18 6 July 2019 I http://www.amateurphotographer.co.uk I subscribe 0330 333 1113
Technique
What has artifi cial intelligence
(AI) got to do with autofocus?
Face- and eye-detection AF have been around for a
little while, and they are useful for social photography.
Eye AF is especially helpful because it gets the most
important part of the subject razor sharp, leaving you
to focus on aspects such as the composition and timing
of your shots.
One issue with some face-detection systems is that
they incorrectly ‘see’ faces where there aren’t any,
which leads to focusing errors. For this reason, it’s
a good idea to turn the feature off when you’re not
going to use it.
Thanks to increased use of artifi cial intelligence and
greater processing power it seems likely that we’ll see
more subjects being detected automatically in the near
future. Sony has now extended its Eye AF system to
cover animals’ eyes, or at least cats’ and dogs’ eyes,
and Olympus’s OM-D E-M1X can detect cars, trains
and aeroplanes, while Panasonic’s Lumix S1 and S1R
can detect animals. They all work well, making it easier
to get those subjects’ sharp.
Even though modern AF systems are incredibly
smart, there are occasions when it’s better, or perhaps
easier, to focus manually. Cameras usually search the
distance for their target, which means that it can be
quicker to focus manually on a macro subject, for
example. And if you’re shooting through something like
water spray from a fountain, or foliage, the AF system
is likely to get very distracted, thus making manual
focusing a better choice here too.
If you’re using a DSLR, don’t struggle with the
tiny viewfi nder image while you focus. Instead,
activate live view so that you can magnify the focus
point and use focus peaking to get the subject
completely sharp.
Why use the
back button?Even with high frame
rates, it’s important to
time your shots carefully.
Back-button focusing
can help with this by
taking away the shutter
release’s AF activation
duty. Instead, you use
the AF-on button on the
back of the camera (or
via a customised button)
to control the AF system,
and only press the
shutter release when
you want to take a shot.
This is especially useful
when you’re capturing
action. With motorsports,
for example, you might
want a car to appear at
the apex of a bend. To
help with this, you can
set the focus with the
AF-on button and it
won’t shift when you
press the shutter release
to take the shot.Most cameras offer a collection of
AF-point selection modes that help you
to choose the point(s) you want to use.
These include options such as single-
point, multiple-point, all-point or
auto-point, zone and tracking. The
smaller the selection area, the more
precise you can be with the focusing.
However, if your subject is moving, it’s
worth selecting multiple-point mode or
zone mode as it’s easier to keep at least
part of a larger area over it.
In the past I would have avoided using
an all-point or auto-AF point mode that
puts the camera in full control over which
AF point is used (it always just focused on
the nearest or most central object), but
Sony’s recent AF systems have changed
that. Its Wide and Zone Focus Area
modes work brilliantly and in continuous
autofocus (AF-C) mode in models like theA6400 and Alpha 7 III, they’re great for
tracking a moving subject. But it’s worth
noting that they usually target the nearest
part of the subject and if it stops moving,
it’s prone to focusing just in front of it.
Sony’s Lock-on AF and Real-Time
Tracking modes work in a similar way to
other manufacturers’ tracking modes, but
are better than most. You start by placing
the point over the subject, and then press
the shutter release to activate the focusing.
Once you start shooting, the camera is in
control over where the AF point goes. It
usually follows the subject, but if it strays
you have to stop shooting to reposition it.
Most tracking AF modes aren’t as fast
or accurate as Sony’s, but can be useful
with slower or motionless subjects. For
example, you can place the starting AF
point over a critical part of a subject and
it should stay with it as you recompose.I like to focus and recompose –
is this OK?
When cameras had one AF point we often had to
focus and recompose the shot before taking the image.
However, simple trigonometry tells us that this can lead
to focusing errors. If you’re shooting a portrait with an
85mm f/1.4 lens wide open, for example, the depth of
fi eld is so shallow that using the central point to focus
on the eye and then recomposing can result in
the eyebrow or nose being sharper than the pupil.
Consequently, it’s much better to compose the shot
and use the AF point that’s closest to the object you
want to be sharpest.Eye AF can help you
nail the focusing
with a wide range
of people shots© MICHAEL TOPHAMNever use auto-AF point mode: true or false?
When shooting a portrait wide open with an 85mm lens it’s
best to select an AF point rather than focus and recomposeA small AF point gives you precise control over the focus