How to determine infinity focus distance?

Discussion in 'Digital Cameras' started by My View, Dec 31, 2004.

  1. My View

    My View Guest

    How can I determine at what distance the Canon 300D kit 18-55 lense (or any
    lens for that matter) starts to focus at infinity.
    I am setting up a table of hyperfocal distances and I want to ignore
    distances beyond which the lens will be focusing at infinity.
    Is it 63 metres or 200metres or 1 kilometre??? Is there an equation that
    will calculate this?
    My View, Dec 31, 2004
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  2. First, you should understand that there's a contradiction in your
    question. The lens "focuses at infinity" (meaning it gives the sharpest
    image of things a long way away" only when it is actually set to the
    infinity position, not somewhere nearer.

    Now consider depth of field. Given a particular criterion of
    acceptable sharpness, usually expressed in terms of the size of the
    "circle of confusion", the depth of field is defined to be the range of
    subject distances that are acceptably sharp. This doesn't mean that
    the whole range of depth is equally sharp; it's not. And the near and
    far limits of the depth of field are, by definition, right at the
    minimum limit of acceptable sharpness.

    The hyperfocal distance is the lens focus distance that makes the far
    limit of depth of field equal to infinity. In other words, everything
    from some near limit all the way out to infinity is acceptably sharp,
    and this is the setting that maximizes the range of depth that is so.
    Thus, it's useful when you absolutely need this. But it guarantees that
    objects at infinity are just acceptably sharp, not good and sharp. You
    *will* get sharper images of distant objects if you set the lens closer
    to the infinity mark than at the hyperfocal distance.
    Having said all that, there is a formula that will calculate hyperfocal
    distance, which should be found in any good lens or photography
    reference book. But it depends on focal length *and* aperture *and*
    your chosen circle of confusion size. You'd normally pick some fixed
    value for CoC, then calculate a 2D table of hyperfocal distance as a
    function of focal length and aperture setting.

    Dave Martindale, Jan 1, 2005
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  3. My View

    Don Guest

    Don, Jan 1, 2005
  4. SNIP
    Or on the web as a small Windows application at:
    which directly gives the "point blur diameter at infinity" as you move
    the slider through the distance setting. That also allows to set the
    CoC to one value and play with the other settings for the amount of
    infinity defocus, while giving a warning if the aperture setting
    causes the diffraction circle to exceed.

    Bart van der Wolf, Jan 1, 2005
  5. SNIP
    Yes, it is good to stress that there is only an infinitesimal narrow
    plane of optimal focus, and the rest is 'acceptable enough' for a
    given output magnification. The standard CoC recommendation is a
    somewhat arbitrary figure, usually chosen as giving an 'acceptable'
    blur at a given output size (often something like 5x7 or 8x10 inch at
    standard viewing distance).

    However, with Digital imaging on a sensor, we do have a restriction
    that was not present in film. The sensor pitch will pose a physical
    limit to what can be resolved. One could adopt a very high standard
    CoC of what's ultimately acceptable/achievable by choosing a CoC of
    twice the sensor pitch. As long as the out-of-focus detail is smaller
    than pitch x 2, it will look as good as something in perfect focus.

    So by setting the CoC to 0.0148 for the OP's 300D (7.4 micron pitch),
    the maximum resolution (assuming proper AA-filtering) range is
    achieved, without any sort of magnification restriction other than
    inherent sensor resolution (assuming the lens outresolves the sensor).
    Hyperfocal calculations will then exactly give the maximum,
    uncompromised, range of best attainable focus.

    Bart van der Wolf, Jan 1, 2005
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