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.
To solve this issue one needs to work on lighting by lessening the depth of field. At the same time one needs to darken the background and highlight the subject.
A photograph with very low contrast is known as flat and the opposite image will be the one with very high contrast. In the case of low contrast image, the histogram will be centered in the middle with no shadows or definite highlights. Similarly in case of high contrast image, the histogram will feature spikes at either ends with no gradient transition in between them. Hence, one can control the contrast of the image by the histogram and adjust the settings.
The cameras focus by finding the distance with the largest contrast difference and this is done within the focal point. Hence, one must make sure that the camera is focusing on the area with strong contrast difference, bright against dark. One can also press the shutter down half-way. By doing this the focus will be locked in case there is a bright, high contrasting subject nearby in the same proximity. This will also lock the focus on high contrast subject allowing you to recompose. If the camera has a ‘spot focus’ mode, then the live view must be over an area with strong contrast.
Besides this there are many cameras available which add contrast to the images by default or one can also adjust the contrast by using levels dialog box in Photoshop.