Scene range vs dynamic range

Discussion in 'Digital Photography' started by Robert Feinman, Jun 30, 2005.

  1. Apparently the term dynamic range lends itself to much misunderstanding.
    So, I propose we try some new terminology so that the various factors
    in evaluating image capture can be discussed.

    What I think most photographers are interested in is the scene range.
    That is the ratio of the brightest portion of a scene that can be
    captured without clipping compared with the darkest region which can
    be captured without "too much" noise (or grain).
    The bright limit is fairly easy to determine with slide film and digital
    sensors, it is the point where any more exposure does not produce any
    change in the value stored in the image. For slide film it is clear
    film, for digital it is the max readout (255 or whatever the scale is).

    On the dark side things get a little more complicated, with no exposure
    at all film will produce some density (usually inherent fog) and digital
    will produce a non-zero value (various noise sources).
    At this point the user or manufacturer needs to establish a criterion
    for how much above this residual is necessary to consider the resulting
    values as "real" information. For film it is usually some fixed density
    increment like .2 above fog or whatever. For digital there seems to be
    no totally agreed on standard, but there are several reasonable choices

    Now the difference between these two limits set the amount of scene
    range that can be captured. For slide film this is usually around 6 or 7
    stops, for black and white it is around 9-10 stops (the zone system, for
    example). For color negative film it seems to be about 10+ stops.
    For digital capture people quote anywhere from 6 - 14 stops. It is not
    clear to me if these difference are due to the wide range of
    capabilities of digital sensors or the different criteria used to
    evaluate an image.

    Now once we have established the scene range we have the other factor
    which is how fine a gradation between values can be record. Film being
    an analog medium the answer should theoretically be infinite, but in
    most cases it is usually considered to be some small finite density
    change like .02 or .03 on the film. For prints the value depends on the
    acuity of the eye and is usually taken to be about 1/2% change in
    brightness for medium ranges and greater for the darkest areas. So
    people commonly state that a print can show about 50 or so discrete

    For digital the issue gets confused by the software used to evaluate the
    captured data. Notice the scene range discussion is independent of the
    software. So if the camera software only delivers 8 bits then the scene
    data will be divided into one of 256 possible values. More bits allows
    for more steps, however the maximum and minimum values don't change.
    When looked at this way the discussion of "dynamic range" is clarified.

    The maximum brightness of a scene can be reported as 256 or 4096 or
    100%, but this does not affect where the loss of detail occurred.
    The number of bits has a use when trying to make adjustments to the
    digital original after capture. If an image is represented by a small
    number of bits and their values are shifted by a large amount then gaps
    in the values can occur and the scene gets to look blocky or pixelated.

    There are many discussions on how many bits are needed to avoid this and
    how visible the effect is, but theoretically more bits allow more
    extreme editing.
    To use an analogy the scene is represented by a foot long ruler, the
    darkest portion is at 0 and the brightest portion is at 12". The object
    measured remains a foot long regardless of whether the markings on the
    ruler are 1 inch apart or 1/16 inch apart. It is just that somewhere in
    the middle of the ruler we can make a more accurate measurement with the
    finer divisions. This is the difference between accuracy and precision.

    Dynamic range is the correct scientific term for what is meant by the
    "useable" ratio of the biggest to the smallest signal values whether
    these are from light or sound or radio waves or whatever. However, for
    a working photographer I think the concept of scene range might be less
    confusing and more useful in practice.
    Robert Feinman, Jun 30, 2005
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  2. Hello Robert

    I have to say that this is one of the most interesting posts on this
    subject I've read and I will print it out and read it again too.

    I'm a bit surprised that it has gone unanswered, but then again
    sometimes a more indepth approach won't get read fully and, as a
    consequence, not grasped.

    I've been to your site. You have great images there - a lot of work!

    For DSLR/SLR Amateurs & Novices:
    Sharp Shooter, Jul 4, 2005
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  3. Robert Feinman

    Marvin Guest

    My prediction is that when (if ever) your proposal takes effect, it will be widely
    misunderstood. For one hting, the marketers at camera makers will want to bend the
    definition to make their products look good.
    Marvin, Jul 4, 2005
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