Autofocusing is one of the best things that's ever happened to the SLR
camera. I didn't think so while testing early examples when the AF SLR
era began back in 1985, but a lot of progress has happened in the ensuing two
decades. Today's AF SLR cameras, film and digital, will focus more quickly
and accurately than most photographers can. Naturally, the higher-end pro models
outperform the lower-priced models, but even the entry-level AF SLRs can handle
a wide range of shooting situations. Here are some tips to help you get the
most out of your SLR's AF system.
1. Complex Subjects
The first AF SLR cameras had a single AF "hot" area, indicated by
a frame or brackets in the center of the viewfinder. To focus, you aimed the
AF frame at the portion of the subject you wanted the camera to focus on, then
pressed the shutter button halfway to activate the AF system. The camera would
then focus on whatever appeared in the AF frame. If you wanted to compose with
the subject somewhere besides dead-center, you had to use the AF lock, then
recompose as desired. With most cameras, the focus would lock once established,
as long as you kept the shutter button depressed halfway; with others, there
was a separate AF-lock button.
photographing a large bird in flight, the camera will focus on
the near wing tip, leaving the rest of the bird out of focus.
The solution here is to use spot AF to focus on the bird's
head--or focus manually. While an image with a sharp head
and an out-of-focus near wing looks better than an image with
a sharp near wing and an out-of-focus head, the image will look
better still if the entire bird is sharp. So it's a good
idea to stop the lens down to increase depth of field (if possible,
given the light level and ISO speed in use) for this type of shot.
Photo by Mike Stensvold
Today's AF SLRs have multiple AF areas--up to 45 in some pro models.
These provide a wide "hot" focusing area that makes normal shooting
easier, because you don't have to dead-center a subject to focus on it:
Just make sure the subject appears within the wide AF area indicated in the
viewfinder, and the camera can focus on it. However, keep in mind that AF systems
lock onto the closest thing they "see." If there's something
in the "hot" area that's closer to the camera than your subject,
the camera will focus on the closer object rather than the desired subject.
You can avoid this by using spot AF, which employs a single AF sensor to measure
just a small area, to focus on your chosen subject--just like with those
original AF SLRs!
With some cameras, spot AF is restricted to the center AF point; with others
you can activate any of the camera's AF points individually--handy
for spot-focusing on off-center subjects, but a bit of a pain in practice. Some
Canon cameras have a feature called Eye-Controlled Focus, which allows you to
activate a specific AF sensor just by looking at it in the viewfinder--much
easier and quicker than manual activation of a specific point. With cameras
that don't offer spot AF, you can solve the focus-on-the-closest-object
problem by focusing manually.
2. Center Point for Action
While wide multi-point AF areas make it easier to keep a moving subject in the
"hot" zone, with most cameras the center AF sensor is more sensitive
than the peripheral ones, and thus will achieve focus more quickly (and in dimmer
light) than the outer ones. And the camera can focus more quickly when its CPU
has to process data from only one sensor instead of data from several sensors.
So for fast action subjects, it's best to activate the center AF sensor
only. It is tougher to keep a fast-moving subject in a small AF area than in
the wide AF area, but with practice you can do it. The quicker focusing response--and
improved action photos--make the practice well worthwhile.
the center AF sensor instead of all the AF sensors can make for
quicker autofocusing on action subjects. Photo by Karel Kramer/Dirt
3. Give It a Moment
When photographing fast-moving action subjects, naturally you'll
want to use the camera's predictive continuous AF mode. But don't
just aim, then suddenly press the shutter button fully to shoot. Predictive
AF works by taking successive AF readings, then calculating the subject's
speed and direction of motion from those readings. The camera's onboard
CPU then uses this data to predict the subject's position at the moment
the exposure is made, and adjusts focus accordingly, thus compensating for the
distance the subject travels during the brief lag between the moment you fully
depress the shutter button to make an exposure, and the moment the exposure
is actually made. (This lag is not great, but a fast-moving subject can change
its distance from the camera quite significantly in even a small fraction of
a second.) If you fully depress the shutter button to make the shot the moment
you point the camera at the subject, the AF system might not have time to do
its predictive magic--or even focus at all.
So, for action shots, first, select continuous AF mode. Next, select the center
AF sensor. Acquire the target (aim the viewfinder's AF target at the subject).
Press the shutter button halfway down to activate the AF system. Continue to
track the subject in the finder as the camera focuses. A beat before the subject
arrives at the point at which you want to record it, fully depress the shutter
button to make the shot. If shooting a sequence, continue to hold the shutter
button all the way down, and pan the camera to track the subject (it's
a good idea to follow through even if doing single shots). Bear in mind that
some cameras focus only for the first frame of a sequence, while others attempt
(with varying degrees of success) to refocus for each frame--it's
likely that only the first frame or two of a rapid action sequence will be sharply
the AF system before the subject reaches the point at which you
want to record it, then fire a beat before it arrives there. Handy
tip: Subjects that go up will briefly stop before coming back
down; if you shoot then, you won't need a fast shutter speed
to freeze the subject. Photo byRon Leach