On most (if not all) DSLR's and a few advanced point and shoots there is a button that has on it a +/- symbol. This symbol is used to make adjustments to the exposure of an image when you are in either the Shutter or Aperture Priority modes. Now, you might be wondering why you would need to make adjustments when you are in a type of auto mode as the ones mentioned previously. It really comes down to this: No matter how advanced or how much you spent on your camera it will on occasion need some help. Mr. Nikon and/or Mr. Canon simply cannot account for every lighting situation. Remember, when the camera tries to read a scene it wants to make sure the overall image is medium toned. That means if you are taking a photo of a bride on a bright sunny day you will end up with a grey wedding dress. Just FYI, the brides don't like that very much. How do you fix that? Exposure Compensation of course (unless you are in manual and I will explain that in a bit). Exposure Compensation allows you to tell the camera that the overall scene is either brighter or darker than what it thinks. Then, the camera knows to adjust it's shutter or aperture depending on what mode you are using.
Note: If you are shooting in fully manual this will have no effect on exposure.
Example: You are doing a portrait session at noon on a bright day. While you can't always pick the time of day the shoot happens you can control how the scene is set up. Realizing that the best way for this to be a successful shoot you find some shade to put your subject. The problem is, the background is a group of bushes being fully lit by the sun. Since you are in aperture priority mode you set your aperture to 5.6 so that you can blur the background some and snap a shot. When you look at the view finder you see that the shrubs are perfectly exposed but your subject is WAY to dark. Realizing this you push your Exposure Compensation button and move the exposure a stop or two to the + side. This tells the camera that it needs to let in more light. Remember, you are in aperture priority so the camera slows the shutter speed to allow in more light. Now, your subject is properly exposed and the bushes in the background are really bright. This would be a good one to turn into one of those high key images we discussed in an earlier blog post. In fact, I will use the same photo from that blog post to show you what I mean. I took two at the time of the photos shoot because I wanted a silhouette as well as a high key. So, take a look here:
Just for reference, if you are ever shooting at noon try and find some shade and a shaded background for your subject.
Why doesn't the exposure compensation work when you are in manual mode? Well, you are in manual mode and you are making all the decisions. Just change your shutter speed or aperture for the desired effect or exposure.
I hope this clears some things up but if you have more questions feel free to give me a call or send an e-mail.
Great examples of what that +/- button does.
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