Portrait lens
A portrait lens is a photographic lens that is particularly suitable for portraits due to its focal length , light intensity and optical design (correction) .
Focal length and angle of view
Portraits or half-length pictures of people look particularly natural if they are created with a working distance between well over one and about three meters, i.e. at a distance at which normal, everyday interaction with other people also takes place. For a full-format portrait photo, this results in an angle of view between around 15 and 30 °, significantly smaller than that of typical normal lenses with around 45 to 55 °.
While portrait photos can be quite good with normal lenses, wide-angle lenses are unsuitable for classic portraits, as the working distance for a full-size portrait is very small and this leads to strong distortion ("moon face" or large nose). Cropping by blurring against a noisy background is difficult or impossible with short focal lengths.
In the area of so-called full - format cameras (roughly equivalent to analog 35 mm cameras with a 36 mm × 24 mm 35 mm format ), typical portrait lenses therefore have a focal length between about 80 and 135 mm. They are so-called "light" or "small" telephoto lenses . With this focal length range, the display is perceived as pleasantly distortion-free, but not yet as “flat”, as is the case with a telephoto lens with a longer focal length or a smaller angle of view . In medium format cameras , longer focal lengths are used for portraits depending on the larger recording format, while shorter focal lengths are used for the smaller sensors of digital single-lens reflex cameras, depending on the format factor .
Light intensity
An important design option for portraits is the so-called cropping using a very shallow depth of field . For this purpose, large initial apertures are required, which is why portrait lenses in the small image area usually have initial apertures of 1: 2.8 or more; lenses with an initial aperture of 1: 1.2 are available here. With the aperture open, the face or the entire person can be optically separated from the background and, if necessary, the foreground. With a precise focus on the pupil (s), special emphasis can be placed on the eye area.
Other special features
When designing portrait lenses, manufacturers often value a pleasant bokeh and may compromise other properties such as image field flattening or edge sharpness. Especially in the portrait area, modern zoom lenses, even if they have sufficient light intensity, often have unsatisfactory imaging properties, which is why special portrait lenses are still justified.
Some lenses for portrait photography, such as the Rodenstock Imagon or the Minolta Varisoft, are equipped with special functions for soft focus effects. Other lenses use built-in apodization filters to create a very soft bokeh. Lensbaby lenses , for example, also allow special effects .
As part of digital image processing , various portrait effects are offered by image processing programs and smartphone apps .
history
The first special design for a portrait lens was the comparatively fast so-called Petzval lens from 1840.