Re: Field of view

Discussion in 'Digital Photography' started by Paul Furman, Mar 8, 2009.

  1. Paul Furman

    Paul Furman Guest

    Alfred Molon wrote:
    > What is the formula for calculating horizontal and vertical field of
    > view out of focal length, crop factor and aspect ratio of the sensor?


    In addition to the suggestions given, the distance you are focusing on
    can change the fov, especially when doing closeups & macro work. The
    focal length is stated for infinity *and* often rounded to some industry
    standard like 50mm, 135mm, 300mm but might actually be 52mm, 138mm, 290mm...

    --
    Paul Furman
    www.edgehill.net
    www.baynatives.com

    all google groups messages filtered due to spam
    Paul Furman, Mar 8, 2009
    #1
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  2. Paul Furman

    Matt Ion Guest

    Alfred Molon wrote:
    > In article <dPWsl.15271$>, Paul Furman
    > says...
    >
    >> In addition to the suggestions given, the distance you are focusing on
    >> can change the fov, especially when doing closeups & macro work. The
    >> focal length is stated for infinity *and* often rounded to some industry
    >> standard like 50mm, 135mm, 300mm but might actually be 52mm, 138mm, 290mm...

    >
    > Sorry, I don't follow you here. Are you saying that the focal length in
    > the exif data is not accurate?


    I dunno about accuracy, but with my cameras and lenses, it's always
    listed to the nearest millimeter; rounding isn't any rougher than that.
    Matt Ion, Mar 9, 2009
    #2
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  3. Alfred Molon <> wrote:
    > In article <dPWsl.15271$>, Paul Furman
    > says...


    >> In addition to the suggestions given, the distance you are focusing on
    >> can change the fov, especially when doing closeups & macro work. The
    >> focal length is stated for infinity *and* often rounded to some industry
    >> standard like 50mm, 135mm, 300mm but might actually be 52mm, 138mm, 290mm...


    > Sorry, I don't follow you here. Are you saying that the focal length in
    > the exif data is not accurate?


    It wouldn't suprise me if at least some of the fixed focus "prime"
    lenses which change their effective focal length as they focus close
    didn't bother with calibrating and reporting that, but simply reported
    the canonical fixed focal length of the lens.

    --
    Chris Malcolm
    Chris Malcolm, Mar 9, 2009
    #3
  4. Alfred Molon wrote:
    > In article <>, Chris Malcolm says...
    >
    >> It wouldn't suprise me if at least some of the fixed focus "prime"
    >> lenses which change their effective focal length as they focus close
    >> didn't bother with calibrating and reporting that, but simply
    >> reported the canonical fixed focal length of the lens.

    >
    > By the way, how do zoom lenses in a camera measure the focal length
    > and report it back to the image processor, so that it can write it in
    > the exif?


    Most likely a mechanical position sensor, and a simple look-up table.
    Some compact cameras only have a limited number of zoom positions, so it
    may be counting steps to a stepper motor.

    David
    David J Taylor, Mar 9, 2009
    #4
  5. Paul Furman

    Paul Furman Guest

    Chris Malcolm wrote:
    > Alfred Molon <> wrote:
    >> In article <dPWsl.15271$>, Paul Furman
    >> says...

    >
    >>> In addition to the suggestions given, the distance you are focusing on
    >>> can change the fov, especially when doing closeups & macro work. The
    >>> focal length is stated for infinity *and* often rounded to some industry
    >>> standard like 50mm, 135mm, 300mm but might actually be 52mm, 138mm, 290mm...

    >
    >> Sorry, I don't follow you here. Are you saying that the focal length in
    >> the exif data is not accurate?

    >
    > It wouldn't suprise me if at least some of the fixed focus "prime"
    > lenses which change their effective focal length as they focus close
    > didn't bother with calibrating and reporting that, but simply reported
    > the canonical fixed focal length of the lens.


    I think that's normal expected behavior for the real focal length to
    change while focusing. Some macro lenses give corrected exif data for
    the aperture but none attempt to calculate the focal length, I guess
    because the definition of focal length is 'at infinity'. Lord only knows
    how the focal lengths of microscopes are figured <g>. Obviously I'm in
    over my head at this point!

    I've heard it's common to round off the actual focal length also.
    You never see 27.63mm lenses... just 28mm. Perhaps zooms are calibrated
    more carefully... I doubt it. Also consider fisheye lenses or those with
    noticeable barrel distortion: the center is a whole different
    magnification than the corners... it's all somewhat arbitrary, or a
    matter of accepted benchmarks.

    --
    Paul Furman
    www.edgehill.net
    www.baynatives.com

    all google groups messages filtered due to spam
    Paul Furman, Mar 10, 2009
    #5
  6. Paul Furman

    Bob Larter Guest

    Chris Malcolm wrote:
    > Alfred Molon <> wrote:
    >> Sorry, I don't follow you here. Are you saying that the focal length in
    >> the exif data is not accurate?

    >
    > It wouldn't suprise me if at least some of the fixed focus "prime"
    > lenses which change their effective focal length as they focus close
    > didn't bother with calibrating and reporting that, but simply reported
    > the canonical fixed focal length of the lens.


    That's correct (at least with Canons). I've also noticed that Canon
    zooms only report back the shot FL in quite large steps.

    --
    W
    . | ,. w , "Some people are alive only because
    \|/ \|/ it is illegal to kill them." Perna condita delenda est
    ---^----^---------------------------------------------------------------
    Bob Larter, Mar 13, 2009
    #6
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