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Thread: How does exposure compensation work?

  1. #1
    Senior Member erictan8888's Avatar
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    Default How does exposure compensation work?

    hi,

    i understand a bit about shutter speed and aperature and ISO....
    fast shutter speed close fast, thereby do not allow much light in...
    slow shutter speed close slow, hence allow more light in...

    big aperature allow more light in (shallow DOF) and vice versa


    but when we set exposure compensation of +0.3 or +0.7, we are compensating the brightness, meaning that we want to take care of the fact that we think the picture is going to be underexposure, so we dial in +0.3 or +o.7 for that matter....

    question here is:
    what does the camera do when we dial in a +0.3 (or any other values) for exposure compensation?

    the shutter speed, ISO, aperature all unaffected, so how does the camera make the picture "brighter" when we dial in a +0.3 ?

    thanks
    Hope to learn from everyone here....

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    Under DPreview's glossary, there's a detailed article on exposure compensation.

  3. #3

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    exposure compensation is not for manual mode. this is the mode where shutter, aperture and iso is fixed. but in the other modes, at least one of these can be changed.

    in programme mode, the camera just picks a shutter/aperture combination that will take in more or less light. e.g. if the camera would normally shoot 1/100, f5.6 and you dial in +1.0 ev, the camera would choose something like 1/50 f5.6 or 1/100 f4.

    if you use exposure compensation in shutter mode, the camera will open or close aperture more than the meter tells it to. in aperture priority mode, the camera will adjust the shutter speed up or down.
    Last edited by pai; 27th December 2004 at 03:34 PM.

  4. #4

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    As what pai said, exposure compensation only works on one of the Auto mode:
    Full Auto, Program Mode, Shutter Priority or Aperture Priority
    where your camera has to determine the exposure.
    Eg. If you shooting SNOW scene. 90% that your camera's metering will be wrong. Why? Your metering will try to capture as much detail as possible. But in a snow scene, most of the parts will be white in colour and your camera tends to UNDERexpose it. So to say, your white snow become gray snow. To get it corrected, use the positive (+) compensation.
    It applys to any situation where midtone is hard to determine.

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    Moderator catchlights's Avatar
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    Exposure Compensation should work on all mode, it just like changing you ISO setting, so it tell the camera meter to give a + or - to the current reading.

    Most pple thinks it works on Auto or P mode only, it because this is the only way to overwrite the current meter reading.

    Let say you use a ISO 400 film, and decide to rate it at ISO 200, either you set ISO manualy at 200 or leave it at DX and give +1 EV.

    Hope this help.

  6. #6

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    Quote Originally Posted by erictan8888

    the shutter speed, ISO, aperature all unaffected, so how does the camera make the picture "brighter" when we dial in a +0.3 ?

    thanks

    when you dial the EV on creative modes expect manual mode, the camera output will be affected. on Aperture mode your shutter will be 1/3 stop slower to allow 1/3 more light in, making your pic 1/3" brighter than your camera metered mode. same for shutter mode, your aperture will be 1/3" faster to allow more light in.

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    Senior Member erictan8888's Avatar
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    in other words, for EV value of zero, the aperture and shutter speed that the camera has chosen is meant for that environment and lighting right?

    so if i dial in a +1, then the camera will adjust the shutter speed slower by one stop OR adjust the aperture wider by one stop right?
    last time i was still wondering how the camera make the picture brighter when i dial in a +1 for EV value.... thought the camera had some other ways of allowing more light in.... hee hee

    ok.. think i understand liao... .thanks guys.... you have been very helpful....
    Hope to learn from everyone here....

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    Moderator catchlights's Avatar
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    Quote Originally Posted by erictan8888
    in other words, for EV value of zero, the aperture and shutter speed that the camera has chosen is meant for that environment and lighting right?
    Er... yes, meter reading as it is.

    Quote Originally Posted by erictan8888
    so if i dial in a +1, then the camera will adjust the shutter speed slower by one stop OR adjust the aperture wider by one stop right?
    Yes also, speed or aperture, depend what mode you are on.

