Lens Focal Length

Focal length

In pho­tog­ra­phy, the most essen­tial char­ac­ter­is­tic of a lens is its focal length, which is a mea­sure­ment that describes how much of the scene in front of you can be cap­tured by the cam­era. Tech­ni­cal­ly, the focal length is the dis­tance between the sec­ondary prin­ci­pal point (com­mon­ly and incor­rect­ly called the opti­cal cen­tre) and the rear focal point, where sub­jects at infin­i­ty come into focus. The focal length of a lens deter­mines two inter­re­lat­ed char­ac­ter­is­tics: mag­ni­fi­ca­tion and angle of view.

focal length animation
Stepped ani­ma­tion of focal lengths rang­ing from 140 mm to 10 mm on an APS-C dig­i­tal cam­era body. On a full-frame cam­era, the equiv­a­lent focal length range would be 210 mm to 15 mm. The camera’s posi­tion and direc­tion of view remain sta­t­ic: notice how this results in most­ly unide­al com­po­si­tions.

Focal length and magnification

The focal length of a lens deter­mines its mag­ni­fy­ing pow­er, which is the appar­ent size of your sub­ject as pro­ject­ed onto the focal plane where your image sen­sor resides. A longer focal length cor­re­sponds to greater mag­ni­fy­ing pow­er and a larg­er ren­di­tion of your sub­ject, and vice ver­sa.

Sub­ject size is direct­ly pro­por­tion­al to the focal length of the lens. For exam­ple, if you pho­to­graph a soc­cer play­er kick­ing a ball, then switch to a lens that is twice the focal length of the first, the ren­dered size of every ele­ment in your image, from the per­son to the ball, will be dou­bled in size along the lin­ear dimen­sions.

It’s impor­tant to under­stand that the degree to which the focal length mag­ni­fies an object does not depend on your cam­era or the size of its image sen­sor. Assum­ing a fixed sub­ject and sub­ject dis­tance, every lens of the same focal length will project an image of your sub­ject at the same scale. For exam­ple, if a 35 mm lens casts a 1.2 cm image of a per­son, that image will remain 1.2 cm high regard­less of your camera’s sen­sor for­mat. How­ev­er, on a Micro Four Thirds for­mat cam­era, the image of that per­son will fill the height of the frame, where­as it will occu­py half the height of a full-frame image sen­sor, and about one-third the height of a medi­um for­mat image sen­sor. As you progress from a small­er sen­sor to a larg­er one, the 1.2 cm high pro­jec­tion of the per­son remains unchanged, but it occu­pies a small­er part of the total frame. There­fore, although the absolute size of the image will stay con­stant across vary­ing image sen­sor for­mats, its size in pro­por­tion to each image sen­sor for­mat will be dif­fer­ent.

Focal length and angle of view

The angle of view describes the breadth, or how much, of a scene is cap­tured by the lens and pro­ject­ed onto your camera’s image sen­sor. It’s expressed in degrees of arc and mea­sured diag­o­nal­ly along the image sen­sor. The rela­tion­ship between the angle of view and the focal length of a lens is inverse­ly pro­por­tion­al. The longer the focal length of a lens, the nar­row­er its angle of view; the short­er the focal length of a lens, the wider its angle of view.

The angle of view for any lens of a giv­en focal length will change depend­ing on the size of its image sen­sor. For exam­ple, a 50 mm lens has a wide angle of view on a medi­um for­mat cam­era, a nor­mal angle of view on a full-frame cam­era, and a nar­row angle of view on an APS-C cam­era.

All lens­es of equal focal length will have the same angle of view when paired to image sen­sors of the same size. All lens­es of equal focal length will project images of the same size when view­ing an iden­ti­cal sub­ject at the same sub­ject-to-cam­era dis­tance.

Normal focal length lenses

A “nor­mal” lens is defined as one whose focal length is equal to the approx­i­mate diag­o­nal length of a camera’s image sen­sor. In prac­tice, such lens­es tend to fall into a range of slight­ly longer focal lengths that are claimed to pos­sess an angle of view com­pa­ra­ble to that of the human eye’s cone of visu­al atten­tion, which is about 55°.

