Which is more sensitive, One Shot or AI Servo focusing? This is a question I was asked the other day by someone who thought they were seeing differences in the ability of the camera to lock onto a subject in low light situations depending on which AF mode they use.

Well, there is a simple answer, but it's not the answer you'll most likely be wanting!

One Shot is more likely to achieve focus in low light situations. And there is a simple reason why as well... In AI Servo, the camera does not have as long to perform the AF operation becasue it is constantly re-calculating. As such, the AF sensors don't get chance to gather as much light in AI Servo and therefore focus is less likely to lock, or be as accurate. In One Shot AF, the focus sensors can have a bit longer to collect light and therefore they can perform a more accurate AF operation. 

While this won't help if you're trying to focus on a moving subject in low light, at least you know that it's not an issue with your camera and that that is exactly how it's meant to be.