In film days, ISO was referred to photographic film’s sensitivity to light. In digital cameras, ISO works on the similar parameters. Light hitting the photodiode is converted into a single which is amplified. Higher the ISO, higher is the signal amplified. Today’s digital cameras have high range of ISO from ISO 100 to ISO 3200 or even higher. Higher the ISO, higher is the sensitivity of sensor to light.
This is yet another important feature to capture perfect shots underwater by adjusting light. One may think that by using the maximum ISO, maximum motion freeze and depth of field can be achieved. However, this is not advised as the best quality image is achieved at lowest ISO settings.
Using a higher ISO increases noise (grainy looking texture), reduces color sensitivity and can help in decreasing dynamic range. One can use the higher ISO when there is not enough strobe or ambient light. A higher ISO assists in achieving more depth of field and allows saving some strobe battery power by reducing strobe recycle time.
Lower ISO settings produce images that are less grainy with sharp focus and rich color saturation. So, if underwater one may use ISO 50 and 100 for close up and macro work. The lower ISO should be used most of the time as majority of underwater photography is about detailing. As far as underwater landscapes are concerned, ISO 200 and 400 is used.
There are a few guidelines one must follow while setting the ISO underwater. It is advisable, always to know the ISO settings and hence one must avoid the AUTO-ISO. Always use the base ISO like ISO 100 when there is excellent ambient light. At higher ISO settings, digital SLR cameras perform better than compact cameras.
One must understand that noise is a combined product of high ISO and low light. Hence, shooting at a high ISO in bright light conditions will give better results than shooting at high ISO in dark conditions.