As many of you know, I've been doing mostly action photography lately. On weekends I hang out with a group of freerunners, and in the last ten months I've learned a few techniques that I'd like to share.
Know your subject
You have to know what you're taking pictures of before you can reliably capture the best moments. Anyone who's in the air, for example a basketbal player or a snowboarder, won't be there for long. They go up, they hang for the briefest of moments, then they're reclaimed by gravity. Knowing where and when they'll hit that apogee means you can be ready for it, and capture it more often.
The same applies for fast moving objects that don't leave the ground. If you're photographing cars in a race it will help to know where they'll be going the fastest, braking hardest, or making the most dangerous passes.
Know where the interesting stuff is going to be, and be there first.
Pre-focus whenever possible
No matter how fast your camera is there's a good chance your subject will be faster, especially as they move into the frame or emerge from behind an obejct. Someone leaping over a wall or snowboarding into the air from behind a jump will hit that photographic sweet spot fast, and then the moment's gone. Don't rely on your camera's auto-focus to work fast enough: do it yourself.
Familiarity with your subject will help. Learn their paths, know where they will appear, and focus the camera on that spot by holding the shutter release button half-way down. You can also focus and then flick the camera into manual focus mode, so you don't have to re-focus every time.
It can help to shoot with a smaller physical aperture (higher F number), which increases the depth of field, minimizing the negative effects of missing the focus zone. Be careful though not to shoot too slow, or your subject will blur!
It should go without saying, but shoot fast, and shoot wide open, until you come to grips with the process. Zooming in will typically result in a smaller aperture, and when it starts getting dark your job gets harder. Things that move fast will tend to blur, especially if you leave your camera on the automatic settings. In most situations the camera will try to slow the aperture and make longer exposures, not knowing that your subject is racing past as fast as it can go.
I generally shoot in Ap mode, with the aperture wide open, letting the camera choose the fastest shutter speed to accomodate. This will freeze everything, giving blur-free shots as often as possible.
During daylight I shoot the lowest ISO possible, raising it to accomodate the evening's failing light as required.
Sometimes, use a flash
Have your flash handy. Even if there's lots of light you might benefit from some more. You can rarely control the ambient light, so sometimes you have to add your own.
If you're shooting someone against the daytime sky you'll only get silhouettes unless you use some fill flash. It's not always enough to rely on the on-camera flash, so if you can use an external one, do so. You can also benefit from a remotely triggered flash, offering light that's not as harsh, and less likely to reflect directly back at the camera.
Watch the framing
As with any photographic endeavour you should be careful with your framing. When I shoot I typically want the subject to be large in the frame, so I'll get close or zoom in to achieve that. Sometimes though they'll escape the boundaries of the frame, and I'll get a great shot of the background and some feet, or I'll crop their head out of the shot.
You can always crop more, later, but you can never add the cropped bits back if the original shot is missing them.
Sometimes too it can be nice to show more background. Your own preferences will dictate the balance between subject and background. Add as much context as you need to, to tell the story.