Autofocus is just one of those things; super important but really boring to talk about. Nothing screams amateur like an out of focus picture. We rarely think about autofocus until we miss the shot. If you understand your AF settings before you shoot, you will get a much better percentage of keepers :)
There are two basic settings for autofocus, WHERE to focus, and HOW to focus.
WHERE to focus has a few options. You can have the camera pick where it THINKS you want to focus, or you can pick a specific point. I ALWAYS have my camera on single point.
I pick the point right in the middle for a few reasons. The biggest reason is that it is the most sensitive to light, which means it is the most accurate and the fastest AF point in low light. The outside perimeter require more light to work, which means in low light, you may miss the shot. Pick the center point, focus, then recompose. Simple.
Next is HOW to focus. Put the AF point on your subject, hold the focus button down and once the camera focuses, should it lock focus or keep focusing? This is continuous (servo in Canon lingo) VS. Single (single shot in Canon speak)
In continuous focus mode, as long as you hold your AF button down, the camera will continuously focus on whatever your point is on. This is perfect for a bride walking towards you. If you have ever had the first picture in a series in focus, and the rest out of focus as they walked towards you, you probably had your camera set on single shot. This means, once your camera acquires focus, it stays focused at the original point until you let go of the AF button and refocus. I leave my camera on continuous focus 99% of the time. I will talk about the other 1% at the end (because that 1% is REALLY important in a specific situation)
So, the last mind blowing autofocus trick is SET YOUR FOCUS TO BACK BUTTON!!!! There is NO reason to EVER us your shutter release button to focus!! I have no idea WHY camera manufactures use the top “take a picture” button to also focus! Every camera I have ever bought has engaged auto focus by pressing the shutter release button halfway. What that means is that you cannot take a picture without first focusing, even if just a split second. So if you focus on the subject in the center of your frame, then recompose, your camera will refocus on something else because you can’t disengage AF without turning your camera on Manual focus, and who has time for that?!
If you set your AF to the back button, then the top shutter release “take a picture” button does ONLY that, take a picture. To focus, you simply press the button on the back of your camera with your thumb. To stop focusing, let go. Simple. So you can focus on a person in the center of your frame, let go of the AF button, then recompose and your focus won’t change. SOOOO easy!
The higher end cameras have a dedicated AF-ON button on the back of the camera but almost every DSLR can use another button to AF, you just have to set it in the controls menu.
BONUS: I said I use single AF 1% of the time. This is the 1%. If it is SO dark that my camera cannot acquire focus, you can set your camera to single shot AF and your auto focus assist light will come on when you press the AF button. If you have your camera set to continuous, your AF-assist light will not work.