Aperture is the size of the lens opening that can be made smaller or larger. The function of the aperture is to adjust the quantity of light entering the camera. Aperture functions just similar to the iris of an eye. The size of an aperture is known as F-stop. Each lens has a largest aperture like F2.8. The largest aperture allows the passage of most amount of light.
Aperture and F-stop are always in indirect proportion i.e. as aperture increases, F-stop decreases in number and the amount of light entering through the lens increases. This implies that a small F-stop is a large aperture and vice verse. This means that F2.8 is a larger opening than F22.
The transition from landscape photography to underwater photography brings forth many challenges and difficulties. One of them is color loss. When light is transmitted from air to water, water acts as a filter and absorbs large amount of light passing through it. Hence, it is said that color loss is tied to absorption. Water absorbs light wavelengths of all energies starting with red, orange, yellow and so on. These are absorbed before blue and green. Hence, many underwater photos appear bluish green.
Exposure is the total amount of light the camera lets in for any shot that you take. It is the light falling on the photographic medium while taking a photograph. Exposure is one of the most important techniques which should be mastered by an underwater photographer. Exposure adjustment controls the light intensity of the image subject and adjusts it to fall on the appropriate region to a yield a desired exposure.