Camera Focus and Autofocus Quiz

The two main autofocus modes are single and continuous. Single (AF-S or One Shot) locks focus once for still subjects, and continuous (AF-C or AI Servo) keeps adjusting for moving subjects. RAW and JPEG are file formats, and bulb is a shutter mode.

AF-S locks focus on a single point and holds it once acquired. It is well suited to subjects that remain still because it does not waste processing power tracking position changes that are not happening.

AF-C continuously recalculates focus as the subject moves through the frame. This ongoing tracking is essential for sports, birds, and any subject whose distance from the camera changes during the sequence.

Autofocus can hunt or fail in low light, at the very close distances of macro work, and on low-contrast or cluttered subjects. In all of these cases, turning the focus ring by hand gives precise control over exactly where the focus falls.

Choosing a focus point tells the camera which part of the frame to make sharp, such as the near eye in a portrait. Picking the wrong point leaves the subject soft and the background crisp. It has no effect on exposure, noise or white balance.

Back button focus moves autofocus from a half press of the shutter to a separate button on the back of the camera. Focusing and taking the picture become two actions, so you can lock focus once and keep shooting, or switch between still and moving subjects without changing modes.

Focus peaking highlights the sharpest edges in the viewfinder or screen with a bright color. As you turn the focus ring by hand, you can see exactly what is in focus, which makes manual focus much easier. It does not affect exposure or color.

Eye autofocus detects an eye in the frame and focuses on it, since the eyes are what viewers look at first in a portrait. Many cameras detect both human and animal eyes. It is not useful for landscapes or architecture, which have no eyes to find.

Front focus means the camera or lens focuses slightly in front of the intended subject, so the subject itself is a little soft. Focus landing behind the subject is called back focus. Both show up most with wide apertures and shallow depth of field.

With shallow depth of field the sharp zone is very thin, so even a small focusing error puts the subject outside it. Deeper depth of field gives more margin for error. That is why wide-aperture portraits need careful focus on the eye.

Contrast-detect autofocus moves the lens back and forth until the sensor registers maximum edge contrast at the focus point, which corresponds to sharp focus. It is accurate but slower than phase-detect because it requires testing multiple positions.

Phase-detect autofocus measures the convergence of two light paths simultaneously to calculate both the direction and magnitude of focus correction in a single step. This allows near-instant and predictive tracking of fast subjects.

Focus lock lets you focus on the subject, hold focus (usually with a half press of the shutter), then move the camera to recompose. It is useful when the subject sits away from any focus point, such as near the edge of a rule of thirds frame.

Using the wrong autofocus mode is a common cause of soft shots. Single AF locks once, so a subject that moves after locking ends up out of focus; continuous AF can wander on a still subject. Matching the mode to the subject fixes this. Battery level and white balance do not affect focus.

A lens focuses exactly at one distance, the plane of focus. Things at that distance are sharpest, and sharpness falls off gradually in front of and behind it. Depth of field is the zone around that plane that still looks acceptably sharp.