Narrow beam (spotlight) vs wide beam (floodlight), vs focusing (by adjustable lens or reflector):
Beam angle is chosen to suit the conditions and activities of the planned dive.
Cave divers frequently choose narrow beam lights when diving in good visibility as these provide good illumination over relatively long distances, which is useful when navigating into an unfamiliar space where it is actually possible to see a fairly long distance ahead.
Some feel that in poor visibility, or at short range, a narrow beam is not much use, and a wider beam spreads the light over a more useful area.
Others feel that in low visibility settings a wide beam angle is distracting as it tends to reflect back to much light from floating particulates (back-scatter).
Video lighting usually requires a wider beam as the view angle of the video camera must be accommodated for good results.
In the case of very wide angle video, such as the GoPro and similar, an extremely wide angle beam is necessary.

Beam angle may be as tight as 8 to 10 degrees in extreme cases, or as wide as around 140° with a plain flat glass port.
Even wider angles are possible (exceeding 180°) with a domed port on the light, or a "test tube" light without reflector.
LED lights usually use an internal "lens" to produce a focused beam.
This actually focuses the beam by internal reflection on a parabolic surface.
Several lens options may be available for different beam angles for a given LED, but most models of light use only the one chosen by the manufacturer.

A few underwater lights have a focusing facility by which the beam angle can be adjusted.
There are two ways this can be done, depending on the basic emission pattern of the light source.
HID lights, which generally produce non-directional light, are usually focused by a reflector, which can be slid longitudinally over the light.
They produce a beam with a central bright area and a diffuse general illumination around it.
This is good for illuminating a combination of near and far areas at the same time, but not good for wide angle video, as the hot-spot will confuse the automatic exposure software and the result is generally overexposure in the hot spot and underexposure around it.

Other lights use a lens system over the front of the light, which may be focused by axial movement of the lens, which may be on a screw thread for fine control.
Narrow beam (spotlight) vs wide beam (floodlight), vs focusing (by adjustable lens or reflector):
Beam angle is chosen to suit the conditions and activities of the planned dive. Cave divers frequently choose narrow beam lights when diving in good visibility as these provide good illumination over relatively long distances, which is useful when navigating into an unfamiliar space where it is actually possible to see a fairly long distance ahead. Some feel that in poor visibility, or at short range, a narrow beam is not much use, and a wider beam spreads the light over a more useful area. Others feel that in low visibility settings a wide beam angle is distracting as it tends to reflect back to much light from floating particulates (back-scatter). Video lighting usually requires a wider beam as the view angle of the video camera must be accommodated for good results. In the case of very wide angle video, such as the GoPro and similar, an extremely wide angle beam is necessary.
Beam angle may be as tight as 8 to 10 degrees in extreme cases, or as wide as around 140° with a plain flat glass port. Even wider angles are possible (exceeding 180°) with a domed port on the light, or a "test tube" light without reflector. LED lights usually use an internal "lens" to produce a focused beam. This actually focuses the beam by internal reflection on a parabolic surface. Several lens options may be available for different beam angles for a given LED, but most models of light use only the one chosen by the manufacturer.
A few underwater lights have a focusing facility by which the beam angle can be adjusted. There are two ways this can be done, depending on the basic emission pattern of the light source. HID lights, which generally produce non-directional light, are usually focused by a reflector, which can be slid longitudinally over the light. They produce a beam with a central bright area and a diffuse general illumination around it. This is good for illuminating a combination of near and far areas at the same time, but not good for wide angle video, as the hot-spot will confuse the automatic exposure software and the result is generally overexposure in the hot spot and underexposure around it.
Other lights use a lens system over the front of the light, which may be focused by axial movement of the lens, which may be on a screw thread for fine control.
Dive light