Introduction to Camera Shutters

As you’ve pre­vi­ous­ly learned, the pri­ma­ry role of the shut­ter is to con­trol the length of time that light is per­mit­ted to pass through the lens to the image sen­sor. Beyond that, the shut­ter con­trols a crit­i­cal aes­thet­ic ele­ment, the per­cep­tion of move­ment in pho­tog­ra­phy.

Shutter Priority auto-exposure mode, Bulb, and Time

Shut­ter Pri­or­i­ty mode is an auto­mat­ic expo­sure mode in which the pho­tog­ra­ph­er selects the desired shut­ter speed, and the cam­era attempts to achieve opti­mal expo­sure by vary­ing the aper­ture. Shut­ter Pri­or­i­ty mode is com­mon­ly indi­cat­ed as S or Tv (for time val­ue) on most cam­eras’ mode dials. Shut­ter Pri­or­i­ty mode is dif­fer­ent from oth­er auto­mat­ic expo­sure modes because it allows pho­tog­ra­phers to con­trol the per­cep­tion of motion, either by freez­ing move­ment, show­ing move­ment, or min­i­miz­ing cam­era shake.

Your cam­era may offer two addi­tion­al shut­ter-spe­cif­ic func­tions known as Bulb and Time. Press­ing the shut­ter but­ton in Bulb mode (often labelled B) will acti­vate the shut­ter and keep it open for as long as the but­ton remains pressed. Releas­ing the but­ton will ter­mi­nate the expo­sure. Press­ing the shut­ter but­ton once in Time mode (often labelled T) will open the shut­ter, and press­ing it again will close the shut­ter. Both Bulb and Time modes are intend­ed for long expo­sures. They are often used with wired or wire­less remote shut­ter releas­es to avoid vibra­tions induced by touch­ing the cam­era dur­ing expo­sure.

Focal plane shutters

As the name sug­gests, the focal plane shut­ter is posi­tioned just in front of the plane of the image sen­sor. In mod­ern cam­eras, focal plane shut­ters con­sist of two sep­a­rate met­al “cur­tains” (bet­ter described as over­lap­ping blades) that trav­el ver­ti­cal­ly dur­ing expo­sure. When primed for expo­sure, the top (“sec­ond”) cur­tain is in a retract­ed posi­tion above the frame, and the bot­tom (“first”) cur­tain is ful­ly extend­ed upward, block­ing light from the image sen­sor. Upon full depres­sion of the shut­ter but­ton, the image sen­sor acti­vates, and the first cur­tain retracts down to uncov­er it; after a pre­cise inter­val, the sec­ond cur­tain extends down to ter­mi­nate the expo­sure, and the image sen­sor deac­ti­vates.

The process is slight­ly dif­fer­ent in mir­ror­less cam­eras because their elec­tron­ic viewfind­ers and dis­plays receive a real-time video feed from the image sen­sor in place of a DSLR’s opti­cal viewfind­er. When tak­ing a pic­ture, the cam­era momen­tar­i­ly clos­es, or “primes,” the shut­ter before engag­ing as per the descrip­tion above.

In both styles of cam­eras, the shut­ter cur­tains accel­er­ate to a con­stant veloc­i­ty regard­less of the shut­ter speed. At slow­er shut­ter speeds, the first cur­tain retracts to uncov­er the image sen­sor ful­ly, and after a pre­cise inter­val, the sec­ond cur­tain clos­es. Beyond a cer­tain thresh­old, faster shut­ter speeds are achieved by tim­ing the sec­ond cur­tain to start clos­ing before the first cur­tain has ful­ly uncov­ered the image sen­sor. The hor­i­zon­tal slit formed between the two cur­tains expos­es the image sen­sor as it trav­els across its sur­face. Increas­ing the shut­ter speed nar­rows the slit by reduc­ing the inter­val between the first cur­tain start­ing to open and the sec­ond cur­tain start­ing to close. On mod­ern cam­eras, the pre­cise engage­ment and fast move­ment of the shut­ter cur­tains can form very nar­row slits that achieve expo­sures of 1/8000 sec­onds. 