For instance, on full-frame cam­eras, whose image sen­sors mea­sure 36×24 mm, the diag­o­nal length is approx­i­mate­ly 43 mm, and yet, the 50 mm lens is con­ven­tion­al­ly con­sid­ered nor­mal. On APS-C cam­eras (24 × 16 mm), whose diag­o­nal spans about 28 mm, a 35 mm focal length is regard­ed as nor­mal pri­mar­i­ly because its angle of view is sim­i­lar to the 50 mm lens on the full-frame for­mat. There­fore, nor­mal focal lengths will dif­fer as a func­tion of the camera’s image sen­sor size. In fact, as you con­tin­ue read­ing, keep in mind that descrip­tive terms such as “ultra-wide,” “short,” “long,” et cetera, implic­it­ly refer to the angle of view of a lens.

For any giv­en cam­era sys­tem, nor­mal lens­es are gen­er­al­ly the “fastest” avail­able. Adjec­tives such as “fast” and “slow” always describe lens speed, which refers to a lens’ max­i­mum aper­ture open­ing. For instance, a lens with a ƒ/2 or larg­er aper­ture is gen­er­al­ly con­sid­ered fast; a lens with a ƒ/5.6 or small­er aper­ture is deemed to be slow. How is speed rel­e­vant to aper­ture? Recall the reci­procity law: larg­er aper­tures per­mit more light into the cam­era, there­by allow­ing you to use faster shut­ter speeds, and vice ver­sa.

normal 50mm lens on fullframe camera in Toronto Chinatown dumpling restaurant
Toronto’s Dumpling House Restau­rant, 2012. This shot of the dumpling chefs was achieved with a 50 mm lens set to ƒ/4 on a full-frame cam­era.
Normal 50mm lens on full-frame camera in Catedral de Barcelona
Cat­e­dral de Barcelona, 2013. This pho­to­graph was achieved using a 50 mm lens set to ƒ/2 on a full-frame cam­era.
Normal focal length lens on APS-C format Tokyo Imperial Palae
Impe­r­i­al Palace, Tokyo, 2018. This pho­to­graph was tak­en using a 28 mm lens set to ƒ/5.6 on an APS-C for­mat cam­era, achiev­ing a true “nor­mal” angle of view for the sys­tem.

Short focal length lenses

In gen­er­al, a short focal length—or short focus, or “wide-angle”—lens is one whose angle of view is 65° or greater. Recall from above that angle of view is deter­mined by both focal length and image sen­sor size, which means that what qual­i­fies as “short” is pred­i­cat­ed upon a camera’s image sen­sor for­mat. There­fore, on full-frame cam­eras, the thresh­old for wide-angle lens­es is 35 mm or less, and on APS-C cam­eras, it’s 23 mm or less. Lens­es with an angle of view of 85° or greater are called “ultra wide-angle,” which is about 24 mm or less on full-frame and 16mm or less on APS-C cam­eras.

Wide-angle lens­es rep­re­sent the only prac­ti­cal method of cap­tur­ing a scene whose essen­tial ele­ments would oth­er­wise fall out­side the angle of view of a nor­mal lens. Con­ven­tion­al sub­jects of ultra wide-angle lens­es include archi­tec­ture (espe­cial­ly inte­ri­ors), land­scapes, seascapes, cityscapes, astropho­tog­ra­phy, and the entire domain of under­wa­ter pho­tog­ra­phy. Wide-angle lens­es are often used for pho­to­jour­nal­ism, street pho­tog­ra­phy, auto­mo­tive, some sports, and niche por­trai­ture.

