What is a photographic portrait?
In photography, a portrait is loosely defined as a representation of a person whose face and expression form an integral part of the image. While the predominant subjects of portraits are people, they may also feature animals, such as pets. Personally, I’ve taken many portraits of my pets.
Many beginner photographers incorrectly assume that portraits are limited to scales that depict a person from just above their head to their chest or shoulders. Although the visual scale of a portrait is loosely defined, we can set several basic limits. For example, although the eyes are important for facial identity and expression, they occupy a relatively small part of the face. Therefore, an extreme closeup of one eye is not a portrait. Conversely, an extreme long-shot—being a photo where some combination of great distance or angle-of-view renders the subject in small relief against their surroundings—is also not a portrait because the face and expression are lost in their surroundings. Any scale of representation that lies between extreme closeups and extreme long-shots can be a portrait and lends creative flexibility to your expression.
What makes a lens suitable for portraits?
What is a portrait lens? You can capture a portrait with any photographic lens. However, this doesn’t mean every lens is a portrait lens. Traditionally, portrait lenses have several properties that make them more suitable for that role than other lenses.
One of these properties is a relatively fast (that is, large) maximum aperture. This would mean an aperture of ƒ/2.8 or greater for a zoom lens and an aperture of ƒ/2.0 or greater for a fixed-focal-length lens. (And keep in mind: lower f‑numbers represent larger apertures.)
The aperture serves two purposes. First, it affects exposure by limiting how much light can pass through the lens. And second, it affects the depth of field, which describes the degree to which areas that lie outside the plane of focus appear acceptably sharp.
Photographers exploit the depth of field to achieve effects such as deep or shallow focus. We use a large depth of field to attain acceptable sharpness in the fore‑, middle‑, and background of the picture. Conversely, selective focus photography features a narrow or small depth of field characterized by a sharply focused subject and a blurry background and foreground.
Lenses with large maximum apertures—represented by small f‑numbers and called “fast” lenses—give portrait photographers the option to capture photos with a shallower depth of field than slower lenses can obtain. Portrait photographers often use a shallow depth of field because it creates a striking visual separation between the subject and their surroundings. It’s beneficial in candid situations, which differ from studios or other controlled locations because the background is either impossible or impractical to change to your liking. Your only option for minimizing background distractions becomes rendering them out of focus.
Superior image quality and portrait lenses
Another desirable property of portrait lenses is high image quality. This is a fairly complex subject that warrants several dedicated videos, but I’ll briefly touch upon two important components for portraiture: good sharpness and pleasing bokeh.
Portrait lenses and sharpness
Sharpness describes the ability of a lens to resolve fine detail of a subject that’s in focus. In practice, it’s characterized by the fine details and edges in the scene being rendered as fine details and edges in the photograph. When everything is focused, a sharp lens renders distinct details across the frame. In contrast, a lesser lens may produce images with a loss of sharpness towards the corners, where details may appear smeared, blurred, or split into their constituent colours, as if by a prism. Such loss of sharpness is caused by the presence of optical aberrations, to which no lens is immune.
Most modern lenses can easily produce sharp photos that show crisp edges and defined details across the frame when their apertures are set to the range of ƒ/5.6–11. However, portrait photographers often take photos close to their lens’s largest aperture to achieve focus separation between the subject and background. This presents a challenge for lens makers because the aperture’s size strongly impacts image sharpness. Optical aberrations are most pronounced when a lens is set to its largest aperture, and aberrations decrease as the aperture is stopped down.
While no lens is immune to sharpness-degrading aberrations, and every photographic lens has more aberrations at larger apertures than smaller apertures, smart engineering, superior glass, and precision assembly of your lens will have a measurable impact on its overall sharpness, including at its largest aperture setting. A high-quality lens that produces sharp images, even at large apertures, allows you to achieve a shallow depth of field and precisely render the subtle details of your subject’s face, especially in the eyes and eyelashes.
Portrait lenses and bokeh
Another important trait of a good portrait lens is how well it can render blurry parts. Photographers use the term “bokeh” to describe the visual and aesthetic characteristics of the out-of-focus areas in photos. Beginner photographers are often surprised to discover that all lenses aren’t created equal in terms of the objective and subjective attributes of their defocus blurring.
Bokeh can exhibit various objective qualities that are influenced by the optical design of a lens. Bokeh can be round, oval, or polygonal—in which case it’s taking on the shape of the lens’s aperture diaphragm. Swirly bokeh appears to swirl or rotate about the optical centre of a lens. Catadioptric lenses—commonly called mirror lenses—create very distinct donut- or ring-shaped bokeh, which are especially visible in out-of-focus highlights. Lenses with aspherical glass elements render bokeh that looks like the concentric rings of an onion.
Bokeh can also feature various subjective qualities that photographers often describe using words such as “smooth” and “creamy” when describing pleasing qualities or “nervous” and “busy” to describe undesirable qualities. A hideous and distractive type of defocus blurring is called “Nisen” or double-line bokeh.
Applying this information towards your next portrait lens purchase takes a little research. Every photo retailer makes it trivial to filter their lens inventory by maximum aperture, and even if they didn’t, that number forms part of the name of virtually every lens you can buy. Searching for a particular lens’s image quality takes a little bit more effort, and you’ll have to refer to the wealth of lens and camera review websites vying for your eyeballs. My personal favourite site for concise lens reviews is OpticalLimits.com.
Choosing an appropriate focal length for your next portrait lens is where matters become incredibly subjective, and I’ll be covering that in the second part of this two-part series.