I was using a 50mm f/1.8 II lens on my Canon 40D yesterday, and I was trying to capture some action shots using continuous high-speed drive mode and AI Servo autofocus mode. I was using a shutter speed as high as 1/4000, but I didn't seem to be getting the burst rate I expected (should be better than 6 frames / sec).

On a hunch, I tried changing to a lens with a USM focus motor (17-85mm f/4-5.6 IS USM), and even though I couldn't keep the shutter speed up as high (due to the smaller aperture), the burst rate seemed to improve.

I didn't have time to actually measure the burst rates, so my experience is somewhat anecdotal, but I was wondering if the focus speed of the lens could have been impacting the burst rate of the camera. One of the benefits of Canon's USM motors is faster focus speed, but I was also under the impression that the AI Focus mode on the 40D is supposed to take shots even if it's not able to "lock" focus. Am I out to lunch, or can my burst rate be affected by the focusing of the lens?

  • 2
    \$\begingroup\$ f/5.6 is less than 2 stops smaller than f/1.8. That means that your shutter speed should be about 1/1000 sec - still much faster than 1/6 sec (expected frame rate). This means that the fact that it is a slower lens does not play a role here. \$\endgroup\$
    – ysap
    May 6, 2011 at 13:47
  • \$\begingroup\$ Is it a menu setting to control focus lock and shutter release? It is on Pentax, not so sure with Canon. \$\endgroup\$
    – Joanne C
    May 6, 2011 at 14:44
  • 1
    \$\begingroup\$ @ysap f/5.6 is more than 3 stops smaller than f/1.8, FWIW. I agree that's not the issue. \$\endgroup\$
    – coneslayer
    May 6, 2011 at 15:31
  • \$\begingroup\$ @coneslayer - you are obviously right, I was probably (not...) thinking linearly instead of squared. \$\endgroup\$
    – ysap
    May 6, 2011 at 19:10

2 Answers 2


Obviously, the lens cannot focus any faster just because the drive mode is set to burst. The time it takes to focus is the time it takes to focus.

What you can do is tell the camera to take the shot without waiting for focus to lock. This is called Release Priority as apposed to Focus Priority. To change that you have to go to the Setup menu and find the custom setting which lets you choose between them. Your camera probably has that option but not all do. Of course, the shots you fire before focus is locked will me slightly out of focus.

You can always to manual focus to make things more predictable.

  • 1
    \$\begingroup\$ This is really close -- in fact, it gave me the clue I needed to find out what's going on. It turns out that the implementation of AI Servo mode in the 40D uses release priority for shot #1 and focus priority for subsequent shots. There is, to the best of my knowledge no way to adjust this behavior, short of upgrading. \$\endgroup\$
    – D. Lambert
    May 12, 2011 at 16:09

We shot round 2 yesterday, and I went back to the 50 to try again. This time, I shot in "AI Focus" mode instead of "AI Servo" mode, and the 50 worked great. This left me feeling a bit unsatisfied, however, because I couldn't really explain why I was seeing the results I was seeing. The user manual, for instance, implies that AI Servo should be ideal for moving subjects.

@Itai's answer turned out not to be entirely correct, but it gave me exactly the clue I needed to figure out what went wrong in AI Servo mode. A little googling turned up a very helpful article with information that's purportedly straight from Canon. The key information begins around item 3 -- paragraph A indicates that the first shot in AI Servo mode uses Release Priority, and paragraph B indicates that subsequent continuous shots use Focus Priority. Although this information is given specifically for the EOS 1, circa 1992, I've yet to find anything indicating that this has changed, and given Canon's tendency to respect backwards behavioral compatibility, I don't expect that this behavior would change without the addition of a menu option to maintain the prior behavior. Custom Function III-1 seems to be close to to this sort of setting, but I don't think that's really the same thing as release priority vs. focus priority.

Further searching turned up an answer that seems to confirm this and extend understanding to encompass what I saw yesterday:

AI SERVO (at least in the iterations before the 7D, i.e. 20D, 30D, 40D, and perhaps 50D (dunno never had one)) will keep shooting at full speed as long as its getting (what it thinks is) focus lock for every shot. With the 7D (and perhaps 50D) there are two C.Fn's that handle how AI Servo AF behaves.

C. Fn III-2 "AI Servo 1st/2nd image priority" 0: AF priority/Tracking priority 1: AF priority/Drive speed priority 2: Release/Drive speed priority 3: Release/Tracking priority

of the 4 choices, Choice 2 is the fastest because even with the first shot of the sequence the AF system does not wait for AF Lock, its priority is shutter release.

of the 4 choices, I use 0 for the simple reason that the 70-200 f/4L IS USM which is the main reason for such speed for me can actually keep up. 2nd fastest lens is probably my 100mm f/2.8 Macro USM, followed by the 17-55 f/2.8 IS USM. The speed of the lens's AF motor plays a major role in how the AI SERVO will work. If the lens is slow and your priority is choice 0 for example, you won't likely be able to maintain full 8fps on the 7D. I know on my 40D and 30D before it, slow lenses like the 75-300 III would no go faster than 3fps in AI SERVO, even in High Speed Continuous motor drive. Simply put AI SERVO was originally designed to keep shooting as long as AF is found - no lock, no shot.

What I used to do with my 40D before I got the 70-200L was I'd set my AF to ONE SHOT and High Speed. It ended up doing the same as option C.Fn III-3 Option 1. It locks for the first shot, then just shoots more of the same focus. AI SERVO on the 7D is a lot better tho because now it can still give a damn about focusing the subsequent shots - something ONE SHOT AF is not meant to do.

C.Fn III-3 "AI Servo AF tracking method" 0: Main focus point priority 1: Continuous AF track priority

This function deals more with tracking priorities... i.e. if something gets between you and the subject over the course of a sequence... like a tree that gets between you and your dog. __________________ Alan "NuReality" Fronshtein

This answer supports the idea that the motor speed of the lens is part of what may have been slowing me down with AI Servo, since that mode depends on focus lock for continuous shooting on shots 2-n. It sounds like later Canon bodies, including the 7D, have an implementation of AI Servo that includes better control over the use of release priority vs. focus priority.


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