Shutter Speed and Exposure

long exposure toronto portlands energy centre winter
The Toron­to Port­lands Ener­gy Cen­tre, 2012. I made this pho­to­graph using a Canon EOS 7D and the EF 70–200 mm ƒ/2.8 L lens. The 30-sec­ond expo­sure allowed me to show a more dynam­ic plume of steam.

Shutter speed and exposure time

The pur­pose of the shut­ter is to con­trol the spe­cif­ic dura­tion that light from the lens projects on to the image sen­sor. Assum­ing all oth­er para­me­ters of expo­sure are equal, using a “slow­er” (longer dura­tion) shut­ter speed will increase expo­sure and result in a brighter pic­ture, and using a “faster” (short­er dura­tion) shut­ter speed will reduce expo­sure and result in a dark­er pic­ture.

Most cam­eras with ful­ly man­u­al and semi-auto­mat­ic con­trols per­mit pho­tog­ra­phers to set the shut­ter speed with­in a pre­de­fined range. The avail­able range varies depend­ing on the make and mod­el of a cam­era. A typ­i­cal sequence of shut­ter speeds is (in sec­onds) 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, and 1/8000. This sequence fol­lows a geo­met­ric pro­gres­sion where the dif­fer­ence in expo­sure between adja­cent val­ues is one stop (1 EV), each expos­ing the image sen­sor for either dou­ble or half the dura­tion of its neigh­bour. Most cam­eras will also per­mit the selec­tion of inter­me­di­ate steps in one-half and one-third stop incre­ments.

The rela­tion­ship between the dura­tion of light strik­ing the image sen­sor and the result­ing total expo­sure is direct­ly pro­por­tion­al. For exam­ple, a shut­ter speed of one sec­ond is dou­ble (+1 EV) both the dura­tion and bright­ness of a one-half sec­ond expo­sure. Mean­while, a shut­ter speed of 1/500 sec­ond receives half (–1 EV) the light of a 1/250 sec­ond expo­sure, and one-quar­ter (–2 EV) the light of 1/125 sec­ond expo­sure. Beyond being pro­por­tion­al, this rela­tion­ship is also straight­for­ward, espe­cial­ly when com­pared to aper­ture stops.

The most com­mon­ly used shut­ter speeds are mea­sured in frac­tions of a sec­ond. Some cam­eras sim­pli­fy those frac­tions by show­ing only the denom­i­na­tor. It’s espe­cial­ly true in the opti­cal viewfind­ers and top LCD pan­els of DSLRs and on the ded­i­cat­ed shut­ter dials of retro-styled cam­eras. For exam­ple, 1/8 sec­ond is shown as 8, 1/125 sec­ond as 125, 1/1000 sec­ond as 1000, and so on. The prac­ti­cal rea­son for such short­hand is that it saves space; how­ev­er, as a con­se­quence, it’s sim­ple to mis­take your slow frac­tion-of-a-sec­ond expo­sure for one that’s sev­er­al sec­onds long. If this describes your camera’s behav­iour, famil­iar­ize your­self with how it dis­tin­guish­es between frac­tion­al and mul­ti-sec­ond expo­sures. For instance, Canon and Fuji­film cam­eras indi­cate full sec­onds by fol­low­ing the num­ber with straight dou­ble quotes, as in 5” for five sec­onds.