35 mm wide-angle lens on full-frame camera, Čertovka Canal, Prague.
Twi­light on the Čer­tov­ka Canal, Prague, 2016. This long expo­sure pho­to­graph was tak­en using a 35 mm lens on a full-frame cam­era secured on a tri­pod.
35 mm wide-angle lens on full-frame camera, Ward's Island Ferry Dock, Toronto.
Shel­tered Piano, Ward’s Island Fer­ry Dock, Toron­to, 2012. This documentary/street pho­tog­ra­phy style pho­to was tak­en with a 35 mm lens on a full-frame cam­era.
Spirit of Ecstasy, 16 mm super wide-angle lens on full-frame camera.
Spir­it of Ecsta­sy, 2016. I made this pho­to­graph short­ly after com­plet­ing por­traits of the cou­ple. It was tak­en with a 16 mm super wide-angle lens set to ƒ/2.8 on a full-frame cam­era. The Rolls-Royce stat­uette was about 10–15 cm from the lens.
16 mm super wide-angle lens on full-frame camera, Gardiner Museum architecture in Toronto
Toronto’s Gar­diner Muse­um of ceram­ics, 2012. This image demon­strates how short focus and super wide-angle lens­es can be used to pho­to­graph archi­tec­tur­al inte­ri­ors. Care was tak­en to lev­el the cam­era to ensure that ver­ti­cal ele­ments ren­dered as ver­ti­cal par­al­lel lines in the pho­to. This image was tak­en at 16 mm using a full-frame cam­era.
16 mm super wide-angle on full-frame camera, Port de Barcelona central building
Cen­tral build­ing of Port de Barcelona, 2013. Like the pre­vi­ous pho­to­graph, this was tak­en using a 16 mm lens on a full-frame cam­era; how­ev­er, the cam­era was angled up towards the build­ing, which cre­ates a “key­stone” effect: the par­al­lel ver­ti­cal lines in the scene ren­der as con­verg­ing lines in the image.
16 mm super wide-angle lens on APS-C camera, north Newfoundland bay.
Sun­set at Nod­dy Bay, New­found­land, 2017. This land­scape pho­to­graph was tak­en with a 16 mm lens on an APS-C cam­era. The cam­era was tilt­ed up to give promi­nence to the sky, and to place the cab­in and water near the bot­tom third of the frame.

There are two types of wide-angle lens­es, rec­ti­lin­ear and fish­eye (some­times termed curvi­lin­ear). The vast major­i­ty of wide-angle lens—and oth­er focal lengths, too—are rec­ti­lin­ear. These types of lens­es are designed to ren­der the straight ele­ments found in a scene as straight lines on the pro­ject­ed image. Despite this, wide-angle rec­ti­lin­ear lens­es cause ren­dered objects to pro­gres­sive­ly stretch and enlarge as they approach the edges of the frame. In pho­tog­ra­phy, all fish­eye lens­es are ultra wide-angle lens­es that pro­duce images fea­tur­ing strong con­vex cur­va­ture. Fish­eye lens­es ren­der the straight ele­ments of a scene with a strong cur­va­ture about the cen­tre of the frame (the lens axis). The effect is sim­i­lar to look­ing through a door’s peep­hole, or the con­vex safe­ty mir­rors com­mon­ly placed at the blind cor­ners of indoor park­ing lots and hos­pi­tal cor­ri­dors. Only straight lines that inter­sect with the lens axis will be ren­dered as straight in images cap­tured by fish­eye lens­es.

15 mm fisheye lens on APS-C camera, Manhattan skyscrapers
Louise Nevel­son Plaza, Man­hat­tan, 2007. A 15 mm fish­eye lens on an APS-C cam­era was used to cap­ture this pho­to­graph. The cur­va­ture of the build­ings, which we know should be straight, give away the nature of this lens. Notice how the por­tion of the build­ing beneath the white line, which almost per­fect­ly inter­sects with the lens axis (red dot), has no dis­cern­able cur­va­ture.
15 mm fisheye lens on full-frame camera, Duomo di Milano.
Look­ing down the nave of the Duo­mo di Milano, 2014. This pho­to was cap­tured with a 15 mm fish­eye lens on a full-frame cam­era, which records more of the lens’ image cir­cle, and ren­ders the scene with greater cur­va­ture. What we intu­itive­ly know are sup­posed to be straight columns show stronger cur­va­ture than in the APS-C exam­ple above.
15 mm fisheye lens on full-frame camera, Civico Tempio di San Sebastiano ceiling, Milan.
Civi­co Tem­pio di San Sebas­tiano, Milan, 2014. The effects of the 15 mm fish­eye (on full-frame cam­era) lens are much less obvi­ous in this pho­to­graph because there are few­er straight lines.

