Today’s Question: Is it more difficult for autofocus systems to grab focus at wider (lower numerical) f-stops? Specifically, does the camera spend more time hunting at f/4 than at f/11 due to factors such as narrower depth of field?
Tim’s Quick Answer: Actually, you could say that the opposite is true. There are actually two considerations here. First, under normal circumstances your aperture setting on the camera doesn’t impact autofocus, because the camera essentially ignores your aperture setting when it is establishing autofocus. However, the maximum aperture size for a given lens does impact autofocus, with a larger aperture size (smaller f-number) providing an advantage.
More Detail: You may have noticed that it can be much more difficult for your camera’s autofocus to achieve focus when photographing a scene at night. The camera might search back and forth attempting to achieve focus, and it might fail to establish focus altogether. This is due to the lack of available light in the scene.
Similarly, the maximum aperture size (smallest f-number) available for the lens impacts the amount of light that will be available. When you reduce the size of the lens aperture by stopping down the lens to a larger f-number, you are restricting the amount of light that enters the lens. Less light makes it more difficult to achieve autofocus, as noted above.
However, most cameras today ignore the aperture setting you have established when attempting to focus. The aperture is left wide-open until you actually take a photo, at which point the aperture closes down based on the setting you have established, the mirror moves out of the way if you are using a digital SLR camera, and the image is captured.
So your camera is establishing autofocus based on a wide-open aperture. But different lenses offer different maximum aperture sizes. For example, there are a variety of 70-200mm lenses available, and in many cases a given manufacturer will offer an f/2.8 version as well as an f/4 version of the lens. With the f/2.8 version of the lens you can obviously achieve narrower depth of field. But you may also be able to achieve autofocus faster with the f/2.8 depending on the specific circumstances, because more light is being let into the lens when the camera is focusing.
The bottom line is that in general the aperture setting on the camera (for most cameras and lenses) has no impact on autofocus performance. However, the maximum aperture size of the lens being used will have an impact on autofocus performance as well, with a larger maximum aperture size providing an advantage.