There are three main dis­ad­van­tages to focal plane shut­ters. First, they typ­i­cal­ly have a hard lim­it for flash syn­chro­niza­tion. The fastest shut­ter speed at which an image sen­sor is com­plete­ly uncov­ered is known as its X-sync or flash syn­chro­niza­tion speed. The fastest avail­able X-sync speed on mod­ern cam­eras is typ­i­cal­ly 1/250 sec­ond. Using a flash at faster shut­ter speeds results in uneven illu­mi­na­tion of the image sen­sor as it’s being exposed by a trav­el­ling slit. Cam­eras with phys­i­cal shut­ter speed dials often mark the X-sync speed with an “X.”

Sec­ond­ly, focal plane shut­ters suf­fer from a phe­nom­e­non known as shut­ter shock, which are the minute high-fre­quen­cy vibra­tions cre­at­ed by the shut­ter cur­tains as they accel­er­ate into motion and decel­er­ate to a stop. In DSLRs, the effect is exag­ger­at­ed by the mirror’s slap, which is the vibra­tions induced by the speedy rais­ing and low­er­ing of the mir­ror that gives SLRs their dis­tinct sound. While mir­ror­less cam­eras don’t expe­ri­ence mir­ror slap, they suf­fer from extend­ed shut­ter shock. As a rule of thumb, if you can feel the vibra­tions through your hand, they have the poten­tial to affect your image below a spe­cif­ic shut­ter speed.

Last­ly, focal plane shut­ters may induce an image dis­tor­tion known as a rolling shut­ter when pho­tograph­ing fast-mov­ing sub­jects at shut­ter speeds faster than X-sync. Imag­ine shoot­ing a fast-mov­ing sub­ject with a sta­t­ic cam­era. Expo­sure of the image sen­sor is accom­plished through a nar­row slit. As the nar­row slit trav­els across the focal plane, it expos­es minute­ly dif­fer­ent instances of time in the subject’s pro­gres­sion with­in the frame. The result­ing rolling shut­ter dis­tor­tions vary depend­ing on your subject’s direc­tion of motion. When the subject’s move­ment with­in the frame is per­pen­dic­u­lar to the shut­ter cur­tains, it may appear slant­ed; sub­jects mov­ing in the direc­tion of the shut­ter cur­tains will appear elon­gat­ed or stretched; and, sub­jects mov­ing in the oppo­site direc­tion of the shut­ter cur­tains will appear fore­short­ened or com­pressed. These phe­nom­e­na occur incred­i­bly rarely in prac­ti­cal pho­tog­ra­phy.

Leaf shutters

The leaf shut­ter, or cen­tral lens shut­ter, con­sists of sev­er­al met­al blades, or leaves, in a cir­cu­lar arrange­ment that is sim­i­lar to an iris diaphragm. Dur­ing expo­sure, the blades open and close very quick­ly. When opened, they retract to clear the aper­ture and allow prop­er expo­sure. When closed, the edges over­lap to ensure that no light pen­e­trates the assem­bly. In mod­ern pho­tog­ra­phy, leaf shut­ters are used pre­dom­i­nant­ly in lens­es designed for medi­um and large for­mat cam­eras. They also appear in sev­er­al fixed-lens con­sumer-ori­ent­ed enthu­si­ast cam­eras, such as the Sony DSC-RX1R II and Fuji­film X100F. There are sev­er­al sig­nif­i­cant dif­fer­ences between leaf and focal plane shut­ters.

First, the leaf shut­ter is built direct­ly into the lens and is locat­ed near the iris diaphragm. This adds to the mechan­i­cal com­plex­i­ty of a lens; lens­es that fea­ture leaf shut­ters are typ­i­cal­ly more expen­sive than sim­i­lar lens­es with­out them. Fur­ther­more, a leaf shut­ter does not pre­clude a cam­era from hav­ing a focal plane shut­ter; a cam­era with a focal plane shut­ter can be fit­ted with a lens using a leaf shut­ter.

Two, leaf shut­ters ful­ly expose the entire record­ing sur­face at all avail­able shut­ter speeds. The prac­ti­cal ben­e­fit is that flash syn­chro­niza­tion is avail­able through­out the shut­ter speed range. Since mod­ern medi­um for­mat leaf shut­ters can attain expo­sure dura­tions as fast as 1/2000 sec­ond, this gives them a tremen­dous advan­tage over cam­era sys­tems with focal plane shut­ters with regards to flash pho­tog­ra­phy.

