A perfectly exposed photo with the wrong thing in focus is still a missed shot.
Single-shot AF (AF-S) locks focus once — best for stationary subjects. Continuous AF (AF-C) keeps refocusing as long as you hold the shutter, tracking movement — essential for sports, kids, pets, wildlife.
Focus points matter as much as the mode. A common habit — "focus and recompose," locking focus center-frame then reframing — works fine at narrow apertures, but at wide apertures (f/1.8-f/2.8) it's prone to subtly missed focus, since recomposing shifts the focal plane just enough to blur a subject's eyes. The more reliable habit: move your focus point to where your subject actually is.
Most cameras from recent years include automatic eye/face detection, which has made "focus and recompose" largely unnecessary for portraits — turn it on and let the camera find the eyes.
Some cameras add an automatic option that switches between single and continuous AF by itself (AF-A on many Nikon bodies, AI Focus AF on Canon DSLRs). It's convenient but less predictable: a subject that starts moving just after focus locks can be missed. Choosing AF-S or AF-C yourself means you always know what the camera will do.
The AF area mode decides where the camera looks for focus. Single-point uses one point that you place — precise, and best for still subjects. Zone (or expanded) area uses a small group of points, useful for a subject moving within part of the frame. Wide or full-area lets the camera choose, usually the nearest or most contrasty subject, which is why it sometimes focuses on a branch in front of a bird. Subject detection — eyes, faces and, on newer bodies, animals or vehicles — works on top of these modes.
Manual focus still has its place: dark or low-contrast scenes where autofocus hunts back and forth, shooting through glass or foliage, and macro or night-sky work where you set focus once and leave it. Focus peaking and a magnified view, available on most mirrorless cameras, make it far easier than it used to be.
Focus falls on a plane at one distance. Depth of field is how far in front of and behind that plane things still look acceptably sharp, and it gets shallower with a wider aperture, a longer focal length and a closer subject. For a tight portrait at f/1.8 it can be only a few centimetres deep — enough for the nearer eye to be sharp while the far eye is already soft. The Depth of Field Calculator shows the near and far limits for your lens, aperture, distance and sensor before you shoot.
A head-and-shoulders portrait by a window with a 50mm lens at f/1.8. The first attempt uses the centre point, then recomposes to put the face off-centre: the eyes come out slightly soft and the ear behind them is sharp. Turning the camera to recompose moved the plane of focus just behind the eyes, and at this aperture that's enough to miss. The second attempt uses AF-S with the single point moved onto the nearer eye and no recomposing: the near eye is crisp, the far eye slightly softer, which reads naturally. With eye detection switched on for the third attempt, the camera follows the eye as the subject shifts between frames, and nearly every frame is usable. When the subject turns further away from the camera, stopping down to f/2.8 adds enough depth to keep both eyes acceptably sharp.