Depth of field is governed by four factors: aperture, focal length, subject distance, and sensor size. A wider aperture, a longer focal length, a closer subject, and a larger sensor all combine to produce a shallower depth of field. Hyperfocal distance is the focus point that maximizes depth of field, putting everything from half that distance to infinity in acceptable sharpness—an essential tool for landscape photographers. The maximum blur spot that the eye still perceives as sharp is called the circle of confusion, around 0.03 mm on a full-frame sensor, and is the basis for depth-of-field calculations.
Prime lenses offer wider maximum apertures, better optical quality for the price, lighter bodies, and a certain compositional discipline, while zooms provide unmatched flexibility. A 50mm f/1.8 prime, often called a "nifty fifty," is the classic learning lens on full frame: affordable, sharp, fast, and small. The widest maximum aperture in mass production today is f/0.95, found in lenses from Leica, Voigtländer, Canon, and others, with extremely shallow depth of field and a price tag to match. A "fast" lens is one with a wide maximum aperture, typically f/2.8 or wider.
Crop factor describes the ratio of a smaller sensor's dimensions to full frame. APS-C is roughly 1.5× and Micro Four Thirds is 2×, so a 50mm lens on APS-C produces the same field of view as 75mm on full frame. Importantly, depth of field and diffraction depend on actual focal length and aperture rather than the equivalent field of view, so f/2.8 remains f/2.8 regardless of sensor size. Focal length compression is a perspective effect: telephoto lenses visually compress the distance between foreground and background, while wide angles exaggerate the gap.
Focus modes split into single-shot (AF-S or One-Shot), which locks focus when the shutter is half-pressed for stationary subjects, and continuous (AF-C or AI Servo), which tracks moving subjects. Back-button focus decouples focusing from the shutter, allowing the photographer to lock focus and recompose freely or shoot bursts without refocusing. Eye-detection AF locates human or animal eyes in real time and continuously tracks the nearest one, a major breakthrough for portrait and event work. Focus stacking combines several images focused at different distances to extend depth of field beyond a single shot, used heavily in macro, product, and landscape photography. The nodal point of a lens is its optical center; rotating a panorama around this point, using a panoramic tripod head, prevents parallax errors during stitching.
Other focus-related issues include focus shift, in which the plane of best focus moves as the lens is stopped down—more common in older designs and fast primes—mitigated by stopping down one or two stops. Focus breathing refers to a small change in focal length or field of view as the lens focuses closer, minimized in cinema and fast primes. Parfocal lenses hold focus as you zoom, while varifocal lenses require refocusing after zooming. Teleconverters extend reach—a 1.4× increases focal length by 40 percent and costs one stop of light, while a 2× doubles focal length and costs two stops, with reductions in autofocus speed. Tilt-shift lenses correct converging verticals in architecture, allow selective focus planes in product work, and create miniature-style effects. AF microadjustment calibrates a lens's autofocus to a specific body to fix front- or back-focus issues, while live view with focus peaking or focus magnification offers precise manual-focus aids for macro, landscape, and astrophotography.