Portrait lens
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| the Ross Portrait Lens, a design similar to the Petzval lens |
Portrait lens is the general term for a lens thought suitable for making a pleasing photograph showing a subject's face and perhaps some of their shoulders.
If a photographer tries to make such a portrait from too close a position (say, under 2.5 feet or 75 cm) the subject's nose can appear unattractively exaggerated. On the other hand, an image made with a long telephoto from extreme distances will show the sides of the head to an unnatural degree, making it appear ballooned out. Thus, a "portrait" focal length is one allowing the photographer to frame a head-and-shoulders shot from some middle distance—perhaps 5 feet or 1.5 meters. (The desired length is a matter of taste: note the much larger minimum focussing distances specified in the Ross advertisement.)
A distance of 1.5 meters requires focal lengths of about 1.5 to 2.5 times the image diagonal of the film or sensor format. In other words, 45mm would be considered a "portrait" focal length on the sensor size of a Micro Four Thirds camera; while a 300mm lens would give the equivalent angle of view on a 4x5 view camera.
Another consideration for portraiture is that the photographer will likely wish to blur the background to de-emphasize distracting detail. A portrait lens typically offers a relatively large entrance pupil (the physical diameter of the aperture) which greatly facilitates this. For this reason, portrait lenses offering speeds of f/2.0 or wider may become some of the most sought-after items in a particular manufacturer's lens lineup.
Beyond simply offering selective focus to a photographer, portrait lenses may be explicitly designed to provide pleasing bokeh—the perception of softness and smoothness in the out-of-focus area. This may call for a large number of iris leaves, so that the aperture is almost circular rather than clearly polygonal when the lens is stopped down. As for the plane that is in focus, "clinical" sharpness would show up wrinkles and other skin blemishes, and some designs may avoid it via the deliberate undercorrection of spherical or other aberrations. Some manufacturers go further with special-purpose soft-focus portrait lenses that may provide adjustable amounts of spherical aberration, or aperture stops with multiple perforations, resulting in a slight halo around highlights.
The first portrait lens was the Petzval lens, invented in 1841 by Jozef Maximilián Petzval. This was a portrait lens by virtue of its considerably greater aperture. Previous lenses had demanded exposure times measured in minutes, but the Petzval lens could reduce exposure time to a more tolerable 30 seconds or so. The residual aberrations of the Petzval design give it a softness which a few portrait photographer continue to find useful today.
35mm cameras with portrait lenses
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| Helios-40 85mm f1.5 | Canon FDn 100mm f2.0 | Pro-Tessar 115mm f4.0 |
| image by Siim Vahur (Image rights) | image by Voxphoto (Image rights) | image by John F Richardson (Image rights) |
