Contrast is a measure; a difference between the brightest and the darkest area in an image. This is yet another issue which photographers face while transition from landscape photography to underwater photography. The contrast is resolution or difference of the photographic subject against the background of the image. Hence, contrast is all about distinguishing between the lightness and darkness or color contrast of a photograph.
Contrast is not a well versed feature when it comes to underwater photography. Many a times it can be seen that you have captured photos of good specimens, well focused and posed, but fail to standout due to their dull surroundings.
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.