If you use your camera in some automatic modes, such as Program or one of the semi-manual modes, such as aperture priority or shutter priority, you will not give full control over everything: You can still control the exposure with exposure compensation. Here's how it works.
What your camera's light meter looks like
If your camera calculates the exposure settings you want to use, this is a big assumption: when you figure it all out, what's in front of it is pretty gray. In other words, all lights and darks balance each other to a medium gray.
This is the photo that your camera is trying to capture.
And it's a pretty good rapprochement. Here are some of my average-brightness photos:
It's not perfect, but your camera thinks she's trying to take a photo of a boring gray wall for many scenes in the right park.
But not for every scene. Here are a few photos of me that were averaged on average.
This time, all scenes are a fair bit brighter than the middle gray. If this happens, if you leave your camera in automatic mode, the pictures are underexposed so that you will see about the right image, not the left, that is exposed properly.
Not ideal, as vast amounts of image data are lost in these dark black shadows. This is where the exposure compensation comes into play.
Using the exposure compensation
Exposure compensation is one way to change the exposure of the camera that the exposure meter will suggest if you do not believe it will produce the desired results. If the scene is lighter than mid-gray, you will need to overexpose the image a bit. If it's darker, you'll need to underexpose the shot.
RELATED: What is a "stop" in photography
Like everything that has to do with exposure, exposure compensation is measured in stops. A single stop means doubling the amount of light falling on the sensor. However, this does not mean that your photo appears twice as bright.
Exposure compensation is available in the aperture priority, shutter speed, and program modes of most cameras. You can also use it in fully automatic mode, but this is not guaranteed.
If you look through the viewfinder or the rear screen, an exposure compensation graph will be displayed.
 0 is the measured value of the scene without compensation. -1, -2 and -3 are one, two or three apertures underexposed, while +1, +2 and +3 one, two or three apertures are overexposed. To adjust the exposure compensation, you usually hold down the exposure compensation button (it is the half black, half white square in the picture above), and turn the primary adjustment dial, though the operation between the cameras may vary. Check the manual if you are not sure.
In the picture above I now sign a stop. This is how the different compensation values look in practice.
If things are really bright, they should be underexposed by a single stop. When things are really dark, you are overloading a stop. One or two stops of the exposure compensation are usually sufficient to adapt to each scene. I'm not sure if I ever had to use an plus or minus three exposure correction.