The lens is the eye of the camera, gathering and bending light so that a sharp image lands on the sensor. Focal length, measured in millimeters, describes the distance from the optical center of the lens to the sensor when the lens is focused at infinity. Short focal lengths, such as 24 mm or wider, capture expansive views useful for landscapes and architecture, while long focal lengths such as 200 mm narrow the field of view and magnify distant subjects, which is helpful for wildlife and sports.
Lenses are commonly divided into two categories. Prime lenses have a fixed focal length and tend to deliver sharper images while offering wider maximum apertures, making them favorites for low-light work and creative background blur. Zoom lenses cover a range of focal lengths in a single lens, trading some optical sharpness and maximum aperture for the convenience of being able to reframe a shot without swapping lenses.
Depth of field, often abbreviated DOF, is the range of distance in a scene that appears acceptably sharp. Wide apertures produce shallow depth of field, which photographers use to isolate a subject from a softly blurred background. Narrow apertures produce deep depth of field, keeping both near and far elements in focus for scenes like expansive landscapes. Hyperfocal distance is a useful concept here: it is the closest focus distance at which, when set to a given aperture, everything from half that distance all the way to infinity appears sharp, which is a powerful technique for maximizing sharpness in landscape photography.
Focus can be achieved automatically or manually. Autofocus systems adjust the lens elements automatically and typically offer modes suited to different subjects. Single, or AF-S, focus locks on stationary subjects, while continuous, or AF-C, tracks moving subjects to keep them sharp. Manual focus allows the photographer to turn a focusing ring for precise control, which is especially helpful in low light, when focusing on fine detail, or when shooting close-up macro subjects. The aesthetic quality of the out-of-focus areas produced by a lens is known as bokeh, and lenses with wide apertures and longer focal lengths are prized for producing smooth, pleasing bokeh.