AUTOFOCUS KNOW-HOW
The Canon Magazine 93
EOS S.O.S
Autofocus explained
How does autofocus work, and why is lens aperture so important?
A
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utofocus works by looking for
the closest subject that can be
rendered in focus. It does this
by evaluating the subject for contrast,
maximum contrast is when the subject
is in focus. With DSLR cameras the
individual AF point is sensitive to a line
of contrast, either vertical or horizontal;
cross-type AF points are sensitive to
both horizontal and vertical lines. If you
have no contrast the camera will not be
able to focus, you’ll know this if you’ve
tried focussing on a blank painted wall.
Early implementations of focus in
Live View worked by maximizing the
contrast of the subject, changing focus
iteratively to achieve max contrast.
When more AF points are active, the
camera has more information to
process, and it is possible that it will
find a closer subject than the one you
intend. This is often seen on EOS M andEOS R cameras where the AF points
cover almost the entire image area.
In reduced light levels, subject
contrast is reduced, making it harder to
detect accurate focus. If your lens has a
slower max aperture, say f/5.6 or more,
it becomes harder for AF to function.
DSLR AF sensors need the light to be
perpendicular to the AF point to detect
contrast. It’s for this reason that using a
tele-converter on some lenses, or using
long lenses with f/6.3 or slower
apertures often causes the camera to
focus more slowly, if at all. Bright light
on high contrast subjects helps, but in
low light AF performs better with fast
aperture lenses.
Taking a look at the EOS R, AF can
be used successfully in very low light
levels, but it needs the help of fast
aperture lenses. Remember an f/2.8
aperture lets 2x the light in an f/4aperture, and f/1.4 lets in 8x the
amount. For the best performance you
need well lit, high contrast subjects and
fast aperture lenses. Automation gets
ever more intelligent and works well,
but giving it the best chance is the path
to sharp shots.Using fast aperture lenses enables the
camera to AF accurately in the lowest lightFast moving sports and wildlife
is best captured with fast lenses
to allow the camera AF system
to perform to the highest level