Re: Circle of Confusion

Discussion in 'Digital Photography' started by Marvin Margoshes, Jul 4, 2004.

  1. From: "Donald Gray" <>
    Newsgroups: rec.photo.digital
    Sent: Saturday, July 03, 2004 7:11 AM
    Subject: Circle of Confusion


    > I saw this being discussed in an earlier thread...
    >
    > FWIW, here are my thoughts on it..
    >
    > When I studied photography in an other life time, (50 years ago, so my
    > memory ain't so good these days), I under stood that the CoC was not a
    > fixed size but varied according to the viewing distance and actuity of
    > the viewers eyes.
    >
    > I seem to remember ...
    >
    > ... The CoC is the point when dot or object can no longer be
    > detected/defined by the eye because of the distance of the eye to the
    > object/dot. It becomes a 'circle of a confused' image in the eye. When
    > applied to a lens and film (or digital), I guess it is the smallest
    > dot that the combination of quality of glass (actuity) and the
    > resolution (grain or pixel) that the image of the dot/object falls
    > upon.
    >
    > ... The smallest dot that can be detected by the eye with 'average,
    > normal' eye sight, the CoC on white paper, under average lighting
    > conditions could be expressed as 1/500 of an inch when viewed from 36
    > inches.
    >
    > So, a dot 3 inches in diameter on a white sheet is huge when viewed
    > from 3 feet but when viewed from several hundred yards (metres) away,
    > the do cannot be resolved by the unaided human eye.
    >
    > my tuppence worth...
    >
    > The other definition of the "Circle of Confusion" is what politician
    > run all day....
    > --
    > Donald Gray
    > Putting ODCOMBE on the Global Village Map!
    > www.odcombe.demon.co.uk
    > You do not have to email me, but if you wish to...
    > Please remove the SafetyPin from my email address first
    > Thanks


    I learned about the circle of least confusion when I took and optics course,
    also in the '50s, and I still have the textbook and refeshed my memory. It
    is not a matter of individual perception, as you think. It is subject to
    mathematical treatment. For a non-mathematic discussion, you might look at
    http://www.molecularexpressions.com/primer/java/aberrations/astigmatism/.
    You can probbaly find a full mathematical treatment elsewhere on the Web.
     
    Marvin Margoshes, Jul 4, 2004
    #1
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  2. "Marvin Margoshes" <> writes:

    >I learned about the circle of least confusion when I took and optics course,
    >also in the '50s, and I still have the textbook and refeshed my memory. It
    >is not a matter of individual perception, as you think. It is subject to
    >mathematical treatment. For a non-mathematic discussion, you might look at
    >http://www.molecularexpressions.com/primer/java/aberrations/astigmatism/.
    >You can probbaly find a full mathematical treatment elsewhere on the Web.


    This is not the same as the Circle of Confusion used in depth of field
    calculations.

    Marvin is talking about the fact than when a lens forms an image, it
    does not focus light from a point in the subject to a point in the image
    plane. Instead, it forms a blurry spot which can also be called the
    "circle of confusion". The "circle of least confusion" is just the
    best-defined (smallest diameter) circle of confusion as you adjust
    focus. So this circle of confusion is an *observed or calculated*
    property of how a particular lens forms an image under certain
    conditions.

    But in general photography, objects in the field of view are at
    different distances from the camera. Things in the plane of best focus
    are imaged to a circle of least confusion on the image plane, but other
    objects necessarily have a *larger* circle of confusion in the image
    because they are not in focus. As they get further from best focus, the
    circle of confusion changes from having something like a Gaussian
    profile to a (much larger) plain disc.

    Depth of field calculations attempt to predict *how much* something can
    be out of focus and still look acceptably sharp. To do this, they start
    from an assumption about the maximum allowable size of the circle of
    confusion, then calculate the near and far object positions that result
    in a circle of confusion of this size (using geometric optics only,
    ignoring diffraction and lens aberrations). So the "circle of
    confusion" term that appears in depth of field calculations is more
    accurately described as "the maximum allowable size for the circle of
    confusion".

    And the appropriate size for this circle of confusion diameter limit
    *does* depend on the size of the print and viewing distance, because it
    depends on how blurred something can be before the person viewing the
    print thinks that it is unsharp. It's a useful number because once you
    make some assumptions about viewing conditions, you can obtain a circle
    of confusion size that applies to all images viewed under those
    conditions. After scaling the CoC size to account for printing
    magnification, you get a single CoC size for the image in a particular
    camera that applies to all images taken with that camera regardless of
    focal length and f/number. This allows depth of field charts to be
    calculated.

    Dave
     
    Dave Martindale, Jul 4, 2004
    #2
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  3. Marvin Margoshes

    Gordo Laqua Guest

    Rumour has it that on or about 04 Jul 2004, (Dave
    Martindale) wrote:

    > "Marvin Margoshes" <> writes:
    >
    >>I learned about the circle of least confusion when I took and optics
    >>course, also in the '50s, and I still have the textbook and refeshed
    >>


    <<Snip>>

    Time for an old bad joke.....

    Circle Of Confusion is defined as

    "A group of photographers discussing Depth Of Field"




    --
    ------------------------------------------------
    "I didn't get rich by writing a lot of cheques."
    - Bill Gates to Homer Simpson
     
    Gordo Laqua, Jul 5, 2004
    #3
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