All professional photographers know the ability to achieve pin-sharp shots is essential. It doesn’t matter how thrilling the subject or how inspired the photo composition. Even a minute hint of blur can mark the difference between an award-winning shot and a second-rate snap. If the eyes of a portrait are slightly soft and the eyebrows and tip of the nose are sharp, for instance, the image will lack both impact and credibility.
Your digital camera can adjust the focusing automatically at the touch a button. Good quality autofocus systems can move the lens elements into position faster and more accurately than the human hand and eye can. This makes them invaluable for sports photography, documentary photography, natural history, paparazzi and fashion photographers, not to mention anyone with less than 20-20 vision.
Nailing the focus for every shot is far from straightforward, involving considerably more user intervention than a point and shoot approach. This is because the speed, complexity and accuracy of autofocus systems varies dramatically from camera to camera and in different shooting situations.
Ultimately, autofocus is just a mechanical tool that’s prone to errors and suffers limitations like any other man-made technology. The good news is, if you understand how your camera’s autofocus system works and what its shortcomings are, you’ll be well equipped to anticipate and compensate for its failings.
Below we put our heads together and came up with 9 common subjects and shooting situations in which using autofocus will likely let you down…
01 Low contrast
Passive AF have problems locking onto low contrast subjects such as solid blue skies, low contrast subjects against a low contrast background or if the subject’s the same colour as the background. Use manual focus or AF lock instead.
02 Low light
Passive AF struggles in low light, particularly with telephoto and macro lenses where light levels are extra low and depth of field is limited. If your AF assist beam is out of range, use flash or manual focus.
03 High reflectance
Highly reflective subjects such as cars with metallic body paint, mirrors, windows and shiny metal objects will often flummox AF systems. Switch to manual focus or use AF lock to solve the problem.
04 Overlapping objects
AF systems have great difficulty locking onto subjects that are overlapped by closer objects. Classic examples are an animal in a cage or a person behind a pane of glass. Focus manually.
05 Geometric patterns
Subjects dominated by regular geometric patterns, such as rows of windows on tower blocks, can pose problems for passive autofocus systems. Focus manually instead.
06 Differing brightness
When the focus area contains areas of greatly differing brightness, such as when the subject is half in shade, autofocus may not work. The solution? Use manual focus or AF lock.
07 Off-centre subjects
When the subject doesn’t appear in the central focus area (or in one of any additional focus areas), accurate focusing on the subject may be impossible. Use AF lock and recompose the shot.
08 Small subjects
If your subject appears smaller in the frame than the focus area (e.g. if the focus area contained both your foreground subject and distant buildings), a camera won’t know where to focus. Zoom in and use AF lock.
09 Fine details
AF systems don’t perform well when faced with subjects containing lots of fine details, such as fields of flowers or other subjects that are small or of similar brightness. Focus manually instead.
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