Photography Life (formerly The Mansurovs) – How to Use Prime Lenses in Low-Light Environments This is by Romanas Naryškin, rather than Nasim Mansurov. Naryškin is young and his stuff is just OK. I’m linking this to say that the article has a couple of good tips (like shooting in bursts if you have to be sure to get focus and the subject is moving, calibrating your lens if your camera supports it), but that it misses some of the big points.
Use single point (center) focus for best precision or when shooting cameras with poor focus systems
My first DSLR was a Nikon D40. Great camera, but the focus system had just three focus sensors (for comparison, my new D7000 has 39 and some pro cameras have 50 or more). I quickly discovered that the only way to get food focus was to use single point/single area autofocus and pick my focus point instead of using area focus and letting the camera do it for me. Naryškin hints at this when he discusses cross-type focus sensors and mentions that the center sensor is usually of the cross type.
Single point means that you get one dot in the center of the viewfinder. You aim that at what you want to be in focus. Then you either take the picture or press the shutter release halfway down to lock focus then recompose the picture and then press the shutter the rest of the way down. Compared to other focus options such as area or matrix you get complete control and you always know exactly where focus will be. For people and animal pictures I prefer to focus on the eyes, for instance. An example of a time you wouldn’t want to use single point focus is when you’re shooting very fast and don’t have time to pick a focus point. In that situation you could accidentally pick a focus point in front of or behind your subject.
Use continuous focus to track moving objects
The focus system usually locks in the focus point and doesn’t change. This is called AF-S on Nikon and (I think, correct me if I’m wrong, Canon guys) One-Time on Canon.. If the distance between you and the subject changes (because your or the subject moved) the focus will be wrong. The way to prevent that is with continuous autofocus (AF-C on Nikon and AI-Servo on Canon) which updates focus up until the picture is taken. Turning on the Sports scene mode will usually activate continuous autofocus; check your manual.
Another nice thing about continuous autofocus is that it will take a picture even if it doesn’t have definite focus. Some scenes (like tall grass swaying in a breeze) will throw off AF-S and keep it from locking focus and taking a picture.
Note that continuous focus will disable the half button press method of locking focus and recomposing with single point focus (at least on Nikons; I’m not 100% sure about Canons). In that case you have to lock focus using the AE-L/AF-L button or AF-ON button.
Back button focus
Speaking of which, pressing the shutter release halfway sets focus. Pressing it the rest of the way trips the shutter. Some DSLRS have a dedicated AF-ON button that will set focus. With others you can program one of other back buttons for focus lock or AF-ON, and all DSLRs have an AF-L button to lock focus. Even if you don’t use single point focus you’ll find that AF-L makes the shutter button more responsive and gives you manual-like control over focus, with less hunting.
I’ve experimented with configuring the AF-E/AF-L button on my D7000 as an AF-ON button a few times for sports photography in situations where I knew something was going to happen quickly at a fixed distance and liked it. Here are a few articles explaining some of the advantages of back button focus.
On-camera autofocus assist lights – hate ‘em
Naryškin recommends using them. I took Thom Hogan’s advice and turned them off on my cameras for reasons of etiquette. They shine in people’s eyes in dark settings and draw attention to your picture taking.
On-camera AF assist lights are only good to maybe 10 or 15 feet if you’re lucky. They’re often blocked by large lenses or even lens hoods, which I always use. If I really need AF assist I’ll bring along my SB-800 flash. It has a bigger, stronger AF assist lamp that’s red instead of white. Since the flash sits on top of the camera the AF lamp isn’t blocked by the lens.
Become passingly familiar with depth of field
Another term for depth of field is depth of focus. Henri Cartier-Bresson I ain’t. I don’t have depth of field tables memorized, but I’ve got an app on my phone now and I try to consult it now and again. Here’s a video where I explain exactly how my depth of field mistakes destroyed the focus in that video. Distance and small apertures and wide angle lenses are your friends. Back up a little and stop down a little, especially with longer lenses and group shots.