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Can you use autofocus in doing video underwater or is it best to preset the focus?



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Questions and Answers (Q & A's)

From Alan Broder (from Ocean Realm Magazine - January 1994)


That depends on the particular camera and housing you’re using and what you’re doing with your video camera. In general, most underwater video is done in the wide-angle mode and most video housings are designed to be used primarily for this type of shooting. Therefore, they’re equipped with built-in wide-angle lenses and dome ports. When you’re videotaping divers, a wreck, a good-sixes portion of the reef, or a similar-sized picture area, you’ll find that you have an incredible depth of field—something like from several inches in front of the camera out to maybe twenty feet or more away. Presetting the focus for this sort of business is practical. Older housings may require that you choose between preset focus or autofocus before entering the water. If you’re considering buying a housing which does not offer control for switching from auto to manual focus, I’d stop considering it.

Autofocus concentrates on a small part of the center of the frame, and if your subject moves out of the area of the finder with the focusing sensor, the camera will go into search mode and your subject will go in and out of focus until the chip is satisfied that is has found something to focus on. If what is found is at a similar distance from the camera, all is well. But if autofocus locks onto something much closer or much further away, your subject obviously won’t be sharp. When a fish swims in front of your camera, the autofocus will try to grab it, and as soon as it swims out of the frame, it will be searching again. If you’re shooting over a reef, whenever the reef occupies the center spot, the focus will lock onto the foreground. If you raise the camera a hair, or if there is a dip in the reeftop as you are swimming or panning, the autofocus will go looking for Sputnik.

This isn’t fun! When you’re doing wide-angle, it is generally best to use the autofocus to acquire focus—not to maintain it. When you’re settled in the water, find an object about two or three feet from the camera, switch into autofocus and allow the image to become sharp, then witch back to manual focus. You’re now "zone focused" and can do most general wide-angle video taping without refocusing.

When you’re using a longer focal length, as with a dome or flat port and no secondary wide-angle lens attachment, or have zoomed in, you will have much less inherent depth of field, and focus will, therefore, be more critical, You may still elect to zone focus, although you’ll have to be careful to maintain the cameras-to-subject distance within stricter limits. You could try to "follow focus" by using the manual focus control, if one is supplied with your particular housing, to maintain focus as the subject’s relative distance from the camera changes. This is very difficult and generally requires a great deal of practice. You probably can’t do it unless you can play the accordion. Since, in this kind of situation, at least some part of the subject is more likely to remain in the center of the frame, autofocus is a definite option.

What if you decide to zoom in on the subject? In wide-angle, you have a lot of depth of field. You have less at the longer focal length that you’re zooming to, so that when you’re zone-focused, you’ll almost certainly see the sharpness go to hell when you zoom in. You have two practical options. You can—before you make the shot—zoom in, kick it into autofocus, acquire focus, then zoom out. Now you do the actual zoom shot. This is probably the best method if you have time for a "dry run" first. If things are happening too fast for a dry run, it’s probably best to kick in the autofocus and do the shot, keeping the subject in center frame.

Suppose the subject decides to zoom in on you? You’re videotaping on the reef. You see a couple of vague shark-like figures off in the haze. You slowly approach. As you close the distance, you can see them clearly. Wow! They’re—like—Sharks! They see you clearly too. They’re kinda arching their backs—and swimming real weird, like with exaggerated swimming movements. You move in closer. Ooopps! Now you remember what all that arching and funny swimming is about. They’re going to close the distance in a flash! No time… to get…the focus…out…But you’re "Maximum Video Diver"! You’re there to video, and video you must!

Now, I know exactly what’s going through your mind right now: you’re thinking that you can follow focus those babies in! No way! Put that sucker in autofocus! This is going to be your last scene—don’t blow it! Even though the action is going to be real fast, if you can hold the sensor on something with some contrast—like some jagged teeth, for instance…Anyway, if they find the housing and view the tape, everything should be—ah—tack sharp!

Some of the latest cameras have an autofocus in macro feature. If you intend to do macro, you’d be wise to buy one of these models. Using autofocus, you’ll be able to get some good macro sequences, even of moving fish. If you don’t have an autofocus in macro feature, you can try using a diopter in front of the lens in autofocus mode. Manually focusing in camera’s which don’t autofocus in macro requires that you follow focus with the zoom control.. Even if you can play "Flight of the Bumble Bee" on the accordion with one finger—you can’t follow focus in macro!



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