The Aperture and its Effects

Aperture beyond exposure

We have estab­lished that con­trol­ling expo­sure is the aperture’s pri­ma­ry func­tion. Beyond that, the aper­ture influ­ences sev­er­al tech­ni­cal and aes­thet­ic effects that are relat­ed to its cen­tral role. In this sec­tion, you’ll learn about how to deter­mine your max­i­mum aper­ture, aper­ture pri­or­i­ty expo­sure mode, the dif­fer­ence between f-stops and t-stops, lens sharp­ness and dif­frac­tion, bokeh, and vignetting.

How is aperture indicated?

The nomen­cla­ture of most mod­ern pho­to­graph­ic lens­es fol­lows some vari­a­tion of the fol­low­ing for­mu­la:

(brand + mod­el line) (focal length)(max. aper­ture) (ver­sions + fea­tures)

Lens name exam­ples:

  • Canon EF 85mm f/1.4L IS USM
  • AF-S NIKKOR 14–24mm f/2.8G ED
  • Sony E 18–135mm F3.5–5.6 OSS
  • smc PENTAX-DA 200mm F2.8 ED[IF] SDM
Identifying photographic lens names and specifications
Although the focal length and max­i­mum aper­ture indi­ca­tions on the Fuji­non and Canon lens­es are sim­i­lar, they are locat­ed in dif­fer­ent loca­tions: front of the lens on the Fuji­non (A), and top of the lens on the Canon (B).

Amidst the alpha­bet soup you’ll find the two most impor­tant details, the focal length and max­i­mum aper­ture, expressed in that order (indi­cat­ed in bold above). The con­ven­tions that describe the lens names and fea­tures are less con­sis­tent when ana­lyz­ing the descrip­tive mark­ings print­ed on the lens­es. For instance, Canon, Nikon, Olym­pus, and Fuji­film all indi­cate the focal length fol­lowed by the max­i­mum aper­ture, with the lat­ter writ­ten as a ratio of one to the f-num­ber (for exam­ple 1:2.8 is ƒ/2.8 and 1:2.8–4.0 is ƒ/2.8–4.0). Pen­tax and Leica mark their lens­es with the max­i­mum aper­ture as a ratio fol­lowed by the focal length. Sony’s lens­es indi­cate the aper­ture as an unde­fined num­ber fol­lowed by the focal length. Very gen­er­al­ly, it’s help­ful to remem­ber that in the absence of any unit of mea­sure­ment, num­bers with sin­gle dig­its or dec­i­mal points indi­cate the max­i­mum aper­ture and dou­ble-dig­it num­bers refer to the focal length.

The aper­ture inscribed on a lens is typ­i­cal­ly the fastest pos­si­ble for that lens, and known as its “lens speed.” Lens­es can be “fast,” and they can be “slow.” In this con­text, speed refers to the oth­er half of the expo­sure equa­tion: the dura­tion. Giv­en iden­ti­cal light con­di­tions, a fast lens with a large max­i­mum aper­ture per­mits using faster shut­ter speeds. Since a slow­er lens gath­ers less light, an equal expo­sure is attained with slow­er shut­ter speed. On most small for­mat cam­eras (with image sen­sors up to 24×36 mm), a lens whose max­i­mum aper­ture falls in the range of ƒ/1.0–ƒ/2.0 is con­sid­ered fast. Fast lens­es tend to cost more than slow lens­es because they require more glass for their larg­er glass ele­ments, the inclu­sion of unique types of glass to min­i­mize aber­ra­tions and because inte­grat­ing both increas­es design and man­u­fac­tur­ing com­plex­i­ty.

Aperture priority auto-exposure mode

Aper­ture pri­or­i­ty mode is an auto­mat­ic expo­sure mode in which the pho­tog­ra­ph­er selects the desired aper­ture, and the cam­era attempts to achieve ide­al expo­sure by vary­ing the shut­ter speed. Aper­ture pri­or­i­ty mode is com­mon­ly indi­cat­ed as A or Av (for aper­ture val­ue) on most cam­eras’ mode dials. Aper­ture pri­or­i­ty mode is dif­fer­ent from oth­er auto­mat­ic expo­sure modes because it allows pho­tog­ra­phers to con­trol the depth of field.

