Exposure Value and the Photographic Stop

Before push­ing ahead to dis­cuss how your camera’s set­tings affect expo­sure and pic­ture bright­ness, it’s essen­tial to famil­iar­ize your­self with the con­cept of expo­sure val­ue, or EV. In pho­tog­ra­phy, expo­sure val­ue has two relat­ed but sep­a­rate mean­ings: first, as a scale of absolute val­ues, and sec­ond, as a unit of rel­a­tive change.

Exposure value as an absolute scale

An expo­sure val­ue is a num­ber on a scale that rep­re­sents scene lumi­nance, which is the amount of light present in your com­po­si­tion and falling on your sub­ject (be it a per­son, place, or thing). Each num­ber on the scale is derived from all pos­si­ble com­bi­na­tions of a camera’s aper­ture and shut­ter val­ues that pro­duce equiv­a­lent expo­sures at ISO 100.

The math­e­mat­i­cal equa­tion for how expo­sure val­ues are derived is too advanced for a beginner’s guide and unnec­es­sary for under­stand­ing cor­rect expo­sure. Attempt­ing to mem­o­rize the table below is also dis­cour­aged because you would need a sec­ond chart to con­vert the expo­sure val­ues into aper­ture and shut­ter set­tings.

There are two rea­sons for intro­duc­ing the scale. First, cam­era man­u­fac­tur­ers use absolute expo­sure val­ues to indi­cate the oper­at­ing lim­its of their cam­eras’ aut­o­fo­cus and expo­sure meter­ing mod­ules. High-per­for­mance cam­eras tend to have a broad­er oper­at­ing range. Sec­ond, the expo­sure val­ue scale demon­strates the immense dif­fer­ences in the inten­si­ties of light you and your cam­era encounter across a vari­ety of sce­nar­ios. How’s this pos­si­ble for an appar­ent­ly mod­est range of EV –6 to EV 18? The scale is log­a­rith­mic.

Exposure values scale

Click to expand the scale.

Exposure Values Scale

EV (100)Light­ing Con­di­tion
>16Rarely encoun­tered arti­fi­cial sources of illu­mi­na­tion.
16Light-coloured sur­face, such as snow or sand, in direct mid­day sun­light
15Direct mid­day sun­light. Full moon at night, high in sky.
14Hazy sun­light or part­ly cloudy con­di­tions.
13Mid­day, cloudy/overcast
12Sub­jects in open shade dur­ing clear mid­day sun­light. Sky­line dur­ing sun­set.
11Sun­sets. Sub­jects in deep shade.
10Art gallery inte­ri­ors. Land- and cityscapes imme­di­ate­ly fol­low­ing sun­set.
9Flood­lit sport­ing events. Art gallery inte­ri­ors. Neon lights. Fires.
8Bright street lights. Office inte­ri­ors.
7Indoor sports. Stage shows. Shad­ed for­est floors dur­ing day­time. Light out­doors about 10 min­utes after sun­set.
6Bright­ly lit home inte­ri­ors.
5Night home inte­ri­ors. Night car traf­fic.
4Can­dle­light. Christ­mas tree lights. Home inte­ri­ors.
3Mon­u­ments, stat­ues, and foun­tains lit by flood­lights.
2Dis­tant office build­ings at night.
1Dis­tant sky­lines at night.
–2Sub­jects under full moon on snow or sand.
–3Sub­jects under full moon.
–4Sub­ject lit by gib­bous moon.
–5Sub­jects lit by starlight and cres­cent moon.
–6Sub­ject lit by starlight.

Exposure value as units of relative change

An expo­sure val­ue also describes an inter­val on the scale above. Adding 1 EV cor­re­sponds to dou­ble the light inten­si­ty; sub­tract­ing 1 EV cor­re­sponds to half the light inten­si­ty. This rela­tion­ship makes the scale log­a­rith­mic. For exam­ple, EV 15 indi­cates a sub­ject bright­ness half that of EV 16 (–1 EV dif­fer­ence, or 1/(2^1) = 1/2) and six­teen times that of EV 11 (4 EV dif­fer­ence, or 2^4 = 16).

The photographic “stop”

Beyond its util­i­ty in describ­ing the dif­fer­ences in absolute scene lumi­nance, rel­a­tive EV is used to describe a spe­cif­ic change to expo­sure. In this role, it’s com­mon­ly called a stop, or less often, a step.

A stop is a unit of change to the rel­a­tive aper­ture or shut­ter set­tings that either dou­bles or halves the expo­sure. When refer­ring to ISO, a stop either dou­bles or halves the effec­tive expo­sure, mean­ing the result­ing pic­ture bright­ness.

 Does 1 EV refer to a spe­cif­ic posi­tion on the absolute scale or a rel­a­tive unit of change? No stan­dard­ized nota­tion exists for dif­fer­en­ti­at­ing between the two. When in doubt, use con­text to deter­mine the author’s intent. This guide refers to the absolute scale using EV # and to the rel­a­tive scale using # EV.

Some­times, it’s not pos­si­ble to achieve cor­rect expo­sure by chang­ing set­tings using full stops. In such cir­cum­stances, pho­tog­ra­phers rely on frac­tions of a stop. Most cam­eras allow adjust­ing expo­sure set­tings using one-half and one-third stop inter­vals, and some pro­fes­sion­al flash units enable adjust­ments as fine as 1/10-stop inter­vals.

 It’s essen­tial to under­stand that a stop has no absolute val­ue; it always describes a change cor­re­spond­ing to an exist­ing amount.