Now that we are in the digital age, how can you tell if your picture is exposed properly? In years gone by, you would have to rely on your light meter and experience and hope for the best in the darkroom. There were various things you could do in the film room to help yourself out in a pinch when mistakes were made. Today, we have different methods for review and confirmation that our photos came out the way we intended them to.
Introducing Hissy the Histogram. Hopefully your camera has something similar to it. This is a quick and dirty guide to give you some information about the exposure of the image you just took. You might have to read the manual to learn how to activate this screen on your LCD first. Once you've got that down you can begin learning a little bit about what it means. Think of the Histogram as a graph comparing two different concepts, one on the horizontal axis and one on the vertical axis.
The Horizontal axis is the relative brightness of the image; dark being on the left side of the graph and light being on the right side of the graph. If any part of the image is too light you will see that represented on the last pixel on the right side of the graph. The same concept in the reverse applies to underexposed parts of the photo. The vertical axis of the graph represents the relative amount of pixels found at that brightness. Taking the blue graph as an example, you can see two blue humps in the graph. This means there are more of those two 'brightnesses' in the photo than there are any other brightness. Overexposure is represented as the last pixel. You will remember that after you get past your camera's dynamic range, everything will appear white or black.
The higher end cameras will show four separate histograms. One for the overall exposure, one for the green channel, the red channel, and the blue channel. Lesser cameras will show only the overall exposure of the image. The overall exposure is less useful than the broken down histograms but it should only be treated as a rough estimate only.
Photos can be exposed in any way you want them to be exposed. Just because you have a majority of pixels underexposed so much that they're outside the dynamic range of the camera (or so many overexposed that they're also outside the dynamic range) it doesn't mean the photo is bad. High key photos, for example, will have a lot of overexposure. As long as the photo came out the way you wanted it to, that is fine. The histogram is just a tool in your tool bag. It isn't gospel and shouldn't be treated as such. That said, it can help you take better exposed photos if you're just starting out.