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What is exposure compensation?

In the Aperture Priority mode (Av), changing the Aperture size would automatically adjust the Shutter Speed to ensure proper exposure. I have however noticed that in this mode, I can increase or decrease the exposure manually to make the picture brighter or darker. So my question is: if the exposure value is decided based on how we adjust Aperture, Shutter Speed and ISO values, why is it possible and what does it mean to directly change the exposure from the camera's menu?

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marked as duplicate by jrista Sep 7 '12 at 15:59

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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You are taking about Exposure-Compensation I think. Does this help? –  Itai Sep 6 '12 at 12:06
    
@Itai Yes it's exposure compensation. The article helped me. Thank you! –  Meysam Sep 6 '12 at 14:24
    
If you are adjusting this after shooting, it's probably a limited form of post processing, or re-running the conversion from the RAW, if still in the buffer, to JPEG. If the RAW is stored, it probably is unchanged, allowing you to do all the post processing better at the computer. Otherwise, your stored file may be modified. In any case this modification is not an alteration of exposure parameters, but just a change of lightness and darkness figured by programming. There is some range where this case be done in many cases. –  Skaperen Sep 6 '12 at 14:35
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See also What is Exposure Compensation? –  mattdm Sep 6 '12 at 14:44

3 Answers 3

In the AV mode the camera decides on the settings to achieve what it computes as the best exposure possibly subject to constraint on the maximum iso and on the shutter time.

This computation is made according, among other things, to the evaluation mode (that is: the camera will compute the "ideal" exposure based only on the central part of the image, or on a larger area, or on some other more sophisticated approach).

This often works wonders and in many situation is a great help which avoids to miss a shot only because you forgot that you were going from within or from without. But you can find some cases where your interpretation differs from the camera algorithm. Maybe you are shooting a dark scene and you really want to get a dark image (while instead the camera could try to expose as to achieve a lighter scene, e.g. by raising the shutter time).

So you can in this case tell to the camera: look, I really want that you under-expose this image with respect to what you would normally do, so that I can get the desired result. The same applies to over exposing of course.

Note that (often) you can only do this in the AV mode in a range of +/-2 stops. But you can also take notice of the "correct" exposure and then pass to the manual mode and "under"-expose or "over"-expose by manually changing the settings as desired. When I say "under" and "over" I am really meaning that the final judgement on the "correct" exposure is yours and depends on what you are shooting and what you want to capture.

The same reasoning applies to the Tv mode (where instead of deciding the aperture you fix the shutter time and the camera judges the best ISO and aperture settings).

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Exposure compensation (for that is its name) allows you to adjust the exposure of a shot in the semi-automatic modes in situations where you think the light meter might be reporting 'incorrectly'. It essentially alters the zero-point of the light meter.

Consider this situation. You are taking a shot of a man dressed in a light grey suit against a black background, in Aperture priority mode. In matrix/evaluative metering, the camera's light meter will look at the whole scene, and come to the conclusion that it's dark.

Since all light meters 'want' the camera to render any scene as 18% grey, it will adjust the shutter speed accordingly, and you will have a grey wall and a vastly overexposed subject (the man). Centre-weighted metering (assuming the man is in the centre of the shot) will likely return slightly better results, but it will still take the black wall into account and overexpose slightly.

With exposure compensation, you can still have the benefits of an automatically set shutter speed (or aperture in Shutter priority mode), but you can also override it in situations where you know the light meter will likely be fooled. In the situation above, you would set the exposure compensation down a stop or so, and the result would be a well exposed subject with a black background.

What I have described here is somewhat proactive. In reality you are more likely to take a shot, check it on the LCD, note that it is either under or overexposed, then set the exposure compensation accordingly and retake.

Exposure compensation systems are also often used for HDR photography, either manually by the photographer or automatically by the camera in the case of Auto Exposure Bracketing.

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So as others have pointed out, this feature is called Exposure-Compensation (EC). As far as I have figured out, this is how it works in my camera (Canon 650D):

In Aperture Priority mode:

  • If ISO is set to auto, changing EC would change the ISO
  • If ISO is set to a specific value, changing EC would change the Shutter Speed.

In Shutter Speed Priority mode:

  • If ISO is set to auto, changing EC would change the ISO
  • If ISO is set to a specific values, changing EC would change the Aperture Size.

Since nobody mentioned anything about how ISO, Shutter Speed and Aperture Size are changed based on EC values, I thought I would post what I witnessed on my camera as an answer. Exposure-Compensation might have other effects as well, of which I am not aware!

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Indeed, it does have other effects and that depends on the camera. It can control flash power too if flash is set to Auto or On. On plenty of Sony cameras for example, you can choose in the menu if EC affects Ambient (like you described) or Ambient+Flash exposure. –  Itai Sep 6 '12 at 15:12
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Keep in mind that the camera might try to avoid iso speeds too high, since that could lead to a decrease in IQ. The shutter can well go to time long enough to be not hand-holdable: in those cases you either raise the ISO, or use a tripod, or change your lights, or accept a "not perfectly exposed according to the camera" image. –  Francesco Sep 6 '12 at 15:13
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As for how the settings are changed by EC, the number set in EC is an exposure value. +1 EV = +1 stop, so if you're in aperture priority and set +1 EV, it would slow the shutter speed by one stop, i.e. from 1/125 to 1/60. And apropos of nothing, don't use Auto ISO :) –  ElendilTheTall Sep 6 '12 at 16:43
    
Thank you guys for your advice! :) –  Meysam Sep 6 '12 at 19:31

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