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What is a Portrait Lens?
This article covers the main characteristics and features of portrait lenses for 35mm SLR cameras.
So, which lenses are considered portrait lenses and what characteristics should the best of them have?
Photographic lenses for 35 mm cameras with constant focal lengths from 50 to 200 mm are also referred to as portrait lenses. As for classic portrait lenses, their focal length is even less, varying from 70 to 180 mm.
Let's get to the bottom of this.
Portrait lenses use focal lengths from 50 to 200 mm because they offer minimal optical distortion, which is highly undesirable for portrait photography. But even within this range there are limitations.
A 50 mm lens is great for a full body portrait or portraits “just above the knee” (a 35 mm is also suitable for full body portraits). An 85 mm lens is a good choice for portraits above the belt and bust portraits. But in order to shoot a close-up without distortion we will need lenses at 135 mm and up. Lenses with a focal length of more than 200 mm are hardly ever used for portrait photography.
The constant focal length provides considerable advantages essential for portrait lenses:
1. Prime lenses usually have a small number of optical elements. Manufacturers generally try to keep the number of optical elements inside good lenses to a minimum with new ones added only when they are absolutely necessary, in the event that the necessary quality characteristics of the lens cannot be achieved without them. Every single extra optical element leads to more loss at the air-glass and glass-to-air borders, as well as in the glass itself.
2. Lenses with fixed focal length generally have higher lens speeds, which is good for picture plasticity and volume, as well as the blur zone picture (Bokeh).
To be fair, there is a variety of zoom-lenses that can be successfully used for portrait photography, for example, many zoom lens variations with a focal length of 70-200 mm. But they still lose in quality characteristics in comparison with prime lenses, primarily in lens speed and Bokeh characteristics.
We have just described a few more factors specific to the portrait lens. Now let's try to build as complete a portrait lens characteristic list as possible. Here we go:
1. Focal length
As we’ve come to understand, a portrait lens’ focal length lies in the range between 50 and 200 mm.
2. Constant focal length
The focal length should be permanent.
3. Small number of optical elements
The number of optical elements in portrait lenses usually doesn’t exceed 7 or 8. Most often portrait lenses have Planar or Sonnar optical designs. The Planar design allows to achieve a higher lens speed (most lenses with a lens speed of 1 to 1.8 are constructed via a Planar design or its variations). By the way, the simplest monocle lens consisting of a single convex-concave lens (meniscus) is a great example of a portrait lens.
(Fig. 2) Sonnar and Planar optical design.
4. High lens speed
Supports large apertures (0.7 to 2.8) and lesser depth of field (DOF), while accenting specific details (eyes, eyelashes). A picture taken with a large aperture becomes more voluminous and gains additional plasticity; plus, the subject is successfully separated from the background. Portraits are generally shot with aperture values up to 4.0. A high lens speed also allows you to easily shoot portraits in poor lighting conditions. The aperture size directly affects the Bokeh.
5. Flexibility and image volume
A good portrait lens has enough sharpness to highlight areas vital to the subject (eyes, eyelashes, hair, eyebrows), but at the same time it expresses softness and flexibility, smoothness of tonal transitions. With all that in place, the image has lots of volume.
(Fig. 3, by Sergey Borodin) Plasticity, volume and Bokeh. Carl Zeiss Jena Sonnar T 85 mm f/ 2 lens
Contrast for portrait shots should generally be medium or slightly less than medium. This results in the skin being softened by a light optical blur, while skin defects are smoothed. This is facilitated by adding an optical element with additional spherical aberration to the lens’ structure.
This effect is often enhanced by special diffusion filters or improvised devices (shooting through a nylon stocking or through a filter coated with a thin layer of petroleum jelly, etc.).
6. Lens Bokeh
The Japanese term Bokeh — (jp. ぼけ Bokeh — “blur”, “blurred”) refers to optical image blurring applied to the part of the picture that was not in focus (outside DOF) on the photo. This includes both foreground and background blurring.
(Fig. 4, by Sergei Borodin) Bokeh achieved via the Helios 40-2 85 mm f/ 1.5 lens.
The most beautiful and harmonious bokeh is usually obtained at aperture values of 1.0 to 2.8 with 50mm and higher fixed focal length lenses. The evaluation of bokeh is a rather subjective factor, but there are some common annoying elements to it: duplicating lines, rough and sharp tonal transition, ugly blur circles, chromatic aberration at the borders, etc.
7. Number of aperture blades
Incidentally, this parameter is related to the previous paragraph as well. Generally, the more blades the aperture has, the more circular the opening of the aperture becomes, and the more harmonious becomes the bokeh and the better plasticity the resulting pictures will have. The best portrait lenses are often designed to have over 10 aperture blades (e.g. Meyer-Optik Görlitz Orestor 135 mm f/ 2.8).
However, there are exceptions to every rule. There are some excellent portrait lenses consisting of only 6 blades (e.g. Asahi Pentax Super-Takumar 85mm f/1.9). But the result is truly impressive only on open apertures. Any further and the aperture creates a female screw outline, although in some cases that can be justified by the plot. It’s best for the aperture to shift smoothly rather than discretely, with the possibility of setting intermediate values (this is generally possible with iris apertures that are manually controlled by a ring). This is very convenient for both taking pictures and recording video.
It’s worth noting that the introduction of “jumping” apertures shot this particular parameter in the foot back in the day, the mechanism of the jumping aperture was generally not built to use more than eight blades, most often limited to six.
Modern autofocus lenses rarely have more than nine blades, yet expensive models have additional circular adaptation for the aperture.
(Fig. 5) Aperture blades of a portrait lens. The Meyer-Optik Görlitz Orestor 135 mm f/ 2.8
8. The optical quality of the lens
Optical quality is not particularly vital for portrait lenses in its classical "technological" sense. Moreover, certain characteristics and lens aberrations in particular, while undesirable for various other photographic approaches, can actually be useful for portrait shooting.
- Vignetting may give additional volume to a portrait photo.
- Residual spherical aberration adds some softness and flexibility to a portrait image.
- The drop in resolution at the edges of the frame and the curvature of the image field are not critical and can serve to better highlight the picture’s primary subject.
- The high resolution of the lens clearly reveals skin defects and often makes the image look rough.
- Medium and low image contrast benefits the flexibility, softness and tonal transitions of the image. It can be easily increased via image editing software if necessary.
As for manufacturers, every major brand has good portrait lenses, but I would like to specifically outline portrait lenses from these companies: Dellmeyer, Carl Zeiss, Rodentock, Voigtlander, Schneider Kreuznach, Leica, and Meyer-Optik Gorlitz.
9. Portrait Lenses
On a final note, here’s a list of some portrait lenses for SLR and mirrorless cameras with interchangeable optics from the Allphotolenses.com database:
Good luck with your portrait shooting! And remember, a good lens is just a tool, and if properly selected, it can significantly improve the impression your picture makes on the viewer, but that’s it! Coming up with an attractive subject matter, selecting the right model, considering and planning lighting and colors, building up the composition, catching the right emotion and so on – the lens won’t be able to do all of that for you. ;)
(с) 2010 Sergei Borodin