6 July 2019 I http://www.amateurphotographer.co.uk I subscribe 0330 333 1113
Angela Nicholson
Photographer and journalist Angela Nicholson is a former technical
editor for Amateur Photographer and has been reviewing and
writing about photographic kit since 2004. She’s tested everything
from compacts to medium-format cameras and has spent a lot
of time working out how to get the best from a wide variety of
autofocus systems. Visit http://www.squeezymedia.com.© BEN HALL
UNLESS STATED OTHERWISE, ALL IMAGES © ANGEL A NICHOLSON
T
here are a few creative
exceptions, but one of the
fundamental aims of most
photographers is to get their
subject sharp. Thankfully, that’s the
aim of a modern camera’s autofocus
(AF) system too but although they
are sophisticated, they are not 100%foolproof. There isn’t one go-to setting
that guarantees every subject is tack-
sharp in every situation. As a result,
it’s important to understand how
your camera’s AF system works, the
conditions that challenge it, and the
settings or techniques that will help
you to get the results you want.The art
of staying
focused
Understanding how your AF system works
and how to avoid pitfalls will really help you
get sharp shots, says Angela Nicholson
There are two types of autofocus systems
commonly used today: phase detection
and contrast detection. Digital SLRs
use phase-detection focusing when the
viewfinder is in use, but many (recent
Canon DSLRs being a notable exception)
actually switch to contrast detection in
live-view mode.
Mirrorless cameras operate in full-time
live-view mode and use either contrast
detection or a hybrid AF system that
combines contrast detection with phase
detection. Even if they use contrast
detection, it’s usually a heck of a lot better
than the contrast-detection system used
by the average DSLR in live-view mode.
The mechanics of phase detection are
a little different depending upon whether
it’s used with a DSLR’s reflex mirror or
a mirrorless camera’s sensor, but the
principle is still the same. The light exitingthe lens falls on to two pairs or groups of
matched pixels. If the light intensity of the
images from these pixels is the same, the
subject is in focus. If there is a difference,
the subject isn’t in focus and the camera
will then calculate which way to adjust
the lens.
Contrast detection relies on the fact that
a subject has the most contrast when it’s
in focus. It’s an accurate method but it
can’t tell which way to adjust the lens so
it may go one way, then the other. This
means it’s inherently slower than phase
detection. However, mirrorless camera
manufacturers have honed this type of
focusing so it’s much faster than it used
to be. And developments like Panasonic’s
Depth From Defocus (DFD) technology
helps cameras like the Panasonic Lumix
G9 and GH5 get super-sharp shots of
fast-moving subjects.Technique AF MISTAKES
Using a large AF How does my camera’s AF system work?
area makes it
easier to track
subjects that
move erratically