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Thread: Just a thought

  1. #1

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    Just a thought

    When I was out this weekend I had a thought and I will ask it now and try to explain it.

    If I zoom out and manually focus does the focus stay the same when I go to wide angle???

    Thanks
    Shawn O

  2. #2
    ktuli's Avatar
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    Bill S

    Re: Just a thought

    Shawn,

    It actually depends on the lens. Some lenses will retain focus through a zoom. Most will not. In general, only the most expensive lenses will retain focus through a zoom. The easiest way to find out is to try it.

    - Bill

  3. #3
    Moderator Dave Humphries's Avatar
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    Re: Just a thought

    Hi Shawn,

    I agree with Bill's view.

    If the lens auto-focuses, I'd trust that over my eyes any day.
    If manual focus only, set the zoom, then focus as best you can.
    At wider angles, where it can be difficult to judge, I'd go more by choosing an aperture for a given DoF, then make sure the lens focus is set to a distance that covers the depth of subject.

    This is probably also why some cheaper P&S don't allow a manual focus over-ride - they always AF, thereby hiding the design compromise made in the lens that doesn't allow you to zoom in tight, focus and zoom out.

    Good lenses on video cameras obviously have to do it right, as you'd often zoom while recording. The professional ones had a zoom tracking adjustment where you set that up long before 'the red light comes on'.

    Cheers,

  4. #4
    rpcrowe's Avatar
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    parfocal vs. varifocal lenses

    http://en.wikipedia.org/wiki/Parfocal_lens#Photography

    Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image. However, not all so-called "zoom" lenses are actually parfocal.

    Many "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation essentially the same as a parfocal zoom.

    My first experiences with zoom lenses began in 1964 with the 12-120mm Angenieux zoom lens for the Arriflex 16mm motion picture camera. This was a manual focus parfocal lens and I would often zoom in to a longer focal length, focus at that focal length and then zoom back to frame the image. It was quite convenient but, I totally agree with Dave in that, "If the lens auto-focuses, I'd trust that over my eyes any day."
    Last edited by rpcrowe; 1st June 2011 at 02:20 AM.

  5. #5

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    Have a guess :)

    Re: parfocal vs. varifocal lenses

    With Canon's L-Series, most are close to being par focal in my experience - and any slack could well be covered by DoF anyway - but if in doubt, refocus (I do)
    Last edited by Colin Southern; 1st June 2011 at 01:48 AM.

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