Metering Light to Determine Exposure

Light meters

A light meter is a tool that mea­sures light inten­si­ty and aids pho­tog­ra­phers in set­ting the appro­pri­ate expo­sure. Pho­to­graph­ic light meters are avail­able as off-cam­era hand­held devices, and they’re built into vir­tu­al­ly every dig­i­tal cam­era.

The only thing a light meter can do is mea­sure the actu­al bright­ness of an object; it can make no assess­ment of whether the object is light or dark, in deep shade or in bright sun. It’s up to you to eval­u­ate and inter­pret the nature of your sub­ject accord­ing to the infor­ma­tion pro­vid­ed by the meter. By under­stand­ing how light meters work and are cal­i­brat­ed, you’ll be bet­ter able to use them to deter­mine the appro­pri­ate expo­sure for a scene.

– John P. Scha­ef­fer.

The 18-percent grey standard

The 18-per­cent, or mid­dle grey, stan­dard is the math­e­mat­i­cal aver­age of all tones even­ly dis­trib­uted across a scale from absolute black to absolute white. In dig­i­tal pho­tog­ra­phy, the mid­dle grey tone lies at the pre­cise mid­dle of the lumi­nance his­togram, and there are cor­re­spond­ing mid­dle tones for the red, green, and blue colour chan­nels. (All future ref­er­ences to 18-per­cent grey or mid­dle grey refer to the tone inde­pen­dent of its colour). When print­ed, a mid­dle grey tone reflects 18 per­cent of inci­dent light (i.e., light falling upon it). Pho­to­graph­ic expo­sures derived from meter­ing the aver­age of all tones in an aver­age scene are remark­able at obtain­ing aver­age results.

Two types of light meters

There are two types of light meters: inci­dent-light meters and reflect­ed-light meters.

Inci­dent-light meters. Inci­dent-light meters mea­sure the amount of light falling on the sub­ject and are only avail­able as off-cam­era, hand­held devices. The most promi­nent fea­ture of inci­dent-light meters is the translu­cent white hemi­spher­i­cal dome (the “lumi­sphere”) that both enclos­es and pro­vides even illu­mi­na­tion to the pho­to­cell with­in, which mea­sures the light’s inten­si­ty. To obtain an expo­sure read­ing, you hold the inci­dent-light meter at the posi­tion of the sub­ject, ensur­ing that the dome is in the same light, point the lumi­sphere towards the cam­era, and take a mea­sure­ment.

Since inci­dent-light meters mea­sure the inten­si­ty of light strik­ing the sub­ject, they pro­vide accu­rate expo­sure infor­ma­tion regard­less of your subject’s inher­ent bright­ness; so long as the sub­jects in your scene are even­ly illu­mi­nat­ed by the same source of light, dark tones will be ren­dered as dark, grey tones as grey, and light tones as light. Fur­ther­more, inci­dent-light meters are incred­i­bly use­ful for mak­ing accu­rate mea­sure­ments in a con­trolled-light­ing envi­ron­ment, such as a stu­dio, and are espe­cial­ly prac­ti­cal when used to cal­cu­late pre­cise con­trast ratios between dif­fer­ent lights.

Despite the neu­tral­i­ty of inci­dent-light mea­sure­ments, there are sev­er­al inher­ent dis­ad­van­tages you should know about. First, mea­sur­ing the inten­si­ty of light falling upon a dis­tant sub­ject may be dif­fi­cult, imprac­ti­cal, or impos­si­ble. For instance, an inci­dent read­ing of the night sky is impos­si­ble, of a sun­lit moun­tain from an out­look cov­ered by the canopy of trees is imprac­ti­cal, and of a sub­ject under com­plex or mot­tled light­ing is impre­cise. Sec­ond­ly, since inci­dent-light meters mea­sure sub­ject illu­mi­na­tion, the expo­sure val­ues they pro­vide for the aper­ture are in the­o­ret­i­cal f‑stops. If the trans­mis­sion val­ue of your lens is sig­nif­i­cant­ly dif­fer­ent from the the­o­ret­i­cal f‑number it’s set to, the expo­sures won’t be accu­rate (see F‑Stops and T‑Stops).

