How to Choose Your Next Portrait Lens—Part 1

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. 

Large aperture

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. 

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