Dynamic range, RAW, HDR, Grad ND Filters

Discussion in 'Digital Photography' started by bugbear, Sep 4, 2007.

  1. bugbear

    bugbear Guest

    I have a fairly advanced P&S camera - Canon A630.

    I can persuade it to do RAW mode via CHDK


    I could also use CHDK to take shutter speed
    bracketed shots for HDR usage,
    although this only applies to
    static images, which restricts me somewhat.

    I note that it is common to use a grad ND filter
    to reduce burnout in the sky, on "typical"
    landscape shots.

    Could I use an ND grad filter, and raw mode,
    and post-apply a correction, so as to
    increase the dynamic range of the image?

    This trick would be a pseudo HDR,
    using different exposures (via the grad filter)
    in different parts of the image.

    I could then use "normal" HDR tone mapping
    to get my desired final image.

    Comments, or hints from people that have already
    done this would be most welcome.

    bugbear, Sep 4, 2007
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  2. bugbear

    Alex Monro Guest

    Short answer: Yes.

    There are many techniques you can use to extend the dynamic range of a
    digital camera, and the simplest is probably the hangover from film
    usage, the ND grad filter, which darkens the upper part of the frame,
    which is usually the bright sky in landscape shots. You can get
    different types of ND grads, with different levels of attenuation (Cokin
    do ND2, ND4, and ND8), and have "hard" or "soft" transitions from the
    clear to dark sections - hard is probably best for compact cameras.

    There are some ND grads in circular screw in mounts, but these limit
    you to a fixed position for the transition zone, and a more flexible
    approach is probably the square type which slots in to a holder that
    in turn screws on to the lens - this allows you to slide the grad filter
    up or down to match the horizon in your composition. They can also be
    rotated to match a hillside. Even this flexibilty can be rather limited
    if your picture includes irregular mountains or a tree against the sky.

    RAW mode usually adds a few bits to the range of values that can be
    used to describe the intensity of each pixel, typically from 8 bits per
    colour (256 levels) with jpeg to 10, 12 or 14 bits (1024, 4096 or 16384
    levels) per colour with RAW - however, jpeg probably uses a different
    mapping cureve of the numbers to intensities, so you might not see that
    much difference. It will however give you more margin for boosting
    shadows when using curves adjustments for tone mapping before noise
    becomes visible.

    These are techniques I use frequently with my Fuji S9500 in RAW mode.
    I haven't tried the merging multiple shots taken at different exposures
    technique, partly because of the hassle of taking the multiple shots,
    but also because many of the examples I've seen look very "false" - the
    intensity of the highlights just doesn't look right with the brightness
    of the shadows. Doing the tone mapping with a curve from a single
    exposure allows a smoother transition of the intensity boost across
    different parts of the image.

    Some cameras, such as the Fuji S5 Pro, use special sensors with high and
    low sensitivity photodiodes to expand the dynamic range.
    Alex Monro, Sep 4, 2007
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  3. bugbear

    Cooter Guest

    The Photomatix program has a built-in function to do a tone-mapped
    pseudo-HDR from a single RAW file. It does a pretty good job, particularly
    on a slightly underexposed image. They have a restricted trial download if
    you want to give it a try. Don't remember the URL, but Google should find
    Cooter, Sep 4, 2007
  4. There are a number of techniques to extend dynamic range of an image,
    but realize that the graded ND is actually being used to compress
    dynamic range of the recorded image. Remember, any hard copy, and
    most display techniques, will not have as much dynamic range as a RAW
    image of a high contrast scene. The use of high dymaic range work is,
    as you say, to map the desired effect on the final image, which,
    unless you are using high dynamic range projection systems, must
    either compress or clip parts of the image. Which of the two you do
    is the "creative judgement".
    Don Stauffer in Minnesota, Sep 4, 2007
  5. bugbear

    Cynicor Guest


    I'm in love with this software right now. I've been playing with it and
    posting samples at http://trupin.smugmug.com/gallery/3387690.
    Cynicor, Sep 4, 2007
  6. bugbear

    bugbear Guest

    Indeed; I'm trying to avoid blown hightlights in the sky, whilst
    retaining detail in shadows.
    Yes. The only part of my idea I regard as "original" is that
    I intend to "undo" the ND grad, so as to have a "flat"
    high dynamic range image. My hope is that I can then tone map
    this (as you outline) as if it were a "flat" HDR, and
    give no further consideration to the fact that a graduated
    filter was used.

    bugbear, Sep 4, 2007
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