Camera tracks focus automatically — handy for documentary, deadly for controlled cinematography. Deactivate for composed shots.
Autofocus systems electronically track the plane of focus — the camera automatically adjusts the focus group when the subject or camera position shifts. Sounds practical, and it is, but only under certain conditions. On set, you'll quickly notice: the system works by priority, not by your aesthetic intention.
In documentary filmmaking or during handheld-intensive scenes (walk-and-talks, tracking shots), autofocus saves time — you don't need a focus puller constantly sitting at the focus crank. Modern cameras use phase or contrast detection, some combine both. The faster systems (typical in mirrorless cameras) can track movement acceptably without the image constantly "pumping" — meaning the focus oscillates.
But: As soon as you want to control the composition — when an actor is supposed to transition from out of focus to in focus, or when two people in depth perform a focus choreography — autofocus becomes a toxic crutch. The system often picks the wrong eye, or it follows the brightest area in the frame instead of the desired subject. In drama, especially with psychological close-ups or visually complex compositions, you turn autofocus off. Period. Manual focus gives you control over the focus pull, over the moment when blur becomes a narrative tool.
Practical tip: Test the AF system under the actual lighting conditions before shooting. Some cameras have focus peaking or zebra patterns that help. Others offer "eye AF" — this is more reliable when faces are the priority. But even here: in professional productions with clear visual design, you stick to manual. Autofocus is an emergency feature, not a creative statement.