F-stops and T-stops

The chap­ter about Aper­ture and expo­sure stat­ed that all lens­es set to a spe­cif­ic f-stop will, in the­o­ry, trans­mit the same amount of light to the image sen­sor. It was an over-sim­pli­fi­ca­tion; in prac­tice, two dif­fer­ent lens­es set to the same f-stop will trans­mit slight­ly dif­fer­ent amounts of light. Recall that f-num­bers are derived from the focal length divid­ed by the diam­e­ter of the entrance pupil. This sets a max­i­mum the­o­ret­i­cal upper lim­it on light trans­mis­sion. Unfor­tu­nate­ly, this equa­tion doesn’t con­sid­er the light loss incurred dur­ing its trans­mis­sion through the lens. A com­pound lens is com­posed of mul­ti­ple glass ele­ments that both absorb and reflect light. Since no glass is both 100 per­cent trans­mis­sive and 0 per­cent reflec­tive of light, lens­es will always trans­mit less light than the the­o­ret­i­cal max­i­mum implied by the f-stop.

The t-stop (t for trans­mis­sion) indi­cates the mea­sured light trans­mis­sion val­ue of a lens. Two dif­fer­ent lens­es set to the same t-stop will always give the same expo­sure. T-stops are used as the stan­dard aper­ture mark­ings on lens­es designed for cin­e­matog­ra­phy. In film­mak­ing, it’s com­mon to set cam­era expo­sure and light inten­si­ty using exter­nal light meters. For these set­tings to remain cor­rect in a scene filmed with mul­ti­ple lens­es, the lens­es must be cal­i­brat­ed in t-stops. T-stops are cal­cu­lat­ed using the for­mu­la:

t-stop = (f-stop)/(lens trans­mit­tance %)

T-stop val­ues are most­ly obso­lete in mod­ern pho­tog­ra­phy. With through-the-lens (TTL) light meter­ing, cam­eras are using trans­mit­ted light val­ues to deter­mine expo­sure set­tings. Fur­ther­more, t-stops per­vert the cal­cu­lat­ed val­ues for depth of field and hyper­fo­cal dis­tance, which are direct­ly relat­ed to the actu­al f-stop.

Aperture and sharpness

Sharp­ness, or acu­tance, describes the abil­i­ty of a pho­to­graph­ic lens to resolve fine image detail of a sub­ject that’s in focus. In tech­ni­cal cir­cles, it’s deter­mined by pho­tograph­ing test charts to mea­sure how many dis­tinct lines per mil­lime­tre a lens is capa­ble of resolv­ing. In prac­ti­cal pho­tog­ra­phy, it’s defined by sharp edges in the scene being ren­dered as sharp edges in the pho­to­graph. A sharp lens repro­duces details pre­cise­ly across the frame, while a less­er lens may pro­duce images with a loss of acu­tance 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 lens aber­ra­tions, to which no lens is immune.

Your choice of aper­ture has a strong influ­ence on lens sharp­ness. Opti­cal aber­ra­tions are most pro­nounced when a lens is set to its largest aper­ture. The sever­i­ty of aber­ra­tions decreas­es as the aper­ture is stopped down. In gen­er­al, mod­ern lens­es achieve their peak opti­cal per­for­mance, their “sweet spot,” in the range of ƒ/4 to ƒ/8 (or about 2.5 to 3 stops down from the largest avail­able aper­ture). Com­mon sense would dic­tate that aber­ra­tions should con­tin­ue to decrease as a lens is stopped down beyond this range, but the effec­tive increase in sharp­ness nev­er tran­spires; in fact, once the sweet spot is sur­passed, sharp­ness starts to decline due to dif­frac­tion of light.

