C.Fn III -2 AI Servo 1st/2nd Image Priority

Which function has priority when you are pressing on the shutter release when you are using AI Servo? Is it the auto-focus, the AI servo or is it the shutter release? This is where you configure your 7D for your preferences.

AI Servo is the most CPU intensive operation. The Canon 7D has 2 CPUs and they are “real” fast ones, but when we press the shutter and focus at the same time, this can still overwhelm the 2 CPUs. So the question is how will be CPUs be allocated? Remember that the 7D can shoot up to 8 frames per second at 14 bits per pixel. The Nikon D300s can “only” do 1.8 frames per second at 14 bits per pixel and there are 33% less pixels.

There are 4 settings for the AI Servo. This doesn't apply to the One-Shot AF:

0: That's the default. This means that the 7D will focus first and take the time to focus, then when it's done focusing, it will release the shutter. How long does this take? That will depend on the subject [contrast, close/far], the lens [tele, wide, macro…], and the previous focus [close/far]. After that the AI servo will try it's best to keep the focus “right”, then the 7D will release the shutter. You won't be getting the 8 frames per second but if your focus point is on a “contrasty” point, the frames will be in focus. This setting is for when you want all the photos will be in focus.

1: The Auto-Focus is given the priority for the first frame. The first photo will be in focus, then release the shutter at up to 8 frames per second. The other photos could be either in focus or not or not even close. The 7D will keep on taking the photos. What's the use? You have a wide-angle at f/11, close is good enough. This would not be appropriate for a long tele wide open.

2: The shutter release is given priority over the AI Servo tracking. The first photo may not be in focus, then the 7D will fire at up to 8 frames per second, even if the photos are not in focus. The 7D will keep on taking the photos. What's the use? You have already set the hyperfocal distance, so you don't really need to focus anymore, the subject will be in focus as longs as it stays in the range.

3: The shutter release is given priority over focusing the subject for the first photo. The first photo may not be in focus then the AI Servo focus tracking will have priority. The other photos, except the first one, should be in focus.