By Jim Church
The purpose of this lesson is to help you decide when to use autofocus, manual focus or a combination of the two for zone focusing.
Autofocus works best when you have a contrasty subject that stays in the center of the viewfinder. As examples, imagine you are shooting close-up views of a lionfish, hermit crab or a Barracuda's front teeth. In each case, there is enough contrast to maintain focus even if the camera to subject distances change slightly during the shot. Generally, I use autofocus for most close-up subjects. With many fish, I try to keep the eye centered.
With moving subjects, always keep a contrasty part of the subject in the center of the viewfinder. If your subject is a swimming shark, for example, keep the front end of the shark centered so the autofocus 'sees' contrasty gill slits and fins. (If, however, the shark is small and far away, autofocus usually can maintain focus.)
With Cayman's stingrays, again try to keep the edge of a wing, eye or other contrasty part of the ray in the central picture area. With downward shots, having part of the ray's body overlap the sand bottom creates contrast. With upward shots, keep the ray's mouth and gill slits centered because its smooth belly can fool the autofocus.
My favorite autofocus technique is to 'follow a path.' Imagine you are descending into the engine room of a sunken vessel. The light is dim and there are many dark areas that won't show enough contrast for autofocusing. In this situation, look for a contrasty line; such as a handrail leading down a ladder or a beam; and keep it near the center of the picture. The handrail or beam is your 'path.' On the Balboa, off Grand Cayman, use the wreck's beams and hull plate seams as your path. In California kelp, use the graceful kelp stalks. For a shark resting on a sandy bottom, follow the edge of the body so the autofocus will respond to the contrast between the sandy bottom and the shark's body.
Using Manual Focus:
Choose manual focus if your subject lacks contrast and the camera to subject distance is relatively constant. Because the depth of field is so great with ultra wide angle lenses, some underwater videographers preset the focus and don't change it during the dive. For close-up shots, some use manual focus for subjects with irregular surfaces that could confuse autofocus.
Zone focusing gives you the advantages of auto and manual focusing. The idea is to focus for a basic distance. You use autofocus to focus on a target, such as a coral head, a diver ten feet away or even one of your fins. Then switch to manual to lock the focus at this distance. If you focused for approximately ten feet, select subjects within a zone of about 8 to 15 feet. If you wish to shoot at a different distance, repeat the procedure to establish a new zone.
Zone focus for pans. For example, assume your main subjects are two divers, about six feet away and five feet apart. You wish to pan from one to the other. If your camcorder is set for autofocus, it will search for correct focus when you pan across the mid-water background and will jump back into focus when you pan to the second diver. In this situation, zone focus for the first diver and then do the pan.
When you have both near and far subjects in the picture, the one that produces the greatest contrast in the central picture area determines the focused distance. If you shift your aim while using autofocus, the focus can jump back and forth. Thus, it's usually best to zone focus for the dominant subject.
Bubbles and Scratches:
Autofocus can focus on bubbles or scratches on the lens port or an external filter. This can happen if you aim the camcorder at a subject with less contrast than the bubbles or scratches. To break the focus on the port, put your hand in front of the port. The autofocus will shift from the port to your hand; when you move your hand away it will search for a contrasty subject.