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Camera zone focusing is a technique to help you
take pictures with more convenience, accuracy and speed. It uses depth
of field controls to establish a zone of sharpness in which any subject
can be photographed without further attention to focusing; these
controls are coordinated by the depth of field scale built into most
lens barrels. Let's study some example of zone focusing to better
understand how it works.
Suppose you find yourself standing on the sidelines as a parade goes
by. You have a 35mm camera and a 50mm lens. The day is bright and the
sun is in the right direction. Everything seems ideal for effective
photography. However, there is one problem - focusing. The parade is
constantly moving, so the distance to any one person or things keeps
changing. Besides that, when the high school bands or rows of horses
and riders go past, the nearest person is only about 3m (10ft) away,
but the farthest is about 15m (49ft) away. Is it possible to get both
ends of a line sharp in the same picture? And if so, where do you focus
to do it?
This is a typical situation in which zone focusing can make your life
easier. The first thing to do is to consult your depth of field scale.
If your ISO setting is low, and yet you require high shutter speeds to
stop the parade action, you have been shooting been shooting at f/8. By
placing the near distance you want sharp (3m, or 10ft) opposite f/8 on
the scale and then checking the far distance against the other f/8
mark, you determine that the zone of sharpness extends only to about
8.5m (28ft). That is not enough! But by placing the f/11 mark opposite
3 meters (10 feet), you see that the far distance is now well beyond
your requirements. Good- except that f/11 calls for shutter speeds that
are too slow, so this solution is also unworkable.
The three prime factors controlling depth of field are focal length,
aperture and distance. You can't change focal length, since you have
only one lens with you. You have explored the possibilities of changing
aperture without success. That leaves just distance to help you. You
check your depth of field scale again and see that if your preferred
f/8 lens stop is set against 5m (16 ft) as the near distance instead of
3m (10 ft), you have a zone of good focus from 5m almost to infinity.
That's the answer! You change your position so that you are 5m from the
nearest subject, keep your aperture at f/8 and set 5m opposite the f/8
mark on subject, keep your aperture at f/8 and set 5m opposite the f/8
mark on your depth scale, thereby selecting the point of focus
automatically. Now without touching the focusing ring again, you can
shoot picture after picture with ease and convenience knowing that
everything of importance will be in focus.
Another situation calling for zone focusing might arise when you are
taking a picture of your house with a 28mm wide angle lens. you want
not only the house, but the tree in the yard and the white picket fence
as well to be in focus. How can you make sure all these things will be
sharp at once? From the position you have selected for the camera, the
nearest part of the fence is only 1.8m (6 ft) way, while the far end of
the house is more than 30m (100 ft) away. This range would be difficult
for most normal lenses to handle, even at the smallest aperture, but a
28mm wide angle lens ahs more inherent depth of field. Checking the
depth of field scale on the 28mm lens, you find that even at f/5.6, you
can adjust the lens so that the zone of sharpness extends from about
1.8m (6 ft) to infinity - more than enough to ensure accurate coverage.
At smaller stops, depth is even greater.
In situation calling for quick response in making an exposure, zone
focusing is even more useful. Imagine yourself at a basketball game.
You have brought a 135mm lens in order to get close up action at one of
the baskets and the fast action. However, you want to avoid focusing
every time something interesting occurs. Things will be happening too
fast for that, and you will lose many good shots. Zone focusing is the
answer.
Consulting the depth of field scale on your 135mm lens, you find (by
trying various combinations) that setting 10m (30ft) opposite the f/11
mark on the short end of the scale produces 15m (50ft) on the long end.
Armed with this information, you take a seat 10m from where the closest
action will occur, set your lens at f/11, and set 10m against the f/11
mark on the depth scale. Now without touching the focusing ring, you
can shoot action around the near basket as fast as you please, knowing
you have 6m (20 ft) zone of sharpness in which to work.
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