SkaGoat
DIS Veteran
- Joined
- Sep 24, 2009
- Messages
- 922
Most camera AF systems can't autofocus with an aperture smaller than about 6.3.
Thus the camera needs to open the aperture between each burst shot to continuously AF.
It's f/5.6 with Canon. Basically the AF system relies on contrast to figure out if the subject is in focus, if the aperture is too small, there isn't enough light to achieve reliable AF. People will run into this mostly when using tele-converters or extensions tubes.
So basically what happens in a burst is it's opens the aperture to wide open, meters the scene, focuses the subjects, does math to determine where the subject will be when it opens the shutter (if set to continuous focus), pops up the mirror, closes the shutter, stops the lens down, opens the shutter, closes the shutter after either the metered amount of time, or the set amount of time (depending on settings), then drops the mirror, and repeats the process.
It's got to do math to meter the scene (if not in Manual), and also has to calculate where the subject has moved to, all of this in 1/8th of a second to achieve 8 fps.