Reflect­ed-light meters. Reflect­ed-light meters work by mea­sur­ing the light reflect­ed off the sub­ject, there­by mea­sur­ing the subject’s bright­ness. Reflect­ed-light mea­sure­ments are tak­en from the intend­ed posi­tion of the cam­era regard­less of whether the meter is on- or off-cam­era. Vir­tu­al­ly every dig­i­tal cam­era meters expo­sure using reflect­ed-light enter­ing the lens, which is known as through-the-lens (TTL) meter­ing.

Hand­held reflect­ed-light meters come in two broad vari­eties. Wide-angle reflect­ed-light meters take an aver­age read­ing from across a rel­a­tive­ly large area. Spot meters are reflect­ed-light meters that read the bright­ness from a rel­a­tive­ly small por­tion of the scene—typically one degree of your field of view. All hand­held spot meters fea­ture a mag­ni­fied viewfind­er with a clear­ly marked cir­cle that out­lines the meter­ing zone. Spot meters tend to be more expen­sive than both inci­dent- and wide-angle reflect­ed-light meters.

Reflected-light meters and 18-percent grey

Every reflect­ed-light meter is cal­i­brat­ed to pro­vide an expo­sure read­ing that ren­ders the sub­ject it’s point­ed at as mid­dle grey. If you take a reflect­ed-light read­ing off a black square of paper, the meter will pro­pose expo­sure val­ues that will ren­der the paper mid­dle grey in the pho­to­graph, there­by over­ex­pos­ing it. If you take a reflect­ed-light read­ing off a white square of paper, the meter will pro­pose expo­sure val­ues that will ren­der the paper mid­dle grey in the pho­to­graph, there­by under­ex­pos­ing it. The only tone for which reflect­ed-light meters pro­vide objec­tive­ly accu­rate expo­sure val­ues is mid­dle grey. 

On-camera exposure metering

Every dig­i­tal cam­era that fea­tures either expo­sure com­pen­sa­tion or a man­u­al expo­sure mode will fea­ture an expo­sure meter either in the viewfind­er, the top LCD pan­el, or both. The expo­sure meter is a sim­ple lin­ear scale of rel­a­tive expo­sure val­ues marked by notch­es, dots, or num­bers, and on some cam­eras, all of the above. The stan­dard expo­sure index, char­ac­ter­ized by the big cen­tral notch, or a zero, always rep­re­sents the opti­mal expo­sure as deter­mined by the camera’s pro­gram­ming. Flank­ing the stan­dard expo­sure index are marks that rep­re­sent incre­ments of 1/3 EV and 1 EV. The marks on the right side rep­re­sent added (or pos­i­tive) expo­sure, and the marks on the left rep­re­sent sub­tract­ed (or neg­a­tive) expo­sure. The expo­sure lev­el indi­ca­tor is the nee­dle or mark­er beneath the notched scale.

Togeth­er, the expo­sure meter scale and expo­sure lev­el indi­ca­tor serve one of three func­tions depend­ing on the expo­sure mode you’re using.

Nikon cam­eras allow users to choose which side of the stan­dard expo­sure index rep­re­sents pos­i­tive or neg­a­tive val­ues.

 On the Canon 7D II and 1DX II, the scale on the bot­tom of the viewfind­er indi­cates expo­sure com­pen­sa­tion amount and auto­ex­po­sure brack­et­ing range, and the scale on the right of the viewfind­er indi­cates the expo­sure lev­el scale that is linked to the camera’s light meter.

Expo­sure com­pen­sa­tion amount. Expo­sure com­pen­sa­tion allows you to increase or decrease image bright­ness beyond what the cam­era deter­mines is opti­mal expo­sure. (See Expo­sure Com­pen­sa­tion). When the cam­era is set to the Pro­gram, Shut­ter Pri­or­i­ty, or Aper­ture Pri­or­i­ty modes, the expo­sure meter shows the amount of com­pen­sa­tion applied in units of EV. When the expo­sure lev­el indi­ca­tor is set to zero, there’s no expo­sure com­pen­sa­tion, and the cam­era reverts to its stan­dard expo­sure pro­gram­ming.

Expo­sure lev­el indi­ca­tor. In Man­u­al mode, the expo­sure lev­el indi­ca­tor shows how much your set expo­sure val­ues devi­ate from the lev­els the cam­era believes are opti­mal, which always reside at the stan­dard expo­sure index. (See Man­u­al Expo­sure).

