What is Exposure?

Exposure to light

Expo­sure is the vol­ume of light that strikes a pho­to­sen­si­tive medium—this can be either film or a dig­i­tal image sen­sor; this guide con­cen­trates on dig­i­tal photography—and the crit­i­cal ingre­di­ent in deter­min­ing image bright­ness. The total expo­sure received by the image sen­sor is deter­mined by the light intensity—which is a fac­tor of both the lens aper­ture and scene luminance—and the dura­tion of that expo­sure. The fol­low­ing equa­tion express­es this rela­tion­ship:

Exposure = Intensity × Time

The aper­ture is a cir­cu­lar open­ing locat­ed inside the lens; it con­trols the inten­si­ty of light pass­ing through it. The shut­ter is a timed-bar­ri­er sit­u­at­ed to the fore of the image sen­sor; it con­trols the dura­tion that light expos­es the sen­sor. On the cam­era, the aper­ture and shut­ter are the only set­tings for con­trol­ling the inten­si­ty of light reach­ing the image sen­sor.

opening and closing of aperture diaphragm Zenit Helios 44.
The open­ing and clos­ing of a step-less aper­ture diaphragm on a vin­tage Zen­it Helios 44 lens.
horizontal-travel cloth shutter Zenit SLR
High-speed record­ing of a hor­i­zon­tal-trav­el cloth shut­ter in a vin­tage Zen­it SLR film cam­era.

In addi­tion to the aper­ture and shut­ter, most dis­cus­sions of expo­sure will fea­ture a third ele­ment, the ISO. Unlike the aper­ture and shut­ter, which are phys­i­cal mech­a­nisms, ISO is an elec­tron­ic func­tion that sim­u­lates changes to expo­sure. Much like chang­ing expo­sure, the ISO affects the final bright­ness of your pic­tures; how­ev­er, unlike adjust­ing expo­sure, the effect isn’t achieved by vary­ing the amount of light strik­ing the image sen­sor.

Toronto skyline different ISO
Both the ISO 200 and ISO 1600 images were tak­en with the same expo­sure set­ting for aper­ture and shut­ter speed (ƒ/5.6 and 1/15 sec., respec­tive­ly). The only dif­fer­ence is ISO.

Although var­i­ous com­bi­na­tions of aper­ture, shut­ter, and ISO set­tings can result in an image of equal bright­ness, the dis­tinct val­ues select­ed will impart their unique char­ac­ter­is­tics to the final pic­ture. This tri­par­tite rela­tion­ship between aper­ture, shut­ter, and ISO is often called the Expo­sure Tri­an­gle; how­ev­er, giv­en what’s been estab­lished about expo­sure, the name is incor­rect. A more appro­pri­ate name would be the Effec­tive Expo­sure Tri­an­gle. This rela­tion­ship is expressed as:

Effective Exposure = Intensity × Time × ISO

Names and def­i­n­i­tions aside, it’s help­ful to devel­op an intu­itive under­stand­ing for each of the set­tings, since cor­rect expo­sure isn’t exclu­sive­ly about pro­duc­ing a pic­ture of suf­fi­cient bright­ness, it’s also about bal­anc­ing the vari­ables at your dis­pos­al to achieve the desired look.