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What does exposure compensation do?

If I take a photo with a given shutter speed, aperature, and ISO, and then take the same shot with +1EV or -1EV, what is actually happening?

Is this just a gain control on the sensor?

Can you achieve the same thing by changing ISO?

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1  
I learned something from each answer, thank you! I chose the one that most succintly captured what I needed to know. –  seanmc Nov 5 '10 at 2:57

5 Answers 5

up vote 13 down vote accepted

Exposure compensation changes the target exposure. Normally the camera is trying to work the settings to get about 18% grey (reflectance), but with exposure compensation of +1EV you are basically just saying to the camera "I want to expose this scene 1 stop lighter than the normal average."

Changing ISO vs Exposure Compensation

  • Manual shooting

    • Changing the ISO would have the same effect on exposure that Exposure Compensation would have in AUTO mode. However, so would changing your shutter speed or aperture.
  • Automatic modes

    • The camera will change either ISO, aperture, or shutter speed as needed to achieve the correct exposure, so adjusting your ISO will only change the shutter/aperture that the camera sets. Changing the exposure compensation will change the target exposure.
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EC does not work in manual mode. Some cameras let you change the EC in M mode to affect the metering-reading (which is there as a guide) but exposure does not change. –  Itai Nov 3 '10 at 13:48
    
Thanks @Itai, I didn't explain that portion very well. Hopefully my edit helps. –  chills42 Nov 3 '10 at 15:07
1  
As far as my memory goes, AEC on the Av (aperture priority) or Tv (shutter priority) modes does not change ISO, but rather the non-set parameter (shutter or aperture, respectively). –  ysap May 12 '11 at 0:49

What does exposure compensation do?

Although I agree with the technical aspects of the other answers, I still prefer to explain "exposure compensation" in lay terms as a way to impose a disagreement with the camera opinion about the shot. :o)

Every time you point your camera to a scene, it tries to calculate how much light should hit the sensor in order to obtain a well balanced (again, according to the camera calculations) picture but allowing a fast enough shutter speed so the risk of blurring it due to vibration or subject movement is minimized.

In the progam mode (P) The camera uses something like this in order to decide the settings for a scene:

  1. Set the shutter speed to something fast enough (say 1/50s) and ISO to the smallest value (say 100).
  2. Increase the aperture until there is light enough for the scene.
  3. If it reaches the largest aperture then start increasing the ISO speed (assuming it is in Auto mode, not fixed by you) until there is light enough for the scene.
  4. If it reaches the largest ISO then start slowing the shutter speed until there is light enough for the scene.

In the aperture and shutter priority modes (A or Av, S or Tv) the aperture or shutter speed are set by you and the camera defines the other two parameters using the above logic.

That means that if the scene has enough light you will end with sensible settings for the three parameters (ISO, aperture and speed) and things tend to work out well.

But most of the times things are not that pretty. Either you get too much light (beach scenes, snow, backlights etc) or too little (dark places, night shots etc) and the camera calculations starts to work against what you may consider a nice shot.

This is where exposure compensation can help you. By tweaking with it you can either explain to the camera that you want it to consider less light in the calculations (moving the compensation to negative values) or to consider more light (moving the compensation to positive values).

The bottom line is, exposure compensation is an easy way for you to influence in the camera calculations in order to make it do things closer to what you want then what it would assume was "correct".

If I take a photo with a given shutter speed, aperture, and ISO, and then take the same shot with +1EV or -1EV, what is actually happening?

Note that in order to set all the three elements (speed, aperture and ISO) you would have to be in Manual mode (M), where there is no exposure compensation controls.

OTOH, if you fix one or two of them, the rules described above apply.

Is this just a gain control on the sensor?

No, that would be the ISO. Exposure compensation is about how much light will make part of the picture, ISO is one of the ways to control that.

Can you achieve the same thing by changing ISO?

Only if you are in Manual mode, otherwise the change in the ISO would only alter the balance between the other two factors (aperture and shutter speed). In Manual mode the change in ISO would indeed change the amount of light captured, so it would impact in the brightness of the picture.

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Exposure compensation only works in one of the [semi] automatic modes (Tv, Av, P etc.) it's usually used to account for the simplicity of the camera's metering system (the camera assumes everything you try and photograph is middle grey). Using it will cause the camera to adjust the settings (shutter, aperture, ISO depending on your shooting mode) to either increase or decrease the exposure.

There are also times when you want to intentionally over or underexpose for example when shooting for HDR, or when you want to minimise noise (at the expense of highlight colour fidelity and detail) by exposing to the right (ETTR) and EC is a good way of doing this if you don't want to shoot in manual.

Basically don't think of EC as "exposure compensation" think of it as "poor in-camera metering compensation"

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Can you achieve the same thing by changing ISO?

No. Changing the ISO doesn't change the exposure, just like changing the shutter speed and aperture don't change the exposure (unless you're in manual mode). If you're in an auto mode, and you change any of those three, the camera will adjust one of the others to maintain what it thinks is correct exposure.

Stepping back a second, the only things that the camera can change to change the exposure are:

  • Shutter speed
  • Aperture
  • ISO

When you change the exposure compensation to +1 EV you're telling the camera to over-expose the image (compared to what it thinks the correct exposure is) by 1 f-stop.

Once you've told it to over-expose, it will then change either the shutter-speed, the aperture, or the ISO to change the exposure. In aperture priority the camera will decrease the shutter speed, in shutter priority the camera will widen the aperture, in automatic it could do either. If auto-iso is turned on, it might adjust the ISO. In manual mode, exposure compensation has no effect.

For example, if you were in aperture-priority, at f/4, and ISO 200, the camera might think correct exposure would be at 1/100s. If you set exposure compensation to +1EV, you're telling the camera to over expose, so its going to adjust the shutter speed to 1/50s.

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EV affects exposure in every mode but manual. It is extremely handy when working with advanced lighting systems that incorporate TTL.

Changing ISO won't make a difference, unless you push it beyond the range of the camera's ability to make a proper exposure. The camera will compensate with aperature or shutter speed (depending on your mode) countering your ISO change.

EV, in these modes, is THE WAY you control exposure. Very helpful on the fly. Some purists (myself, until recently included) think this is a cheat. Manual is the only way to fly. They are wrong.

Joe McNally, TTL super-star and award winning photog, likens this attitude to purchasing a Ferrari and driving it like a Volkswagen. TTL and EV allow you to move fast, concentrate on your subject, and deliver the same, if not better, exposure control as a full manual setup.

For more info check McNally's site.

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Thanks...now I don't feel so guilty about keeping the button on AV and constantly adjusting the EC! (On my EOS 550 there's little difference between this and manual exposure anyway; it's exactly the same set of button presses, but with their roles reversed between aperture priority and shutter speed priority.) –  whuber Nov 3 '10 at 20:44
    
Very well said. Exposure compensation is definitely the key way to control exposure. I try to avoid full manual mode unless I am using filtration or bulb mode, and use Av w/ EC most of the time. As you said, it is a great way to get quick, on-the-fly changes to metered exposure. –  jrista Nov 3 '10 at 22:36

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