The advantage of interchangeable lens cameras like DSLRs and mirrorless cameras is that you can change lenses for different kinds of photography. There are many different types of lenses, often with specific names. Lenses are usually characterised or grouped by their focal length. It might be more useful to use their angle of view instead, […]
As the name suggests, this is a lens particular suited to portraiture by virtue of its focal length and a fast maximum aperture.
In portrait photography, you get better facial perspective if you stand back a little from the subject. This means you need to use a slightly longer focal length than usual to fill the frame, and an 85mm lens is normally considered the ideal focal length (or 85mm equivalent on a camera with a sensor smaller than full frame). Longer focal lengths can work too, but you quickly run out of space in normal interiors.
Besides, the 85mm focal length also lends itself to wide apertures without excessive weight or costs. Using a wide aperture will throw the background out of focus and highlight attention on your subject. Portrait lenses typically have a maximum aperture of f/1.8, f/1.4 or even f/1.2.
This is one reason why 'portrait' lenses are fixed focal length 'prime' lenses rather than zooms – it's not possible to make zoom lenses with a maximum aperture that wide.