| 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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