A portrait lens is a photographic lens designed and optimised specifically for the demands of portrait photography, combining a focal length and maximum aperture that produce flattering perspective rendering and subject isolation with optical characteristics that render skin tones, fine detail, and out of focus areas in a particularly pleasing and complementary manner. While virtually any lens can be used to photograph people, portrait lenses are distinguished by a combination of specific optical and mechanical properties that collectively make them exceptionally well suited to the particular aesthetic and technical requirements of portrait work.
The focal length of a portrait lens is one of its most defining characteristics, and is typically in the short to medium telephoto range - broadly from around 85mm to 135mm in full frame 35mm equivalent terms, with 85mm and 105mm being the most widely used and celebrated portrait focal lengths. This telephoto range is favoured for portraiture for several important and interrelated reasons. Working at a moderate telephoto focal length allows the photographer to maintain a comfortable working distance from the subject - typically one and a half to three metres for a head and shoulders portrait - that avoids the close proximity that can make subjects feel uncomfortable or self-conscious during a session. At this working distance, the moderate telephoto focal length produces a natural, undistorted rendering of facial features without the exaggeration of nose and forehead relative to ears that wide angle focal lengths introduce when the camera is brought close enough to fill the frame with a face. The resulting facial proportions are flattering and natural, closely approximating the appearance of the subject as seen by the human eye at a comfortable conversational distance.
Portrait lenses are typically designed with a large maximum aperture - commonly f/2, f/1.8, or even f/1.4 - which serves both a practical and an aesthetic purpose in portrait photography. Practically, the large aperture provides excellent light gathering capability that allows portraiture to be conducted in relatively low ambient light levels without resorting to flash or high ISO settings that might compromise image quality or alter the natural quality of the available light. Aesthetically, the combination of a moderate telephoto focal length and a large aperture produces a very shallow depth of field at typical portrait working distances, rendering the background behind the subject as a smooth, softly blurred wash of colour and tone that separates the subject clearly from their environment and draws the viewer's attention directly to the face and eyes. The quality of the out of focus rendering - known in photographic parlance by the Japanese term bokeh - is therefore an important optical characteristic of portrait lenses, and manufacturers invest considerable design effort in ensuring that the out of focus areas of images made with their portrait lenses have a smooth, creamy, and visually pleasing character rather than the harsh, distracting appearance that poorly designed out of focus rendering can produce.
Many portrait lenses incorporate a degree of deliberate residual spherical aberration in their optical design - sometimes described as a soft focus characteristic - that produces a gentle diffusion or glow around highlight areas and fine details in the image when the lens is used at or near its maximum aperture, softening skin texture and creating a flattering, luminous quality that is particularly prized in glamour, beauty, and fine art portraiture. This spherical aberration based softness is distinct from simple unsharpness or defocus, as the image retains its underlying sharpness and detail while being enveloped in a soft halo of diffused light that the eye perceives simultaneously as both sharp and softly beautiful. As the aperture is stopped down from maximum, the spherical aberration is progressively corrected by the restricted aperture, and the image becomes progressively sharper and more defined, giving the photographer control over the degree of softness through aperture selection.
Some portrait lenses feature a dedicated soft focus control - a separate ring or setting that introduces a variable and controlled amount of spherical aberration independently of the aperture setting, allowing the softness to be dialled in to any desired degree while the aperture is set independently for depth of field control. This type of adjustable soft focus portrait lens, exemplified by classic designs such as the Nikon 105mm f/2.8 Soft Focus and the Canon 135mm f/2.8 Soft Focus, offers the most versatile and precise control over the softness characteristic of the portrait rendering, though the widespread availability of post-processing software capable of simulating similar effects has reduced the commercial demand for this specialist lens type in the digital era.