joonrhee wrote in post #13753773
I'm exclusively always using AI Servo on both 5D2 and 1DIV. To me, there is absolutely no difference in focus accuracy. I just have the advantage of being able to shoot moving objects as well as static objects.
You might want to take a look at the topic on this page - http://digitaljournalist.org/issue0905/tech-tips.html - which says....
Q. In poor lighting, say an exposure of f/1.2, 1/15 to 1/30 second, at ISO 800 in evaluative metering with an EOS-1D Mark III, selecting the center AF point manually, I'm observing a fairly significant difference in the AF sensitivity between One-Shot AF and AI Servo AF. In AI Servo mode, on an object with relatively decent contrast, the 1D Mark III is unable to lock focus. When I switch to One-Shot AF, the center AF point quite accurately focuses on the subject and gives a confirmation beep and I am able to capture the frame with extremely sharp accuracy handheld. It is almost as if in AI Servo mode, the AF sensitivity has suddenly switched "profiles" to a rather less sensitive "mode," so as to not be able to lock onto the subject in a fairly low-light situation, whereas One-Shot AF mode has absolutely no issues with that same situation and subject. Does my camera need to go back to Service or is the camera performing according to its design?
A. The light level you describe (ISO 800, 1/15 at f/1.2) is close to the threshold of the EOS-1D Mark III's low-light AF sensitivity. Without the use of flash under these lighting conditions, it would be very difficult to obtain sharp photos at f/1.2 unless the camera was steadied through use of a tripod, and also the camera's reflex mirror should be locked prior to exposure. Assuming that you're prepared to take those considerations into account, then your observation about the difference in focusing capability between One-Shot AF and AI Servo AF for the EOS-1D Mark III is correct. Focus will lock on a subject with readable contrast in One-Shot AF, but AF will fail under the same lighting conditions in AI Servo AF. That is completely normal and to be expected. It's due to differences in the amount of time that light is allowed to accumulate on each pixel in the AF sensor. That period of time, which Canon does not disclose, is longer for One-Shot AF than it is for AI Servo AF, and the result is superior low-light performance for One-Shot AF. This is essentially the performance level that the EOS-1D Mark III is designed to offer in extremely low light, so there would be no benefit in sending the camera in for service on this issue.
I would be surprised if similar limits did not apply to other bodies too. Certainly I am a back button focuser and usually have the camera in AI Servo, but when I need fuss free focusing on a static subject in tricky conditions I tend as often as not to now switch to One Shot for extra security.