graph of aperture settings and its effect on optical aberrations and diffraction
This hypo­thet­i­cal graph demon­strates the rela­tion­ship between the size of a lens aper­ture and its effect on the pres­ence and inten­si­ty of both opti­cal aber­ra­tions and dif­frac­tion. The inten­si­ty of aber­ra­tions in pho­to­graph­ic lens­es is high­est at larg­er aper­tures. As the aper­ture is stopped down, the mag­ni­tude of aber­ra­tions decreas­es; con­cur­rent­ly, the pres­ence of dif­frac­tion increas­es. (It’s impor­tant to note that up to a spe­cif­ic f-num­ber, dif­frac­tion is unseen, either because aber­ra­tions mask it or because it’s too small to be resolved by the camera’s image sen­sor.) The inter­sect­ing point on this graph is what’s known by pho­tog­ra­phers as the “sweet spot” of the lens—the f-stop at which it exhibits peak sharp­ness. In prac­ti­cal terms, stop­ping the aper­ture down fur­ther won’t result in greater sharp­ness, for although aber­ra­tions will decrease, the ben­e­fit is lost to the soft­en­ing effects of dif­frac­tion.

In pho­tog­ra­phy, dif­frac­tion is the phe­nom­e­non of light “bend­ing” slight­ly around the sharp edges of the diaphragm blades, which caus­es it to spread and dif­fuse mar­gin­al­ly more than the light pass­ing through the aperture’s cen­tre. Although dif­frac­tion is present at all aper­ture sizes, it becomes most pro­nounced with small­er aper­tures because a high­er pro­por­tion of the total light strik­ing the image sen­sor is dif­fract­ed. As aper­ture sizes decrease, dif­frac­tion increas­es and the result is reduced image sharp­ness.

Raising the f-stop reduces aberrations and introduces diffraction.
As the aper­ture is pro­gres­sive­ly stopped down from ƒ/1.4 to ƒ/16 there is a reduc­tion of opti­cal aber­ra­tions and then an intro­duc­tion of dif­frac­tion. This par­tic­u­lar lens reach­es its peak per­for­mance at about ƒ/5.6 – ƒ/8, with obvi­ous dif­frac­tion appear­ing at ƒ/11. Dif­frac­tion at ƒ/16 results in a soft­er image that is less sharp, albeit devoid of aber­ra­tions, than at ƒ/1.4.
The scale of diffraction in photography is miniscule.
This is the full image from which the dif­frac­tion ani­ma­tion was cre­at­ed. The green rec­tan­gle rep­re­sents the area vis­i­ble in the ani­ma­tion. Viewed at this scale, the soft­en­ing effects of dif­frac­tion are minute, even at ƒ/16.

Aperture and vignetting

Every pho­to­graph­ic lens pro­duces a cir­cu­lar pro­jec­tion of light. This so-called image cir­cle is always large enough to ade­quate­ly cov­er the image sen­sor for­mat for which the lens is designed. The image circle’s bound­ary, being where the pro­jec­tion ends and shad­ow begins, isn’t sharply delin­eat­ed, but instead, shows a grad­ual dark­en­ing. It’s this zone of tran­si­tion from light to dark that caus­es most lens vignetting.

Lens vignetting, or light fall-off, man­i­fests as the pro­gres­sive dark­en­ing of the image towards the cor­ners of the frame (the cor­ners are clos­est to the image circle’s bound­ary). Most lens­es exhib­it the high­est amount of vignetting at their largest aper­tures, and stop­ping down the aper­ture reduces light fall-off dra­mat­i­cal­ly. This occurs because vignetting is caused by the phys­i­cal obstruc­tion of light by the lens bar­rel, whose shad­ow is ren­dered out of focus—and hence as a grad­ual tran­si­tion to black—due to its prox­im­i­ty to the front­most lens ele­ment. Stop­ping down a lens reduces the depth of field and cre­ates an increas­ing­ly sharp­er image cir­cle boundary—although it nev­er becomes solid—thereby reduc­ing vignetting.