Histogram is a graphical display on the camera that shows the exposure. It shows the details of the image such as shadows, the extent of brightness and if any area in the image is black or white. Histograms are of great help in underwater photography as it is not easy to accurately read the exposure of the image on the LCD screen. This graphical histogram is also known as tonal range of a photo.
The tonal range of the image is graphed on the basis of its brightness value. Tones to the right of the screen are light whereas the one’s to the left are dark. There is no correct histogram technique, as different levels of brightness are displayed for different images.
The histogram is just a tool that gives the information as how it was captured.
A nice image with even exposure will display the bars on the histogram in the form of wide bell curve.
As mentioned earlier viewing histogram underwater while taking shots is very important. Hence, one must learn to display histogram next to the photo after each shot. The left side of the histogram represents shadows; the right side denotes highlights whereas the mid-section shows mid-tones. Many cameras are capable of displaying 3 histograms at once; one for red, green and blue. If the histogram is skewed to the left, the image is underexposed. To avoid this one must slow down the shutter speed. Similarly, if the image is skewed to the right, the image is overexposed. To limit this exposure one must increase the shutter speed.
A macro shot with a black background will have lots of pixels accumulated at the left end of the histogram. This is because of underexposed water which is desirable. Similarly, in wide-angle photography the black effect will be created by sun. Histogram is very important as the graph is more reliable than the eye when it comes to exposure determination. As mentioned earlier, there are red histograms which help in resolving strobe problems. Indeed a valuable setting!