DIGITAL SLR
ESSENTIALS
In this edition of Digital SLR Essentials we explain all you
need to know about getting better autofocus
CANONSCHOOL
Why maximum aperture matters
THE LENS apertures that you see listed alongside AF point info in a camera’s
specs is the maximum (largest) effective aperture needed to unlock the extra
precision. The aperture you set on the camera doesn’t make a difference, it’s
the max effective aperture of the lens that counts. We say ‘effective’, because
it can change if you fit an adapter between the lens and camera. Add a 2x
extender to a 70-200mm f/2.8, for example, and the max aperture effectively
drops by two stops – from f/2.8 to f/4 to f/5.6.
Advanced EOS cameras usually feature several AF points that work down
to f/8. This is useful for wildlife and sports photography, where 2x extenders
are combined with f/4 telephoto lenses – giving an effective maximum
aperture of f/8 that would be too slow for most AF systems to cope with.88 http://www.digitalcameraworld.com
OS DSLRs have two
autofocusing (AF)
systems – the optical
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JUHDWHUSUHFLVLRQGet smart with autofocus
The essential settings and techniques to quickly take sharper pictures
E
MARCUS HAWKINS
PHOTO EXPERT
Marcus has been passionate about
photography for more than 25
years. A former editor of our sister
publicationDigital Camera, he has
written about photography and
cameras for a wide range of clients,
including Canon and Jessops, and
uses a Canon EOS 5 D Mk III.(^) Cross-type focusing
f/ (^5). 6 and f/ 8 horizontal
f/ 5. (^6) and (^) f/ 8 vertical
Vertical^ focusing^
at^ f/^5.^6 and^ f/^8
Dual^ cross-type^
focusing^
f/^2.^8 diagonals
f/ 5.^6 and^ f/^8 horizontal
f/^5.^6 and^ f/^8 vertical
Unlocked^ potential^
The EF^70 -^300 mm^ f/^4 -^5.^6 IS^ II^
USM^ ‘unlocks’^ f/^5.^6 and^ f/^8 AF^
points,^ but^ not^ f/^4 or^ f/^2.^8 ones
Cross-type focusing (^)
f/ (^4) horizontal
f/ 5. 6 and f/ 8 vertical