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Thread: Exposure Compensation vs. ISO setting

  1. #1
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    Exposure Compensation vs. ISO setting

    When using a digital camera, what is the actual difference between exposure compensation and changing the ISO setting?

    E.g. if the metered exposure at ISO 200 is 1/1000th second and f/8, and I want to overexposure by one stop, I would just change to 1/500th second. On the other hand, changing the ISO setting from 200 to 100 would do just the same.

    So, my question is: what is the difference between these to options (if we don't think about depth of field, motion blur, and that kind of things, just the image quality)?

    Is the camera sw somehow handling the cases differently? Does it matter, if RAW or JPEG is used?
    Last edited by jni; 25th May 2008 at 10:08 AM.

  2. #2
    SRH's Avatar
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    Re: Exposure Compensation vs. ISO setting

    jni, your question is the same exact question I was researching when I found this excellent site, and yes, the camera handles EC and ISO very differently, depending on the camera settings.

    Changing ISO amplifies the electronic signal generated by the sensor for a given amount of light.

    EC changes the amount of light that reaches the sensor (when ISO is set to a fixed value)by either changing the exposure or changing the aperture.

    If you have your camera set in full "auto" mode, there may not be a difference between the 2 compensations functions, as my E3 will adjust ISO (in auto) to achieve exposure compensation, over a range of ISO set in the menu, but for fixed ISO, or outside that range, it adjusts speed or aperture, depending on the shooting mode.

    So, what difference does it make? In most circumstances, increasing the ISO setting introduces more noise, whereas slight underexposure can be corrected in processing with minimal quality effect.

    In McQ's tutorials, there is a better explanation of ISO (Noise, part 1) and EC (Camera metering).

    JPEG vs RAW? Depends on your JPEG algortihm and settings. Both options allow you to correct for exposure or noise, but RAW has more original data, so you stand a better chance of correcting defects. Again, McQ has a nice tutorial on RAW.

    (Just a note on your example -- to overexpose by one f stop by adjusting ISO, you would have to change from 200 to 400.)

  3. #3
    xeliex's Avatar
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    Re: Exposure Compensation vs. ISO setting

    SRH, well said.

    I still find it kinda strange that the E3 compensates with ISO versus shutter speed / aperture if ISO is not fixed.

    I wonder if other cameras do the same in full auto...
    Last edited by xeliex; 28th May 2008 at 09:52 PM. Reason: I wonder

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