@cicopo wrote:
YES. You need to use AI Focus for moving targets. If you don't the AF stays locked on the point it first locked onto for each burst.
I'm thinking cicopo meant to write "AI Servo" instead of "AI Focus".
The default focus mode is called "One Shot" mode. In this mode the camera will focus the shutter button is pressed half-way and will stop focusing once it has been able to lock focus. If anything moves AFTER focus is achieved, the camera will not re-focus (that subject would be out of focus because it is no longer at the focused distance.)
Incidentally... as a side-note, if you allow the camera to use any focus point it wants, it will tend to pick the focus point which is able to achieve focus at the NEAREST distance to the camera. If you allow the camera to use any focus point, make sure your intended subject is nearer to the camera than anything else in the area of a foccus point. You can force to the camera to use a specfiic focus point and this offers you more control.
One other nuance of "One Shot" focus mode is that it uses "Focus Priority" which means the camera will refuse to take a shot until it can lock focus (this is why some people will claim their camera refuses to take the shot when they press the shutter button.)
There's another focus mode called "AI Servo". This mode is used in situations where the focus distance between camera and subject is changing (in other words, action photography such as sports, children at play, wildlife, etc.). When the shutter button is pressed half-way, the camera will begin focusing... and it will continuously re-evaluate focus as long as you continue to hold the shutter button half-way pressed. As the subject distance changes, the camera will keep adjusting.
It has a nuance feature called "Release Priority" (recall that "One Shot" mode uses "Focus Priority"). "Release Priority" says that the camera WILL take the shot when you fully press the shutter button and it will do so (and this is the important bit) whether or not the camera has been able to achieve focus. That means if you quickly mash the shutter button down completely, the camera will take the shot before it has even had a chance to focus and you'll end up with out-of-focus shots. So the technique is the half-press, wait for the focus, and then fully press the button to shoot. But as the subject moves, the focus will continue to be updated.
And then there's "AI Focus"
You might think the smart thing to do is to just put the camera into "AI Servo" and leave it there. It turns out that's not a good idea if you know the subject is stationary. "One Shot" mode really is better for stationary subjects and "AI Servo" really is better for moving subjects. There's no one mode which always works best. And that's where "AI Focus" comes in.
"AI Focus" evaluates the subject distance for a few moments to determine if the subject distance seems to be changing. If it is NOT changing, then it behaves as if you selected "One Shot" mode. If it is changing, then it behaves as if you select "AI Servo" mode. In other words, it always tends to just do the right thing on a shot-by-shot basis.
So it's not really a unique focus mode like "One Shot" and "AI Servo" mode... it's a really a mode that causes the camera to auto-select the behavior of focus.
So why not just leave the camera in "AI Focus" mode all the time?
It turns out the camera needs small amount of time to determine if the subject is moving, you might need to capture a shot at a "decisive moment" and if the image is captured merely 1/2 second too late, the moment is gone. For these situations, "AI Servo" really is the better choice. You KNOW the subject is moving so there's no need to ask the camera to evaluate whether the subject is moving.
In other words...
If you know the subject is stationary, select "One Shot" focus mode.
If you know the subject is moving, select "AI Servo" focus mode
If you do not know what type of shot you will encounter next, select "AI Focus" mode.
Tim Campbell
5D III, 5D IV, 60Da
