Autofocus Systems Quiz: AF Tracking and Phase Detect

Phase-detect calculates the direction and amount of focus adjustment in a single measurement step, allowing predictive tracking as a subject moves. Contrast-detect must search back and forth, which is too slow for subjects changing distance quickly.

A single AF point requires the photographer to keep a small area precisely over an erratic subject. Expanding to a cluster of points gives the autofocus system more tracking surface area to work with when the subject moves unpredictably within the frame.

Back button focus moves autofocus from the shutter button to a separate button on the back. You can focus once and then shoot many frames without the camera refocusing, or hold the button to track a moving subject, without changing AF modes. It has nothing to do with ISO or exposure mode.

AF-C continuously recalculates focus as the subject changes distance, predicting where it will be when the shutter opens. Sports subjects rarely hold still long enough for a single focus lock to remain accurate across a sequence.

Tracking sensitivity determines how quickly the AF system abandons the current subject in favor of a new one that enters the focus zone. Low sensitivity keeps the system locked on a subject even when an obstacle passes in front; high sensitivity reacquires new subjects faster.

Eye AF uses object recognition to locate and lock on a visible eye within the frame. When the eye is small, obscured, facing away, or moving too fast to resolve, the algorithm cannot maintain reliable lock.

When you focus on an eye and then turn the camera to recompose, the plane of focus swings with the camera and ends up slightly behind the eye. With deep depth of field that small error is hidden, but with a wide aperture and a large reframe it is enough to put the eye out of focus.

AF microadjustment fine tunes where a camera's phase detect autofocus lands with a particular lens. If a lens consistently focuses slightly in front of or behind the target, a small correction value shifts it back. It does not fix exposure, white balance or distortion.

Birds in flight move erratically and change direction suddenly. A wide or zone tracking area gives the autofocus system a larger catchment to follow the subject through unpredictable movements, reducing the chance of losing lock between frames.

Pre-focusing means focusing on a spot before the subject arrives, such as a finish line or a branch a bird lands on. When the subject reaches that spot it is already sharp, so you can fire immediately without waiting for autofocus.

Autofocus needs enough light and contrast on the subject to work. In low light, an AF assist lamp projects a beam or pattern onto the subject so the camera can lock focus. In bright sunlight it is not needed.

Focus breathing is a slight change in the lens's field of view as focus moves, so the frame appears to zoom in or out a little. It barely matters in still photos but is very visible in video during a focus pull.

An electronic shutter reads the sensor one row at a time, so the top and bottom of the frame are captured at slightly different moments. Fast movement or quick pans make vertical lines lean or skew. It does not change white balance, ISO or depth of field.

Viewers look at the eyes first, and with shallow depth of field there is often not enough sharpness to cover both eyes. Focusing on the eye nearest the camera keeps the most noticeable feature sharp. Focusing on the nose often leaves both eyes slightly soft.

Tracking depends on how fast and smart the camera's AF system is, how quickly the lens motor can move the focus elements, and whether the subject has enough contrast for the system to lock onto. ISO, aperture or file format alone do not determine it.