Zooming and Focusing

Discussion in 'Digital Photography' started by Fedman Kassad, Jan 17, 2004.

  1. Sorry for the maybe silly question, but I noticed that when I zoom in or out
    with my Fujifilm S602Z the image goes out of focus while the zoom is
    actually working, only to go back to normal when I'm done zooming.
    Now is this normal behaviour for my camera (or any other digicam that has a
    zoom for that matter) or is it maybe defective in some way? I can clearly
    remember that in my optical SLR when the focus is ok for a certain distance,
    it stays the same even if I change the zoom factor (and all the way through
    the whole zooming process).
    And if it's just normal for digicams to do so, can somebody explain to me
    why the different behaviour compared to analog cameras?

    Thanks in advance for any insight...

    Col. Fedman Kassad
    Fedman Kassad, Jan 17, 2004
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  2. Yes it's quite normal...

    Both my 602 and Oly 2100UZ do it, except the 602 gets back into focus
    Philip Rawson, Jan 17, 2004
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  3. Fedman Kassad

    Bill Guest

    My S602 does the same thing. I'm sure it's normal. As to why it's different
    than your film camera in this way, I'm not sure. Maybe because the sensor is
    much smaller than the film area in a 35mm camera, so focus is more criticalm
    but that's purely a guess.

    Bill, Jan 17, 2004
  4. Fedman Kassad

    Don Stauffer Guest

    This is a design issue, and is one of the criteria the lens designer
    uses when he designs. There is a sort of curve one can make while
    analyzing a design. If the design is in focus at one (or two, or even
    three) points in the focal length range, you can plot how far out of
    focus it is at other focal lengths. Best designs have a sort of 'S'
    shaped curve, say in focus at one extreme, curve bends away, returns to
    focus somewhere near middle, curves in other direction, and returns to
    focus at other extreme. Cheaper designs hold focus at two points,
    cheapest is in focus only at one focal length, off for all others. It
    is almost impossible to get the curve truly flat, equally good focus at
    ALL focal lengths when focused at one.
    Don Stauffer, Jan 17, 2004
  5. Fedman Kassad

    Samuel Paik Guest

    It is probably normal for THAT digicam model. A zoom lens which
    doesn't maintain focus while changing focal length can be less
    expensive than a zoom lens which does. Since autofocus is the norm
    for many point and shoot cameras, many manufacturers skimp on the lens
    Samuel Paik, Jan 17, 2004

  6. It all depends on the design of the zoom lens. Some are designed so
    that they maintain that same focal distance while the focal length changes,
    and are called 'Constant focus'. Those that don't are called 'Vari-focus'.

    Vari-focus lenses tend to be smaller and easier to make, and the
    demand for constant focus isn't high in the market for cameras with built-
    in lenses.

    - Al.
    Al Denelsbeck, Jan 17, 2004
  7. Thanx for the quite complete answer. It all really make more sense now.
    Too bad that for getting a smaller (and I bet cheaper too) zoom assembly,
    one ends up having to wait for refocusing lags...

    Thanx again and see you around...

    Fedman Kassad
    Fedman Kassad, Jan 18, 2004
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