How do you focus on something that is not there yet?

[Fastfwd13]

Let's say I want to take pictures of a bike going by pretty fast. Do I focus on the asphalt close to where I think the bike will be and then choose f4 or more to give me a bit more DOF than f2.8 could?

[Pete]

Stop down and use f/8 or f/10 to get better DOF, it's doubtful you'll get sharp focus at f/4 unless you get it dead on (which would be a neat trick).

If possible, you could get someone to stand on the track where you expect the bikes to pass by and then focus on that spot. After that, make sure you're in the same space and flip the lens into manual focus.

[SOK]

That's pretty much how I do it! :D

The more DOF you can allow the better...

The other alternative is AI Servo autofocus and track the subject as it approaches, but this may not give you the framing you want....

[Biffbradford]

Find a landmark on the road (a crack, oil spot) and pre-focus on that. Depending on the speed, you may have to push the shutter just a bit early to catch them on your mark. Takes practice.

[bubba zanetti]

When you can take the pebble from my hand, it will be time for you to leave.

[DC Fan]

Let's say I want to take pictures of a bike going by pretty fast. Do I focus on the asphalt close to where I think the bike will be and then choose f4 or more to give me a bit more DOF than f2.8 could?

http://www.kevinlillard.com/racing/20090815a1605.jpg

http://www.kevinlillard.com/racing/20090815a1900.jpg

http://www.kevinlillard.com/racing/20090815a1983.jpg

For these criterium examples, used the standard AI Servo autofocus (http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm) with a Digital Rebel XTi and a variety of lenses. Manual focus or prefocus doesn't work best in circumstances where things move and camera-to-subject distance constantly changes. Bicycle racing photography is action photography, and capturing objects in rapid motion is, of course, going to require techniques that are far different from photographing static objects. When the event starts, bicycle racers aren't going to hold still. That's one of the reason Canon offers AI Servo autofocus. (http://cpn.canon-europe.com/content/infobank/autofocus/autofocus.do)

The specialized setting used in these images was a slow shutter speed, 1/80-1/100, to get the blur to simulate motion through panning the camera to track the subject.