  9. #9
    Senior Member erictan8888's Avatar
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    oh... so if i dial in a +1 for EV, and i am on aperture priority, then the camera will adjust the shutter speed by one stop and NOT aperture by one stop right?

    on the other hand, if i am on shutter priority, and i dial in +1 for EV, then the camera will adjust the aperture wider by one stop izzit?

    thanks
    Hope to learn from everyone here....

  10. #10
    Moderator catchlights's Avatar
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    Quote Originally Posted by erictan8888
    oh... so if i dial in a +1 for EV, and i am on aperture priority, then the camera will adjust the shutter speed by one stop and NOT aperture by one stop right?

    on the other hand, if i am on shutter priority, and i dial in +1 for EV, then the camera will adjust the aperture wider by one stop izzit?

    thanks
    Yes, correct.

  11. #11
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    Quote Originally Posted by erictan8888
    in other words, for EV value of zero, the aperture and shutter speed that the camera has chosen is meant for that environment and lighting right?
    Hi Eric,

    Some nice reading for you.. Once you understand how camera does the metering, then proceed with the compensation topic.. Hope it helps!

    Basic Camera Metering..

    http://www.photozone.de/4Technique/metering.htm

    Metering Compensation

    http://www.photozone.de/4Technique/ec.htm

    Cheers!
    Des

  12. #12

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    Quote Originally Posted by catchlights
    Exposure Compensation should work on all mode, it just like changing you ISO setting, so it tell the camera meter to give a + or - to the current reading.

    Most pple thinks it works on Auto or P mode only, it because this is the only way to overwrite the current meter reading.

    Let say you use a ISO 400 film, and decide to rate it at ISO 200, either you set ISO manualy at 200 or leave it at DX and give +1 EV.

    Hope this help.
    I think there is some ambiguity here. From what you wrote here it would appear that, according to you, automatic exposure compensation should work in manual mode?

    Before I proceed further, I take it to understand that the original question refers to a mechanism, dial (or equivalent) whereby one can set the amount of "compensation" (either plus or minus) over or under the meter gives in term of a "correct" shutter speed and aperture, with a set ISO.

    If that is the case, then exposure compensation cannot be used in manual mode. Changing the ISO speed is not the same as "exposure compensation". Exposure compensation instructs the camera to give more or less exposure to the metered reading. When one sets the aperture and shutter speed manually, the camera cannot overide the manual settings. To think of it, if one wants to use manual mode, why should one bother with "exposure compensation' when one could just adjust the aperture or shutter speed easily?

  13. #13
    Senior Member erictan8888's Avatar
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    thanks guys for your advice and links....
    Hope to learn from everyone here....

  14. #14

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    Quote Originally Posted by student
    I think there is some ambiguity here. From what you wrote here it would appear that, according to you, automatic exposure compensation should work in manual mode?

    Before I proceed further, I take it to understand that the original question refers to a mechanism, dial (or equivalent) whereby one can set the amount of "compensation" (either plus or minus) over or under the meter gives in term of a "correct" shutter speed and aperture, with a set ISO.

    If that is the case, then exposure compensation cannot be used in manual mode. Changing the ISO speed is not the same as "exposure compensation". Exposure compensation instructs the camera to give more or less exposure to the metered reading. When one sets the aperture and shutter speed manually, the camera cannot overide the manual settings. To think of it, if one wants to use manual mode, why should one bother with "exposure compensation' when one could just adjust the aperture or shutter speed easily?
    ya. exposure compensation in the form of a one-action-push-button-method becomes an invalid/redundant factor when we are in manual exposure mode.

    however on most modern electronic SLRs, the meter is still active even when in manual exposure mode. thus if are in manual exposure mode and have set any particular combination of aperture and shutter speed, on the LCD it will still show by how many stops the valuse that u set differs from the metered readings.
    Last edited by user111; 27th December 2004 at 09:29 PM.

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