One of the pri­ma­ry draw­backs of the leaf shut­ter design is that the cen­tral por­tion of the image sen­sor is exposed for longer than the edges, which caus­es both vignetting and dis­tort­ed bokeh. The effect is hard to notice at slow­er shut­ter speeds, or when rely­ing exclu­sive­ly on flash for illu­mi­na­tion (because the flash fires at the pre­cise moment the shut­ter is ful­ly open). It becomes increas­ing­ly promi­nent with faster shut­ter speeds and larg­er aper­tures, which aren’t rec­om­mend­ed.

Electronic shutters

An elec­tron­ic shut­ter is a func­tion of an image sen­sor that allows it to acti­vate and deac­ti­vate its read­ing of light over a set peri­od of expo­sure with­out the aid of mechan­i­cal cur­tains or blades. For exam­ple, if your shut­ter speed is set to 1/100 sec­ond, the image sen­sor will read the light val­ues strik­ing its sur­face for 1/100 sec­ond. It acti­vates upon press­ing the shut­ter but­ton and deac­ti­vates after the set peri­od. Elec­tron­ic shut­ters can attain incred­i­bly fast expo­sures, with some cam­eras reach­ing speeds of 1/32,000 sec­ond! In addi­tion, they’re total­ly silent, and, like leaf shut­ters, free from the vibra­tions induced by shut­ter shock. Unfor­tu­nate­ly, cur­rent­ly avail­able elec­tron­ic shut­ters have sev­er­al dis­ad­van­tages: rolling shut­ter, absolute­ly no flash syn­chro­niza­tion, and the poten­tial for band­ing when used with high-fre­quen­cy lights, such as flu­o­res­cent tubes and LEDs.

Elec­tron­ic shut­ters exhib­it a far more severe form of the rolling shut­ter effect than focal plane shut­ters. It’s a con­se­quence of how CMOS image sen­sors (the most com­mon type in use today) record pic­ture infor­ma­tion, and is most com­mon­ly expe­ri­enced as the “Jell-O” effect in quick-pan­ning dig­i­tal videos. Unlike CCDs, CMOS image sen­sors can­not record the light val­ues from every pix­el on their sur­face simul­ta­ne­ous. Instead, the sen­sor acti­vates a sin­gle hor­i­zon­tal line of pix­els at a time until the entire sen­sor is record­ing light. After a pre­cise inter­val equal to the set expo­sure dura­tion, the image sen­sor fol­lows the same sequen­tial process to ter­mi­nate expo­sure and record the data. This is a rel­a­tive­ly slow process in com­par­i­son to the speed of mechan­i­cal shut­ters. The cur­tains of a focal plane shut­ter can trav­el across the span of the image sen­sor much faster than the sen­sor can cycle the expo­sure and cap­ture of suc­ces­sive lines of pix­els to com­ple­tion, even though each indi­vid­ual line of pix­els may be exposed for a short­er total dura­tion. For this rea­son, dig­i­tal cam­eras that use mechan­i­cal shut­ters (leaf or focal plane) will always acti­vate the image sen­sor before the shut­ter cur­tains open and deac­ti­vate it after the shut­ter cur­tains have closed.

The remain­ing dis­ad­van­tages of elec­tron­ic shut­ters all stem from the rolling shut­ter effect. Most cam­eras dis­able the flash when shot with an elec­tron­ic shut­ter because the results would be sim­i­lar to using a focal plane shut­ter above the X-sync speed (see above). Last­ly, at faster shut­ter speeds, elec­tron­ic shut­ters may cap­ture band­ing in the light cast by LEDs and flu­o­res­cent bulbs and tubes. Lights oper­ate at the util­i­ty fre­quen­cy of the AC cur­rent in your coun­try. In North Amer­i­ca, the AC fre­quen­cy is 60 hz, which means that the pow­er to the light cycles on and off six­ty times per sec­ond. Unlike mid- to high-wattage tung­sten bulbs, which retain enough heat through their off cycles so as not to show a notice­able light loss, LED and flu­o­res­cent lights lose and gain bright­ness six­ty times per sec­ond. When tak­ing a pic­ture under such light­ing con­di­tions, the scene’s ambi­ent light lev­els would fluc­tu­ate as the elec­tron­ic shut­ter scans sequen­tial lines of pix­els. The result­ing pat­tern, a grad­ual tran­si­tion between light and dark across por­tions of the pho­to­graph lit by the light source, is known as band­ing.