Auto-expo­sure brack­et­ing range. Expo­sure brack­et­ing is the tech­nique of tak­ing mul­ti­ple expo­sures of the same scene while slight­ly vary­ing the amount of expo­sure between the indi­vid­ual pho­tos. This tech­nique is incred­i­bly use­ful when cap­tur­ing sub­jects with com­plex light­ing, or when you intend to com­bine mul­ti­ple expo­sures into one high dynam­ic range (HDR) image. Auto-expo­sure brack­et­ing (AEB) auto­mates the process with­in a range defined by the user. When AEB is acti­vat­ed, the camera’s expo­sure meter will show mul­ti­ple expo­sure lev­el indi­ca­tors spec­i­fy­ing the AEB range of the con­sec­u­tive shots.   

Standard metering modes

Reflect­ed-light meter­ing is how cam­eras mea­sure sub­ject bright­ness to deter­mine opti­mal expo­sure. Most dig­i­tal cam­eras offer sev­er­al meter­ing modes that fea­ture vary­ing degrees of sophis­ti­ca­tion. Regard­less of the meter­ing mode, it’s impor­tant to remem­ber that every reflect­ed-light meter, includ­ing your camera’s, is cal­i­brat­ed to pro­vide an expo­sure read­ing that ren­ders the sub­ject as a mid­dle grey tone. The most sig­nif­i­cant dif­fer­ence between the meter­ing modes is in their method of inter­pret­ing the dis­tri­b­u­tion and vari­a­tion of the scene’s tonal range.

Evaluative/Matrix/Multi meter­ing. This is the default and most sophis­ti­cat­ed meter­ing mode on most DSLRs and mir­ror­less cam­eras because it works in a vari­ety of sit­u­a­tions and pro­vides accu­rate results most of the time. It works by divid­ing the frame into mul­ti­ple meter­ing seg­ments and ana­lyzes their bright­ness and, some­times, colour. The result­ing matrix of mul­ti­ple seg­ments is eval­u­at­ed based on com­po­si­tion, colour, and dis­tri­b­u­tion of tones. Nikon cam­eras that fea­ture 3D matrix meter­ing also fac­tor the dis­tance infor­ma­tion pro­vid­ed by the aut­o­fo­cus mod­ules. Some mir­ror­less cam­eras eval­u­ate the scene for the pres­ence of promi­nent faces.

Cen­tre-weight­ed meter­ing. This meter­ing mode is designed pri­mar­i­ly for por­trait pho­tog­ra­phy. It con­sid­ers the aver­age bright­ness of the entire frame but gives the great­est promi­nence to the tones in the cen­tral region. Cen­tre-weight­ed meter­ing isn’t rec­om­mend­ed for sce­nar­ios in which mul­ti­ple shots of var­i­ous sub­jects are antic­i­pat­ed, as the expo­sures will fluc­tu­ate.

Aver­age meter­ing. This mode mea­sures the aver­age bright­ness of the entire frame and sets expo­sure to yield a mid­dle grey tone ren­der­ing of that aver­age. This mode pro­duces par­tic­u­lar­ly accu­rate results for land­scapes with the sun out­side the frame. It pro­vides rel­a­tive­ly con­sis­tent expo­sures across mul­ti­ple shots of dif­fer­ent sub­jects under the same light­ing.

Spot meter­ing. The spot meters found in cam­eras are sim­i­lar to those in hand­held light meters. They read light from a small cen­tral area that typ­i­cal­ly com­pris­es one to five per­cent of the frame. On Nikon and most mir­ror­less cam­eras, the spot meter over­laps with the active focus point. On Canon cam­eras, the spot meter is always found in the cen­tre of the frame. Spot meter­ing is both incred­i­bly empow­er­ing and finicky. When using any of the auto-expo­sure modes with spot meter­ing, the cam­era will set its expo­sure val­ues to pro­duce a mid­dle grey ren­di­tion of what­ev­er sub­ject the spot is point­ed towards, and all oth­er tones present in the scene will align accord­ing to their rel­a­tive bright­ness. For this rea­son, using spot meter­ing in any of the auto-expo­sure modes will pro­duce wild­ly incon­sis­tent expo­sures of diverse sub­jects under the same light­ing. For exam­ple, when using auto-expo­sure, spot meter­ing a black dog will pro­duce an expo­sure where the dog appears grey and the scene is over­ex­posed; spot meter­ing a white dog will pro­duce an expo­sure where the dog appears grey, and the pic­ture is under­ex­posed; and, spot meter­ing a grey dog will yield an expo­sure where the dog appears grey, and the scene is cor­rect­ly exposed. Pon­der this point.