You may have noticed that when your camera takes a picture, graph appears in your LCD panel.
You can use the Histogram function in your camera to see if a photograph is under or over exposed, you can also take a test photo to test the exposure, also known as metering.
If your camera does not automatically display the Histogram after taking the photograph pressing the INFO button on your camera will display it. Otherwise refer to your manual.
An images histogram is an accurate reading of the images exposure. It shows the brightness values or Tones of all the pixels in your image.
The brightness levels are shown in a graph that runs horizontal from 0 to 255, or a range of 256 tonal range.
The height of each bar in the graph represents the number of pixels of a particular brightness level that occurs anywhere in the image.
Keep in mind that the graph does not represent your photo from side to side, instead it is a reading, a graph, of the brightness levels in your photograph.
On the left hand side you have 0 which represents pure black, Middle Gray is 128 right in the middle and Pure White is 255 on the far right side.
Empty space at either the left or right side usually means an image that has no bright highlights or dark shadows.
Empty space at both the left and right side means an image that has very little contrast, the image has a gray dead feel to it.
You can use your Histogram as an advanced light meter.
Generally you want to expose your image as much as possible without clipping the highlights. Meaning that you have pushed the light tones past the right side of the graph.
For the most part a rich/well exposed image will have a full range of pixels along the graph.
Underexposed images will have the graph clumped at the left side of the graph and appear dark in tone. Also notice that graph does not curve down at the left side it just stops. This is clipping, when this happens on the left details on the shadows is lost and no amount of photoshoping will bring it back.
Overexposed images clump the graph on the right side. All the info is on the right side. Notice again the clipping. There was more info, more tones that could have been captured but they were clipped and not recorded.
Normal/correctly exposed photographs should have a full tonal range, a wide distribution of the pixels along the Histogram. This means that all tones in the dark areas (the left) and the light areas (the right) has been captured. Also notice how this photo has some space on the left, this is OK. The camera captures lighter tones with more detail you don’t necessarily want or need a lot of detail on the left.
Ansel Adams photographs are known for using the full tonal range. His photographs need to be appreciated in person internet images don’t do them justice.