Long-focus lenses

Lens­es with an angle of view of 35° or nar­row­er are con­sid­ered long-focus lens­es. This trans­lates to a focal length of about 70 mm and greater on full-frame cam­eras, and about 45 mm and longer on APS-C cam­eras. it’s com­mon for pho­tog­ra­phers to (incor­rect­ly) refer to long-focus lens­es as “tele­pho­to” lens­es. A true tele­pho­to lens is one whose indi­cat­ed focal length is longer than the phys­i­cal length of its body. Due to this ubiq­ui­tous mis­use of the word, there exists a fur­ther clas­si­fi­ca­tion of long-focus lens­es whose angle of view is 10° or nar­row­er called “super tele­pho­to” lens­es (equal to or greater than 250 mm on full-frame cam­eras and 165 mm on APS-C cam­eras). For­tu­nate­ly, super tele­pho­to lens­es are more often than not actu­al tele­pho­to designs. A great exam­ple is the Canon EF 800 mm f/5.6L IS USM Lens, which is only 461 mm long.

Due to their abil­i­ty to mag­ni­fy dis­tance objects, long-focus lens­es present pho­tog­ra­phers with many uses. They are almost uni­ver­sal­ly laud­ed for por­trai­ture because their nar­row angle of view allows for a high­er mag­ni­fi­ca­tion of the sub­ject from a pleas­ing per­spec­tive. As a rule of thumb, a desir­able focal length for a por­trait lens starts at twice the nor­mal focal length for the cam­era sys­tem (about 85 mm for full-frame and 56 mm for APS-C).

Beyond por­trai­ture, long-focus lens­es are use­ful for iso­lat­ing sub­jects in busy and crowd­ed envi­ron­ments. Pho­to­jour­nal­ists, wed­ding, and sports pho­tog­ra­phers exploit this abil­i­ty reg­u­lar­ly. Due to their mag­ni­fy­ing pow­er, super tele­pho­to lens­es are a main­stay for wildlife and nature pho­tog­ra­phers. Last­ly, long-focus lens­es are fre­quent­ly used by land­scape pho­tog­ra­phers to cap­ture dis­tant vis­tas or to iso­late a fea­ture from its sur­round­ings.

56 mm long focus lens on APS-C camera, National Revolutionary Martyrs' Shrine, Taipei.
Chang­ing of the guards at Nation­al Rev­o­lu­tion­ary Mar­tyrs’ Shrine, Taipei, 2016. This pho­to was shot using a 56 mm lens on an APS-C cam­era, the com­bi­na­tion of which rep­re­sents the com­mon­ly rec­om­mend­ed min­i­mum focal length for a pleas­ing per­spec­tive on por­trai­ture.
Late morn­ing cof­fee break, Uni­ver­si­ty of Toron­to, 2014. This pho­to­graph was tak­en with an 85 mm lens on a full-frame cam­era, which togeth­er pro­vide the same angle of view as the image above. Note its abil­i­ty to sub­tly iso­late the sub­ject against the out of focus win­dows in the back­ground.
long focal length lens 115 mm wedding couple flower petals
Coun­try church wed­ding, 2016. Long focal lengths lens­es are com­mon­ly used by wed­ding and por­trait pho­tog­ra­phers because they can iso­late and/or sep­a­rate the sub­jects from (poten­tial­ly busy) back­grounds. Pho­tographed at 115 mm and ƒ/2.8.
200 mm lens on full frame camera wedding couple kissing.
Kiss­ing cou­ple at Sun­ny­brook Estates, Toron­to, 2016. Because long focal length lens­es have a nar­row angle of view, sub­ject iso­la­tion may be achieved with you mak­ing a judi­cious choice about the angle from which you take the pho­to: a small lat­er­al shift or in your posi­tion rel­a­tive to the sub­ject can have a mean­ing­ful impact on what’s vis­i­ble in the back­ground. Shot using a 200 mm lens at ƒ2.8 on a full-frame cam­era.
White Lion, Metro Toron­to Zoo, 2013. A long focus lens’ increased mag­ni­fi­ca­tion allows you to take pho­tos from a safe dis­tance.
Mosport Park 320 mm long focal length lens
Mosport Park, 2007. This 320 mm ƒ/8 image demon­strates that increased mag­ni­fi­ca­tion can bring you “clos­er” to the action; how­ev­er, it also mag­ni­fies oth­er aspects of the scene, such as the heavy atmos­pher­ic dis­tor­tion vis­i­ble at, and just above, the tarmac’s hori­zon.

