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.
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.
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.
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.