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Is your zoom varifocal? - Photography

To paraphrase an estimable poster who undoubtedly knows, "Your 'zoom' lens is in fact 'varifocal', meaning that the point of focus changes as you zoom. ... A true zoom lens does not do this." Alternatively, how many "True zoom" lenses are in use by contributors to this group? I posted a quotation from Mr Canon's polyglot leaflet included with the EF24-70mm f/2.8L USM lens (not cheap): "(!) Be sure to finish zooming before focusing. Changing the zoom ring after focusing can affect the focus." There's one instance of non-true zoom confessions. Any others? Is the effect of any practical ...

  1. #1

    Default Is your zoom varifocal?

    To paraphrase an estimable poster who undoubtedly knows, "Your 'zoom'
    lens is in fact 'varifocal', meaning that the point of focus changes as
    you zoom. ... A true zoom lens does not do this."

    Alternatively, how many "True zoom" lenses are in use by contributors to
    this group?

    I posted a quotation from Mr Canon's polyglot leaflet included with the
    EF24-70mm f/2.8L USM lens (not cheap):
    "(!) Be sure to finish zooming before focusing. Changing the zoom ring
    after focusing can affect the focus."

    There's one instance of non-true zoom confessions. Any others?

    Is the effect of any practical importance in the performance of
    photographic tasks by an intelligent biped?



    --
    Frank ess


    Frank Guest

  2. #2

    Default Re: Is your zoom varifocal?

    On Wed, 2 Feb 2005 13:31:36 -0800, Frank ess <com> wrote: 

    We've already seen that one poster to this newsgroup was taught to
    focus at full zoom and then pull back, for better focus accuracy.
    For what it's worth.

    I didn't know until today that there was such thing as a zoom that
    kept focus as the focal length changed, though I had occasionally
    wondered.

    --
    Ben Rosengart (212) 741-4400 x215
    Sometimes it only makes sense to focus our attention on those
    questions that are equal parts trivial and intriguing.
    --Josh Micah Marshall
    Ben Guest

  3. #3

    Default Re: Is your zoom varifocal?

    On Wed, 2 Feb 2005 13:31:36 -0800, "Frank ess" <com>
    wrote:
     

    Both the Nikon 24-120 models are varifocal and noted as such
    in the small print. Kiron made a number of openly varifocal 35mm
    lenses, which were also marketed under the Vivitar brand.

    --
    Michael Benveniste -- com
    Spam and UCE professionally evaluated for $419. Use this email
    address only to submit mail for evaluation.
    Michael Guest

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  5. #5

    Default Re: Is your zoom varifocal?


    Frank ess wrote:
     


    Is that the same thing as Internal Focus (IF) type? My IF doesn't spin
    the zoom but it seems not possible to zoom in for close focusing then
    zoom out. That would be handy.
    paul Guest

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  7. #7

    Default Re: Is your zoom varifocal?

    On Wed, 2 Feb 2005 21:39:05 +0000 (UTC), Ben Rosengart <br+com>
    wrote:
     
    >
    >We've already seen that one poster to this newsgroup was taught to
    >focus at full zoom and then pull back, for better focus accuracy.
    >For what it's worth.
    >
    >I didn't know until today that there was such thing as a zoom that
    >kept focus as the focal length changed, though I had occasionally
    >wondered.[/ref]

    Presumably the "focus tight then pull back" technique would only work if
    the lens _did_ keep its focus unchanged as it zooms/widens (i.e. if it were
    a "true zoom" as someone else called it). Conversely, focusing when at the
    wide end [of a "real zoom"] and then zooming in (without rechecking focus)
    would be unwise, not because the focus might change in doing so, but
    because the original focus may not have been precise, and this only shows
    up at tight zoom.

    My guess is that in the days when manual focus was the only option (or AF,
    if available wasn't used much), keeping constant focus so the above trick
    could be used would have had a sufficiently large "perceived benefit" to
    make it worth designing in.

    Now that the use of auto-focus is much more common [a possibly unfounded
    assumption], it matters less if the focus changes as the lens is zoomed.
    If it's easier to build a lens without worrying about keeping constant
    focus, then lenses can be made cheaper.


    Regards,
    Graham Holden (g-holden AT dircon DOT co DOT uk)
    --
    There are 10 types of people in the world;
    those that understand binary and those that don't.
    Graham Guest

  8. #8

    Default Re: Is your zoom varifocal?


    "Ben Rosengart" <br+com> wrote in message
    news:panix.com... 
    >
    > We've already seen that one poster to this newsgroup was taught to
    > focus at full zoom and then pull back, for better focus accuracy.
    > For what it's worth.
    >
    > I didn't know until today that there was such thing as a zoom that
    > kept focus as the focal length changed, though I had occasionally
    > wondered.
    >
    > --
    > Ben Rosengart (212) 741-4400
    > x215
    > Sometimes it only makes sense to focus our attention on those
    > questions that are equal parts trivial and intriguing.
    > --Josh Micah Marshall[/ref]

    On the Olympus E20, i always zoomed right in, then zoomed out to achieve
    good composure & clicked the shutter...all without loosing focus! Eyes
    crystal sharp, yet the 28-105 on my canon 20D seems soft no matter what I do
    :(


    Mac Guest

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