Exposure is one of the most important factors in taking a photo. If your exposure is wrong then you will have a very tough time getting satisfying results.
Exposure is essentially the light that is allowed onto the sensors. When film was the main medium of capturing photos, exposure was the light that was allowed to reach the film. Many of you may remember having to put the film in the camera under a blanket or in a darker place in order to not expose the first few pieces of the film.
Exposure with digital SLRs and regular digital cameras can be controlled in many different ways. When shooting on my own I tend to control exposure first by shutter speed. By allowing the shutter to remain open longer there is more time for light to expose what you are capturing. And on the same token, by speeding up the shutter speed you are limiting the light that hits the sensors. It just depends on whether you have too much light or not enough. Note: I change the shutter speed to help with exposure when I am in fully manual mode.
Another way to change the exposure of a picture is to alter the ISO. By increasing the ISO you will increase the sensitivity of the sensor. Yet you are also risking a decrease in quality of your shots. The only times I have increased my ISO has been at indoor events where I am not too concerned about top quality work and I am just trying to capture some snap shots. It is good though to be prepared that your photos will not be as good as they could.
The most simple way to change exposure on the digital SLRs is to use the exposure compensation setting. For my Canon Rebel I am not able to use this setting in Manual mode. But in Aperture priority or Shutter priority modes I am able to change the exposure compensation. There is a scale on the camera's LCD screen that shows where the exposure compensation is set. If I feel the pictures are too dark I am able to highlight the exposure compensation and turn a dial to make set it the way I would like. For a photo that is too dark I can dial the exposure compensation up to +1 or +2, etc. this then will expose the shot one step more than it was shot at. Many times when I have not set the exposure compensation the camera will have a blinking bar at where it recommends one to set it.
Below is a picture that I had to work with the exposure on. Initially upon taking the photo I did not have my camera set the right way to expose the shot the way I would have liked. Therefore I had to go to all of the pains of working with photoshop to try and get the exposure fixed. The short of the story is... it's better to take the photo with the right settings for exposure than it is to try and fix it later.
Exposure is one of the assets of being a good photographer. Many photographers sacrifice the quality of their photos in order to make them properly exposed. Yet there are many different ways to change the exposure of a shot without mutilating the quality of it.