Sunday, February 14, 2010

Learnings (Part 3): Using Predictive Autofocus

Any modern DSLR camera supports (or should support) predictive autofocus tracking. While they might have this capability, there is an art to using it effectively, and therefore getting in-focus shots of moving subjects.

This is actually something I only learnt on the 19th of August, 2009.  Yes, I remember the date, it's an important date.  This is after having a DSLR many years, and never being able to effectively shoot moving subjects.  I would resort to doing things like:
  • Stopping down, therefore increasing the depth of field, such that the moving subject might still be in focus.
  • Increasing the shutter speed ('cause clearly I didn't understand the difference between motion blur and just plain out-of-focus).
  • Pre-focusing in front of a moving subject.
These can give acceptable results, but why not get the camera do what it's supposed to do.

To use predictive autofocus, you need to understand how it works.  Then you need to modify the way you shoot images to give it a chance of working.

These images show the results you can achieve - in this case with the Canon 20D (a camera released in 2004) and a Canon 70-200 f/4 L USM lens.  My daughter was riding on a merry-go-round, and was (very) grumpy because she couldn't get the seat she wanted.  I pity the other child - but then I guess her photos will be out of focus anyway.

"Grumpy Leia with 'Friend' on the Merry-go-round", Canon 20D, 81mm f/4, 1/500s, ISO 400
"Grumpy Leia on the Merry-go-round", Canon 20D, 188mm f/4, 1/500s, ISO 400
100% crop of same image

At 188mm focal length, an aperture of f/4, and a distance of less than 2 metres, that's a depth of field of around 2cm.
The "magical" solution is simply to give the cameras focusing engine time to track the subject.  If the camera can track the subjects movement for a second or so, then at the point when you release the shutter it will be able to predict where the subject will be when the shutter opens.

To get your camera to track movement, you may need to refer to your user manual.  On Canon camera, this mode is known as AI Servo, and is automatically selected if you choose Sports mode.  While the shutter is half pressed (i.e. autofocus activated), then the camera will attempt to keep the subject in focus, and learn the movement of the subject.

If you take the first shot quickly, then the behavior of the camera might be for "release priority" instead of "focus priority", meaning that it takes the shot as soon as possible, instead of trying to lock the focus.  Of course if you release the shutter, and press again then it will be considered another first shot.  Furthermore once you release the shutter, the learned subject movement is forgotten.

Using continuous shooting mode will allow you to keep tracking the subject while the shutter is activated, and you can fire off multiple shots.  You'll quickly fill your buffer and may actually miss the best shots.

What you need to do is separate autofocus from the shutter button, allowing you to focus continually and take each shot when you want to.

On some cameras there is a dedicated autofocus button (named AF-ON on my Canon 7D).  You should hold this down continually, and then release the shutter whenever you want to take a photo.  If desired you can still use continuous shooting drive mode.  When using the dedicated autofocus button, the shutter button will perform metering (exposure evaluation) and then release the shutter.

Other cameras do not have an autofocus button, but you may be able to modify the camera configuration.  This is the case with the Canon 20D which can be modified through custom functions as described on Ken Rockwell site.  Custom Function 4, allows you to configure the functions assigned to the shutter button and AE lock button (the * button).  Mode 1 is the one you want; this will configure AE Lock to activate autofocus, and the shutter button will lock exposure (and release the shutter).  Once you've finished your sports photography, you'll probably want to switch back to mode 0 which is the default.

Now that you understand how to use predictive autofocus, you'll also need to check these things:
  • Using a single autofocus point may give more consistent results (if you can keep that point on the subject).  On a Canon 7D, you can also choose AF Point Expansion.
  • Consider using the centre autofocus point.  This may be more sensitive (depends on the camera).
  • Use an appropriate shutter speed based on lens focal length and the degree of subject movement.
  • Where possible use lenses with better autofocus motors - like the Canon USM (ultrasonic motor) lenses.
  • Use the correct depth of field for your subject.
  • Ensure the lighting is good for accurate focusing.

Additional References:
Read about other things I've learnt here.

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