Aperture and Exposure

Aperture

Inside a lens is a ring of over­lap­ping blades col­lec­tive­ly known as an iris diaphragm, or iris. The expan­sion or con­trac­tion of the iris blades adjusts the size of the open­ing at its cen­tre, which is called an aper­ture. Chang­ing the size of the aper­ture con­trols the inten­si­ty of light pass­ing through the lens.

The most fit­ting ana­logue for under­stand­ing the struc­ture and func­tion of a lens aper­ture is the eye. The iris of an eye reg­u­lates the retina’s expo­sure to light by either dilat­ing or con­tract­ing the pupil. Inside a cam­era lens, a large aper­ture is sim­i­lar to an enlarged pupil; it rais­es expo­sure by increas­ing the flow of light. A small aper­ture is like a con­tract­ed pupil; it reduces expo­sure by decreas­ing the flow of light.

Zenit-E and Helios 44-2 iris diaphragm animation
Front view of the aper­ture diaphragm stopped down from ƒ/2 to ƒ/16 on a Helios 44 lens.

F-Numbers and f-stops

Aper­ture size set­tings are indi­cat­ed by f-num­bers, which are also known as f-ratios or f-stops (the lat­ter used most com­mon­ly in pho­tog­ra­phy lin­go). They are expressed with the hooked f fol­lowed by a num­ber, such as ƒ/2, ƒ/5.6, and so on. Each f-stop cor­re­sponds to a spe­cif­ic aper­ture size in each lens. The range of pos­si­ble f-stops varies depend­ing on the lens. The stan­dard series of f-stops is:

ƒ/1 ƒ/1.4 ƒ/2 ƒ/2.8 ƒ/4 ƒ/5.6 ƒ/8 ƒ/11 ƒ/16 ƒ/22 ƒ/32

They are con­sid­ered stan­dard f-stops for two rea­sons: the dif­fer­ence in expo­sure when shift­ing between adja­cent num­bers is equal to one stop (1 EV); tra­di­tion­al­ly, lens­es with phys­i­cal aper­ture selec­tion rings would indi­cate the avail­able aper­ture range using these stepped inter­vals. Most cam­eras per­mit the selec­tion of inter­me­di­ate f-stops in one-third stop incre­ments.

aperture ring rotation Fujinon 23mm f1.4
The series of full aper­ture stops on a mod­ern lens. For every f-num­ber increase, there is a halv­ing of expo­sure. The amount of light pass­ing through the lens at ƒ/1.4 is 128 times greater than at ƒ/16.
effects of aperture on exposure
This demon­strates the effect of the aper­ture on expo­sure when the shut­ter speed and ISO remain con­stant.

F-num­bers have an inverse rela­tion­ship to the aper­ture size: a low­er f-num­ber indi­cates a big­ger aper­ture that col­lects a con­sid­er­able vol­ume of light; a high­er f-num­ber means a small­er aper­ture that gath­ers less light. For instance, adjust­ing your aper­ture set­ting from ƒ/4 to ƒ/2.8 dou­bles your expo­sure (adds 1 EV); shift­ing from ƒ/4 to ƒ/5.6 halves your expo­sure (sub­tracts 1 EV); going from ƒ/5.6 to ƒ/2 increas­es expo­sure by three stops (adds 3 EV), which is eight times more light (2×2×2=8). Many begin­ners find this rela­tion­ship unin­tu­itive, even con­fus­ing; the fol­low­ing mnemon­ic device could help you mem­o­rize the inverse con­nec­tion (adapt­ed from Dig­i­tal Pho­tog­ra­phy for Dum­mies):

raising the ƒ stops more light

The f-num­ber is a ratio of the lens focal length divid­ed by the diam­e­ter of the entrance pupil, also known as the effec­tive aper­ture. (An entrance pupil is the image of the aper­ture as seen through the front of the lens.) The equa­tion is N=ƒ/D, where N is the f-num­ber, ƒ the lens focal length, and D the entrance pupil diam­e­ter. For exam­ple, a lens with a 50 mm focal length and an entrance pupil diam­e­ter of 25 mm would have an aper­ture of ƒ/2 (50÷25=2). A lens with a 100 mm focal length and an entrance pupil diam­e­ter of 50 mm would also have an aper­ture of ƒ/2 despite hav­ing an entrance pupil diam­e­ter that’s twice as large.

Since no lens spec­i­fies the diam­e­ter of its entrance pupil, but every lens pro­vides you with focal length and f-stop val­ues, we can express the equa­tion as D=ƒ/N. For exam­ple, take a 50 mm lens set to ƒ/4, and you obtain an entrance pupil of 12.5 mm. Set the same lens to ƒ/1.4, and the entrance pupil becomes 35.7 mm. Now, the rea­son for the unin­tu­itive inverse rela­tion­ship of f-stop num­bers and aper­ture size is appar­ent: the entrance pupil can only be larg­er when set­ting the aper­ture to a small­er f-stop.

This also demon­strates that an increase in focal length decreas­es the inten­si­ty of light reach­ing the image sen­sor. How­ev­er, because aper­tures are cal­i­brat­ed in pro­por­tion to focal length, all lens­es set to the same f-stop will the­o­ret­i­cal­ly trans­mit the same amount of light regard­less of focal length. The equa­tion takes every­thing into account and pro­vides you with an aper­ture f-stop that rep­re­sents equal light inten­si­ty across all lens­es.