Introduction

Take a close look at the clownfish photos below. (Click + to expand them.)

Both were shot while snorkeling in shallow water, around three feet (one meter) deep. The light is dim-the sky might be overcast or the sun close to setting. We shot the photo on the left using the camera's built-in strobe; we shot the photo on the right using an external strobe added to the housing. All other conditions were the same.
Given a choice, most people prefer the photo on the right for its vivid colors and clarity. The external strobe, built specifically for underwater use, is much better at bringing out the natural colors of the subjects. You can aim its stronger light to fill in the colors that water removes when it filters sunlight.
Can you take good photos without an external strobe? Usually not. If your subject is at the top of a sun-drenched coral reef or in brilliant blue shallow water, you might get by. But most scenes look much better illuminated by a strong external strobe.

The photos above were taken deeper, at roughly 50 to 65 feet (15 to 20 meters). The difference here is much more obvious, because deeper water filters out much more light and many more colors. The photos on the left were taken using natural light without a strobe. They're dark, saturated with blue, and show very few details. The photos on the right were taken using an external strobe that brings out the color and details of the fish and soft coral.
We recommend that you start using an external strobe as soon as possible. It's not difficult to use, and you'll get better results immediately. There's nothing like sharing beautifully lit images after a dive to make underwater photography more fun.

Which photo do you prefer?

Both photos were taken while snorkeling in shallow water, again in somewhat dim light. We shot the photo on the left without a wide-angle lens; the photo on the right with a wide-angle lens. All other conditions were the same.
Most people prefer the clarity and detail of the photo on the right.
So why does the wide-angle photo look closer, clearer, and more detailed? Doesn't a wide-angle lens make things smaller to fit more of a scene into the frame? Yes-if you shoot in the same position where you were using a standard lens. But the secret of a wide-angle lens is that it lets you get closer to your subject while keeping the subject the same size. The background shrinks in comparison, emphasizing your subject.
Getting close to your subject is one of the most important rules of underwater photography. The further you are from the subject, the more water you have between the subject and your camera. Water absorbs light and dulls detail and color. With a wide-angle lens, you can get in right next to your subject and still fit it into the frame. A wide-angle lens has a larger depth of field than a regular lens, so more of the subject and the background remain in focus. And when you use a strobe with a close wide-angle lens, the strobe's light travels through less water, so its effect is stronger, and the wide-angle lens can gather more of its light.
So, in a nutshell, use a wide-angle lens whenever possible to get closer to your subject. You have less water and fewer floating particles between your subject and the camera, the subject stands out against the background, and the color and details pop out in your photo. We'll go into more detail later about wide-angle photography and getting close to shoot.

Wide-angle lenses have an additional advantage: they can capture light gradation that a regular lens might not pick up. For an example, look at the photos below:

We took the photo on the left without a wide-angle lens; we took the photo on the right with a wide-angle lens. All other conditions were the same for both photos. As before, you can see that the wide-angle photo shows more color and detail. But if you look at the light above the coral in both shots, the wide-angle photo shows a gradation of light in an arc where the sun hits the surface of the water. It adds an extra feeling of depth and drama to the photo.
We'll explain this in more detail later, but for now it's enough to know that a wide-angle lens can capture the gradation effect unique to underwater light where a regular lens might not.
Now that you've seen the benefits of two fundamental yet simple underwater camera accessories, let's try everything out and take our first underwater photo.