In pho­tog­ra­phy, the term for­mat describes the dimen­sions of the imag­ing medi­um. Through­out most of photography’s his­to­ry, pho­to­graph­ic film has been the medi­um of choice. It was avail­able in a vast vari­ety of sizes to suit the dif­fer­ent require­ments of pho­tog­ra­phers. Three broad cat­e­gories of film for­mats came to exist: large, avail­able as sheets, and medi­um and small, avail­able as rolls.

Film formats

Large for­mat refers to any sheet film of 4×5 inch­es (102×127 mm) or larg­er. It was (and con­tin­ues to be) used pre­dom­i­nant­ly by seri­ous artists and pro­fes­sion­als whose work demands high res­o­lu­tion and detail, and pre­cise­ly con­trolled ren­der­ing of per­spec­tive and depth of field. Medi­um for­mat refers to any roll film pro­duc­ing a neg­a­tive frame larg­er than 24×36 mm and small­er than 4×5 inch­es. Pop­u­lar dimen­sions include 6×4.5 cm (com­mon­ly called “645”), 6×6 cm, and 6×7 cm, all derived from rolls of film with a width of 60.96 mm. It was the most com­mon­ly used for­mat up to the 1960s. Small for­mat gen­er­al­ly refers to 35 mm roll film that pro­duces a neg­a­tive frame of 24×36 mm. It’s been the most wide­ly used film for­mat since the 1960s, enjoy­ing mass adop­tion by con­sumers and pho­to­jour­nal­ists due to the vari­ety, ver­sa­til­i­ty, and ease of use of cam­era sys­tems that sup­port­ed it. In 1996, a con­sor­tium of pho­to­graph­ic com­pa­nies intro­duced an even small­er film for­mat known as Advanced Pho­to Sys­tem (APS). APS enjoyed some mar­ket pen­e­tra­tion with ama­teurs but was pri­mar­i­ly shunned by enthu­si­asts and pro­fes­sion­als because of its small­er neg­a­tive size of 16×24 mm, which showed low­er res­o­lu­tion and more promi­nent grain than equal­ly sized prints derived from 35 mm neg­a­tives.

Digital formats

Con­tem­po­rary dig­i­tal pho­tog­ra­phy offers many choic­es of cam­era sys­tems and image sen­sor for­mats. When DSLRs first appeared on the mar­ket, man­u­fac­tur­ing image sen­sors equal in size to 35 mm film (24×36 mm) was cost­ly. The indus­try respond­ed by devel­op­ing com­pat­i­ble cam­eras with small­er image sen­sors based on dimen­sions of the APS “clas­sic” for­mat (16.7×25.1 mm), known as APS‑C. DSLRs with APS‑C for­mat image sen­sors retained lens mount com­pat­i­bil­i­ty with each brand’s exist­ing 35 mm cam­era sys­tems.

Unfor­tu­nate­ly, this pre­sent­ed many pho­tog­ra­phers with a prob­lem relat­ed to visu­al­iza­tion. The 35 mm cam­era had been the most pop­u­lar cam­era for­mat for decades. Dur­ing this peri­od, pho­tog­ra­phers came to asso­ciate spe­cif­ic focal lengths with the angles of view they offered on that format—a 50 mm lens was nor­mal, a 35 mm lens was mod­er­ate­ly wide, an 85 mm lens was for por­traits, and so on. As long as the for­mat remained the same, focal lengths ren­dered scenes in pre­dictable ways. Dig­i­tal cam­eras with sen­sors small­er than 35 mm film dras­ti­cal­ly changed this pre­dictable rela­tion­ship.

A lens designed for the 35 mm film for­mat pro­duces an image cir­cle large enough to cov­er the imag­ing area ful­ly. When the same lens is mount­ed to an APS‑C cam­era, its image sen­sor records light from a small­er cen­tral region of the image cir­cle, which effec­tive­ly crops out a por­tion of the image that would oth­er­wise be cap­tured by the full frame of 35 mm film. This crop­ping results in a nar­row­er angle of view than the lens would pro­duce with a 35 mm full-frame cam­era.

Crop factor and focal length equivalence

The con­cepts of crop fac­tor and focal length equiv­a­lence help pho­tog­ra­phers under­stand the angle of view pro­duced by their small for­mat dig­i­tal cam­eras and lens­es using a stan­dard ref­er­ence, the 35 mm full-frame for­mat.

Crop fac­tor, some­times called the focal length mul­ti­pli­er, is the math­e­mat­i­cal ratio of a 35 mm full-frame image sensor’s diag­o­nal length (43.3 mm) com­pared to the diag­o­nal length of the image sen­sor under con­sid­er­a­tion. Mul­ti­ply­ing the focal length of a lens by the crop fac­tor of a sys­tem pro­duces the equiv­a­lent 35 mm full-frame focal length that would yield a sim­i­lar angle of view. For exam­ple, if your Nikon APS‑C cam­era (crop fac­tor 1.5) came with an 18–55 mm kit lens, its 35 mm full-frame equiv­a­lent focal length would be 27–83 mm (18×1.5=27; 55×1.5=82.3). Crop fac­tor works in reverse, too. To deter­mine what the focal length equiv­a­lent of a 35 mm full-frame lens is on a small­er for­mat cam­era, divide the focal length by the crop fac­tor. For exam­ple, if you want­ed to repro­duce the angle of view of a 28 mm film lens on your new Canon APS‑C cam­era (crop fac­tor 1.6), you would require a 17.5 mm lens (28÷1.6=17.5).