As most beginners and amateurs will discover, there are some curious frustrations that come along. Of these, the most common are taking a photo on a bright day of something such as snow only to have it turn out too dark or taking a picture at night and having it turn out to bright. So, why does this occur?
Unfortunately, most people believe that they can just point and shoot their camera and reproduce a photo depicting exactly what they saw. However, this is not the case when a camera is in its automatic mode. This mode makes the camera decide for itself what the proper settings should be.
Unless your camera is manual, the one you are using will have to decide how much light to allow the sensor to be exposed to. In underexposed photos, too little light has been allowed making the subjects in the photo seem almost invisible. If the photo is overexposed, this means too much light was allowed and several areas of the photo may be completely white.
Basically, your camera makes this decision based on the fact that most scenes, most of the time, are a mixture of light and dark tones that average out about the same. This average is called 18% grey. It uses this rule of thumb to work out what combination of shutter speed and aperture to use. However, this breaks down in some situations.
Consider for example what happens if you take a photo in very bright conditions, perhaps the classic example of a polar bear at the snow on a sunny day. You will want your photo to come out very bright, reproducing what you saw with your eyes. However, it is very likely that the photo will instead render the bear and snow as a kind of dirty grey. That's because your camera is applying its rule of thumb to a situation that is unusual.
The reverse situation can be explained with an example of a dark photograph. Perhaps you wish to capture the magic of a dark street lit by streetlights. You are expecting a photo that is mainly dark with areas of illumination. However, because your camera is making decisions using its rule of thumb, you are more likely to have a photo that is brown in areas that should be black as well as having lighter areas that are white or overexposed.
To capture photos as you see them, you will need to take control of the camera and no longer allow it to make its own decisions. The most common way to do this is to dial the 'exposure compensation' setting on your camera. Add one stop for light conditions and decrease one stop for darker situations. In addition, you can try putting your camera in manual mode and trying different settings. This allows you the opportunity to experiment while using your LCD screen until you find the exposure you desire.