changing ISO vs exposure compensation

Discussion in 'Digital Photography' started by Allan, Dec 2, 2004.

  1. Allan

    Allan Guest

    Hi

    I think this was discussed in part a while back but I would like another
    review.

    On my camera (Panasonic LC70) I can change the ISO setting from 50 to 400. I
    can also set the exposure compensation +/- 2 stops.

    For a given light condition, why would I choose one over the other?

    cheers

    Allan
     
    Allan, Dec 2, 2004
    #1
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  2. Allan

    Owamanga Guest

    Changing just the ISO from 50 to 400 will still give you a properly
    exposed picture (assuming shutter priority, aperture priority or
    program mode), based on the cameras internal metering.

    Modifying the EV compensation will under or over expose your picture
    from what the camera decides is 'correct'. So they do different
    things.
     
    Owamanga, Dec 2, 2004
    #2
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  3. Allan

    Jim Townsend Guest

    It will work, but you might run into problems.

    If you're in a condition where the aperture is already wide open, then your
    shutter speed has to give to get the compensation. +EC will make it stay open
    longer and increase the chance of subject motion blur, or camera shake.

    With moving subjects, if you want sharp pictures, you'll probably have to
    bump up the ISO.

    I haven't done this since I owned a Canon S100 which was fully automatic like
    your LC70. (Except I couldn't change the ISO)

    I found I could get better low light shots by selecting an exposure compensation
    of +2, but these were stills such as city scenes etc..
     
    Jim Townsend, Dec 2, 2004
    #3
  4. Changing the exposure compensation will only change the exposure.
    So - if you expose at +2 stops - the picture will get brighter.

    But if you change the ISO setting, then you will change both
    the exposure and the amplification before doing A/D conversion.
    So - the picture will stay the same brightness.



    /Roland
     
    Roland Karlsson, Dec 2, 2004
    #4
  5. Allan

    eawckyegcy Guest

    Try groups.google.com; it works great.
    www.google.com: false dichotomy
     
    eawckyegcy, Dec 2, 2004
    #5
  6. Allan

    Tom Ellliott Guest

    I think the difference would be:
    1: At asa100 an exposure compensation would give you the low grain/noise of
    100 with limited depth of focus
    whereas
    2: At asa 400 would give you more grain/noise but greater depth of focus.
    If you do not need a lot in focus then open up the fstop.
    If you DO need a lot in focus and can live with the grain/noise then up the
    asa to 400.
    IMHO the above is the answere to your question, that is if I understand the
    question.
    In any event do a test, look at the results and then decide.
    Have fun.
    Yours truly,
    Tom Elliott
     
    Tom Ellliott, Dec 3, 2004
    #6
  7. They do completely different things.

    - You use exposure compensation if you for some reason feel that the
    exposure computed by the built-in light meter is "wrong". For
    instance, if you are shooting someone standing in the shadow in a
    otherwise strongly lit scene, you may use +2 EV to brighten the
    motif (your motif will be correctly exposed, but your background
    will be overexposed).

    - You use ISO adjustment to be able to shift your cameras sensitivity
    to light. For instance, you're shooting an indoor game. At ISO
    100, with your lens wide open your "correct" shutter speed works
    out to be 1/50th second - this is too slow to stop action. If you
    change to ISO 400, your "correct" shutter speed becomes 1/200th
    second - which is much better. Your motif will be correctly
    exposed when you use this approach.

    What seem to confuse a lot of people, is that they can achieve the
    same /effect/ on aperture and shutter speed by using exposure
    compensation insteda of changing ISO. In the indoor game example -
    if you set exposure compensation to -2 EV at ISO 100, you would
    also get the shutter speed shifted from 1/50th to 1/200th second.
    However, in this case the motif would be /underexposed/ by two stops.
    By using adjusting the ISO, you get the same shutter speed, but
    the correct exposure.
     
    Gisle Hannemyr, Dec 3, 2004
    #7
  8. Allan

    JPS Guest

    In message <>,
    .... not to mention the fact that at the same aperture and shutter speed,
    the exposure on the sensor is the same regardless of the ISO setting.
    The ISO setting in that case only affects how the data is amplified
    before it is fed to the analog-to-digital converter. You get more
    readout noise at higher ISOs, but you also get more usable luminance
    levels for your image, making a fuzzy compromise that usually favors the
    higher ISO (unless the camera design is weak). The Canon 10D and 300D
    cameras switch to lower ISOs internally, with the output compensated, at
    their highest ISOs, presumably so that the simple rule of "a higher ISO
    is better than under-exposing" works in practice. Of course, the lower
    ISO may be better if you want to capture specular highlights or other
    areas of the scene that are much brighter than the majority of the
    frame.
    --
     
    JPS, Dec 3, 2004
    #8
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