Available light is something that alters considerably underwater and is dependent on depth, time of day and your location. Your ISO function allows you to shoot pictures in a much wider variety of available light scenarios by giving you the option to change the light sensitivity of your camera. This allows it to cope better with lower light situations. Most cameras straight out of the box will have the camera's ISO controls set to Auto. Whilst this is fine for general on-land photography, it doesn't give us the variety of control we need for underwater use. You'll need to find the ISO control on your camera. Some cameras have this as a dedicated button or dial, others locate it in the menu system, but almost every digital camera does have it. This will usually show you a sequence or scale of numbers running 100, 200, 400,800 and 1600. As you go from say 100 to 200 you are doubling the sensitivity of your camera, and every time this number gets higher, your camera is able to successfully record the subject matter in lower and lower light. If you shoot with the lower settings i.e. 100 ISO, in very low light your camera will be forced to choose a shutter speed that will be too long to freeze both yours and your subjects movement. Your camera will usually warn you if there isn't sufficient light to guarantee a safe shutter speed with a warning graphic, often of a symbol of a hand or camera shaking, usually in red. On the other hand it's not wise to always choose the highest ISO's just to err on the safe side. As whilst choosing a high ISO (800, 1600) can give you the edge in low light, it results in your pictures becoming progressively more grainy or noisy, thus diminishing the overall quality. Hints: Which ISO you use in differing circumstances is one of those things you need to find out with a bit of practice. This can easily be achieved on land, say wandering around the house taking pictures in differently lit rooms. Or seeing how steady you can hold the camera without resorting to using the flash. This will give you a feel for what works and what doesn't, and give you the confidence to take available light shots in most conditions encountered underwater.