Lens Aberrations and Distortion

Lens aberrations

Opti­cal aber­ra­tions are imper­fec­tions in the way lens­es con­verge rays of light to a point. These lens aber­ra­tions can be cat­e­go­rized into two types: the imper­fect con­ver­gence of light to a focused point (affect­ing sharp­ness) and flawed geo­met­ric pro­jec­tion of the scene (man­i­fest­ing as dis­tor­tions or warp­ing). When design­ing a lens, opti­cal engi­neers face a series of com­plex com­pro­mis­es to min­i­mize aber­ra­tions. Since no lens is per­fect, engi­neers must make the best pos­si­ble trade-offs with­in the con­straints of the intend­ed use, fea­tures, pro­duc­tion costs, and tar­get price. While there is lit­tle you can do about aber­ra­tions in your lens­es (apart from stop­ping down the aper­ture), it’s help­ful to under­stand them when con­sid­er­ing a new pur­chase. More impor­tant­ly, as a pho­tog­ra­ph­er, being aware of your lens­es’ lim­i­ta­tions allows you to lever­age their strengths and avoid empha­siz­ing their weak­ness­es.

Types of lens aberrations

Aber­ra­tions are the pri­ma­ry cause of reduced acu­ity in lens­es that are cor­rect­ly focused. There are five types you should know about, chro­mat­ic aber­ra­tion, spher­i­cal aber­ra­tion, cur­va­ture of field, coma, and astig­ma­tism.

Chromatic aberrations

Chro­mat­ic aber­ra­tions man­i­fest them­selves as fringes of colour on high-con­trast edges. They are caused by dif­fer­ent wave­lengths of light (i.e., dif­fer­ent colours) under­go­ing vary­ing degrees of refrac­tion and being focused at dif­fer­ent posi­tions as they pass through the lens. Two types of chro­mat­ic aber­ra­tion exist: axi­al (or lon­gi­tu­di­nal) and trans­verse (or lat­er­al). Axi­al chro­mat­ic aber­ra­tions are caused by dif­fer­ent wave­lengths of light focus­ing at vary­ing dis­tances from the lens. Blue-vio­let light focus­es clos­er to the lens than red, with green com­ing into focus between them. Axi­al chro­mat­ic aber­ra­tions can be min­i­mized by stop­ping down your aper­ture, which brings the wave­lengths into accept­able focus. Trans­verse chro­mat­ic aber­ra­tions occur when dif­fer­ent wave­lengths of light focus on dif­fer­ent posi­tions of the focal plane (i.e., on the image sen­sor). These issues typ­i­cal­ly occur with short-focus lens­es, par­tic­u­lar­ly ultra-wide lens­es. Trans­verse chro­mat­ic aber­ra­tions can­not be reduced by stop­ping down the aper­ture but can be effec­tive­ly min­i­mized in raw files using soft­ware like Adobe Light­room.

Spherical aberrations

Spher­i­cal aber­ra­tions result in soft-focused images that lack fine con­trast. They occur when light pass­ing through the edges of a lens focus­es clos­er to the lens than light pass­ing through its cen­tre. In most lens­es, spher­i­cal aber­ra­tion is con­sid­ered an unde­sir­able tech­ni­cal flaw. How­ev­er, in the past, con­trolled spher­i­cal aber­ra­tion was inten­tion­al­ly used in “soft-focus” por­trait lens­es. This type of aber­ra­tion can be reduced by stop­ping down the aper­ture.

Other aberrations

Field Cur­va­ture occurs when a lens can­not focus a flat sub­ject per­pen­dic­u­lar to its opti­cal axis onto a flat image plane. To some extent, this aber­ra­tion is not prob­lem­at­ic for por­trait, land­scape, or street pho­tog­ra­phy; how­ev­er, it is high­ly unde­sir­able in fields that fea­ture promi­nent flat planes, such as archi­tec­tur­al, tech­ni­cal, and macro pho­tog­ra­phy. The effects of field cur­va­ture can be reduced by stop­ping down the aper­ture.

Coma refers to a lens’s reduced abil­i­ty to ren­der a sharp point image that orig­i­nates away from the lens axis. As the name sug­gests, the result­ing image of such a point source has a shape resem­bling a comet’s tail. Coma can be min­i­mized by stop­ping down the aper­ture.

Astig­ma­tism caus­es a sub­ject point locat­ed away from the lens axis to appear as a high­ly stretched oval at one focus dis­tance, as a high­ly stretched oval per­pen­dic­u­lar to the first at anoth­er focus dis­tance, and as a blur­ry disc in between. There are two types: tan­gen­tial and sagit­tal astig­ma­tism. In tan­gen­tial astig­ma­tism, the elon­ga­tion of sub­ject points occurs along an imag­i­nary line radi­at­ing from the opti­cal axis. In sagit­tal astig­ma­tism, the elon­ga­tion is per­pen­dic­u­lar to this line, appear­ing as if it fol­lows imag­i­nary rings cir­cling the opti­cal axis. Like most oth­er types of aber­ra­tion, astig­ma­tism can be reduced by stop­ping down the aper­ture.

Linear distortion

Lin­ear dis­tor­tions refer to devi­a­tions from an ide­al rec­ti­lin­ear pro­jec­tion. Rec­ti­lin­ear lens­es are designed to ren­der straight ele­ments in a scene as straight lines in the image. There are three main types of dis­tor­tion: bar­rel (con­vex), pin­cush­ion (con­cave), and com­plex (some­times called ‘mous­tache’ dis­tor­tion).

Unlike the aber­ra­tions men­tioned above, dis­tor­tions do not gen­er­al­ly affect image sharp­ness and can­not be min­i­mized by stop­ping down the lens. How­ev­er, bar­rel and pin­cush­ion dis­tor­tions can be effi­cient­ly cor­rect­ed using soft­ware like Adobe Light­room, and most mod­ern cam­eras auto­mat­i­cal­ly apply the appro­pri­ate cor­rec­tions when tak­ing a pic­ture. Cor­rect­ing com­plex dis­tor­tions is also pos­si­ble but requires a cor­rec­tion pro­file that accu­rate­ly maps the struc­ture of the warped geom­e­try.

Bar­rel and pin­cush­ion dis­tor­tions are often asso­ci­at­ed with spe­cif­ic focal lengths. For instance, short-focus lens­es tend to exhib­it bar­rel dis­tor­tion, while long-focus lens­es are more prone to pin­cush­ion dis­tor­tion. Zoom lens­es com­mon­ly show both types of dis­tor­tion, with bar­rel dis­tor­tion appear­ing at the wide end and tran­si­tion­ing to pin­cush­ion dis­tor­tion at the tele­pho­to end of the zoom range. This behav­iour is con­sis­tent regard­less of the absolute focal length of the zoom lens. For exam­ple, both 16–35 mm and 70–200 mm lens­es will show bar­rel dis­tor­tion at 16 mm and 70 mm, respec­tive­ly, and pin­cush­ion dis­tor­tion at 35 mm and 200 mm, respec­tive­ly.

Despite the pres­ence of opti­cal dis­tor­tions in many lens­es, their effects are often sub­tle and hard to notice in most pho­tographs. Dis­tor­tions become most evi­dent in images that fea­ture straight lines run­ning par­al­lel and close to the edges of the frame.