. Miller

    S. Miller Guest

    With other cameras that I've had in the past, turning the lens focus ring
    all the way one direction focuses it at the closest point, and turning the
    focus ring all the way in the other direction (until it stops) focuses it at
    infinity.

    My new Canon Digital Rebel kit w/ lens (18-55 mm, 28-88mm after the 1.6 crop
    factor is applied) appears to work differently, however. To my eye, the
    infinity focus point is somewhat *before* the focus ring gets to the stop.
    If you move it all the way to the stop, items at infinity are no longer in
    focus.

    Is that normal? If so, what is a quick and easy way to focus to infinity
    when I'm not sure I can depend on my eyes? Sometimes I can't look through
    the viewfinder but I know the object I'm shooting is at "infinity" and I
    want to ensure that things like auto-focus don't think otherwise. I
    wondered if Landscape mode might help but I'm looking for a way that would
    also work in one of the creativity modes.

    /s
     
    S. Miller, Oct 2, 2003
    #1
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  2. S. Miller

    Jim Townsend Guest

    The markings on the focus ring are an approximation..

    They aren't accurate down to the inch (Or millimeter :) Also don't forget
    you have a depth of field (The area in focus is between two distances and at
    one precise fixed distance).

    Most lenses have a touch of 'slop' that allows them to go slightly 'past'
    infinity. It allows for variances that might exist in the cameras AF system
    or that might be introduced during manufacture.
     
    Jim Townsend, Oct 2, 2003
    #2
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  3. And expansion/contraction of the lens with temperature change. If the
    stop is *exactly* at infinity at room temperature, you might not be able
    to reach infinity when it's very cold or very hot.

    On the other hand, the stop should only be a tiny bit past true
    infinity. If it's way beyond, I'd think the lens was miscalibrated, or
    the camera's sensor was not mounted correctly (so the lens mount to
    sensor distance is wrong).

    Question: do all lenses focus past infinity on this camera, or is the
    problem limited to one lens? Does this lens do the same thing on
    another camera?

    Dave
     
    Dave Martindale, Oct 2, 2003
    #3
  4. S. Miller

    Chris Brown Guest

    [focusing past infinity]
    Normal and deliberate, AIUI. I believe it's done so that the lenses can be
    used for infra-red photography. The different wavelength means that it will
    focus in a slightly different place to visible light.
    If the lens has a distance scale, line the infinity mark up with the focus
    line. Unless it's extreme telephoto and/or very wide aperture, it won't
    matter if you're a tiny bit out.
     
    Chris Brown, Oct 2, 2003
    #4
  5. S. Miller

    JPS Guest

    In message <blhs0i$56q$>,
    Every Canon-mount lens I've tried on my 10D goes past infinity. You
    can't turn any of them "all the way" and expect to be at infinity.
    --
     
    JPS, Oct 6, 2003
    #5
  6. << My new Canon Digital Rebel kit w/ lens (18-55 mm, 28-88mm after the 1.6 crop
    factor is applied) appears to work differently, however. To my eye, the
    infinity focus point is somewhat *before* the focus ring gets to the stop.
    If you move it all the way to the stop, items at infinity are no longer in
    focus. >>

    S.-

    This isn't exactly quick and easy, but it is possible to hold something like a
    pencil or a popsicle stick in front of the lens, which causes a split-image
    effect. By adjusting focus so the two images of your point of interest come
    together, it is possible to focus it fairly well. (Having three hands will
    help!)

    This "trick" also works with slide projectors.

    Another possibility may be to use the built-in focus indicators as a manual
    focusing aid, if the camera will allow that.

    Fred
    k4dii
     
    Fred McKenzie, Oct 18, 2003
    #6
  7. << This isn't exactly quick and easy, but it is possible to hold something like
    a
    pencil or a popsicle stick in front of the lens, which causes a split-image
    effect. By adjusting focus so the two images of your point of interest come
    together >>

    Shannon-

    I should have tried it first!

    Apparently the size or f/stop of the lens makes a difference. A pencil or
    popsicle stick is too fat. I was able to see the split-image effect with a
    ball-point pen refill, about an eighth of an inch in diameter, held against the
    front of the lens near the center. The optimum size may depend on the focal
    length as well.

    Holding the refill vertical, the image splits left-to-right. Perhaps something
    the size of a stiff wire would work. It needs to be near the lens, but it is
    probably best if it doesn't touch the glass.

    Fred
    k4dii
     
    Fred McKenzie, Oct 18, 2003
    #7
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