Focusing the Canon 40D
ONE SHOT Focus = subject is not moving and will not be moving
AI-FOCUS Focus = subject is not moving but may move suddenly
AI-SERVO Focus = subject is constantly moving
Forum discussion
Jason Hall:
Center point is the most accurate and most often used.
To use it, half press the shutter button to activate the meter and AF system. While it is active you use the joystick to light up the desired AF point. Use that point to focus on your subject and shoot away. When you want to switch back to Camera selected AF point, simply hit the right most button with your thumb (it has the picture of the 9 AF points above it). When you hit it, all 9 point light up telling they are all active.
To manually select again, you just use the joy stick to select the preferred point again. If you just push the joystick and not rock it, it will select the center point. Again this must be done while the AF and Meter are active after the shutter is half press. You do not need to hold down the shutter button to make the changes. You can get to the point where this can be done with out ever taking your eye from the viewfinder.
Example: You are going to take a shot of your child at an event. You want them in the left side of the frame. Select the AF point closest to the exact point you want focused(the eyes are the best spot to go for). After selecting that point with the joystick you put the square on what you want in focus, half press the shutter button and adjust composition while hold the shutter button. Do not adjust zoom after focus is locked. Take the shot.
If you want to learn Photography, there are two things that you must always be looking at and thinking about, even if you are just taking a snap shot. They are Light and Composition. The technical stuff is how you CAPTURE the image, Light and Composition is how you CREATE a photograph. Both sides are equally important, and you must constantly be thinking about them. Even when you are not sure what to think yet, enter point is the most accurate and most often used. :o)
Rob Bernhard:
Leave it on Center AF point and in Av mode. Drop the aperture as far as it will go at wide end so the aperture will stay maximum when zooming in/out. When you pick it up, switch on and the center point is a bulleye.
If you had an 85 1.8 and you were sitting down and the subject was standing up, focus the eyes and recompose, yeah its gonna be off a little. But with your lenses and uses. The recompose is fine. If you dont have time to recompose, focus the torso, 80% of the time, the face will be on the same focus plane. Better than missing the shot all together. The 40D will allow you to switch on and focus with center point very quickly.
Paul Thomas:
Please note that for any focal point, most of the zone of focus will be in front of that point, and two thirds behind. Let's use the example you described above. If we plug in those numbers (and your camera) into the DOF calculator, we get:
Near limit 15.9 ft
Far limit 257.9 ft
Total 242 ft In front of subject 14.1 ft (6%)
Behind subject 227.9 ft (94%) Hyperfocal distance 33.9 ft
Circle of confusion 0.019 mm .
If we distill this, it means that if you focus on something 30 feet away at f4 with a 28mm focal length, everything from 16 feet away from you out to infinity will be in focus. Which means that unless you hit something else by accident, you can't miss.
Okie Bill:
If you are going to capture objects in motion I suggest you use AI-servo and the back focus setting. I do a lot of birds in flight and the best results I have gotten have been using this method with center spot focus. The center spot has the most sensitivity of the modes of canon AF.
If you get the 40D set to High speed continuous shutter and things should fall in place.