Zoom lenses

A zoom lens allows pho­tog­ra­phers to vary its effec­tive focal length through a spec­i­fied range, which alters the angle of view and mag­ni­fi­ca­tion of the image. Zoom lens­es are described by stat­ing their focal length range from the short­est to longest, such as 24–70 mm and 70–200 mm. The focal length range of a zoom lens direct­ly cor­re­lates to its zoom ratio, which is derived by divid­ing the longest focal length by the short­est. Both of the lens­es above have a zoom ratio of approx­i­mate­ly 2.9x, or 2.9:1. The zoom ratio also describes the amount of sub­ject mag­ni­fi­ca­tion a sin­gle lens can achieve across its avail­able focal length range.

As you have learned in the sec­tion on aper­tures and f-num­bers, “an increase in focal length decreas­es the inten­si­ty of light reach­ing the image sen­sor.” This rela­tion­ship is most obvi­ous in zoom lens­es. A “vari­able” aper­ture zoom lens is a lens whose max­i­mum aper­ture becomes small­er with increased focal length. These types of zoom lens­es are sim­ple to spot because they list a max­i­mum aper­ture range instead of a sin­gle num­ber. The range spec­i­fies the max­i­mum aper­ture for the short­est and longest focal lengths of the zoom range. Vari­able aper­ture lens­es are the most com­mon type of zoom lens. A con­stant aper­ture or “fixed” aper­ture zoom lens is one whose max­i­mum aper­ture remains con­stant across the entire zoom range. Fixed aper­ture lens­es are typ­i­cal­ly more mas­sive and more expen­sive than their vari­able aper­ture coun­ter­parts. They are also more straight­for­ward to work with when prac­tic­ing man­u­al expo­sure at the max­i­mum aper­ture since no com­pen­sa­tion for lost light is required dur­ing zoom­ing.

A true zoom lens, known as a par­fo­cal lens, main­tains a set focus dis­tance across its entire focal length range. In the days before dig­i­tal photography—before elec­tron­ic aut­o­fo­cus, even—it was com­mon prac­tice to focus a zoom lens at its longest focal length before tak­ing the pic­ture at the desired (if dif­fer­ent) focal length. This tech­nique is no longer pos­si­ble because con­tem­po­rary vari­able focal length lens­es designed for pho­tog­ra­phy are almost exclu­sive­ly var­i­fo­cal lens­es, which do not main­tain set focus across their zoom range. In prac­tice, most pho­tog­ra­phers do not know the dif­fer­ence because the aut­o­fo­cus algo­rithms in their cam­eras com­pen­sate for the slight vari­a­tions.

Prime lenses

A prime or fixed focal length lens has a set focal length that can­not be changed. There are sev­er­al crit­i­cal dif­fer­ences between prime and zoom lens­es that you should know. Prime lens­es are gen­er­al­ly small­er, faster, and have bet­ter opti­cal char­ac­ter­is­tics than zoom lens­es. Despite this, pho­tog­ra­phers fre­quent­ly opt to shoot with zoom lens­es because of their con­ve­nience: a sin­gle lens can replace sev­er­al of the most pop­u­lar focal length prime lens­es. This is espe­cial­ly impor­tant when you’d pre­fer to pack light, such as dur­ing a trip or a hike.

It’s impor­tant to rec­og­nize that the con­ve­nience and flex­i­bil­i­ty of zoom lens­es can inspire lazy pho­tog­ra­phy. The ease of chang­ing the angle of view encour­ages pho­tog­ra­phers to set­tle on com­po­si­tions that are good-enough, instead of seek­ing out bet­ter per­spec­tives and gain­ing a deep­er under­stand­ing of their sub­jects. What­ev­er lens you have, be it zoom or prime, it’s vital for the devel­op­ment of good pho­tog­ra­phy to con­sid­er your sub­ject from sev­er­al per­spec­tives by walk­ing towards, step­ping away, and cir­cling around them.

The con­stant angle of view of a prime lens forces this type of experimentation—“zooming with your feet”—because the oth­er options are either bad pic­tures or no pic­tures. Fur­ther­more, restrict­ing your­self to a sin­gle focal length for an extend­ed peri­od of time acquaints you to its angle of view and allows you to visu­al­ize a com­po­si­tion before rais­ing the cam­era to your face.

Macro lenses

In pho­tog­ra­phy, the term macro refers to extreme close-ups. Macro lens­es are nor­mal to long-focus lens­es capa­ble of focus­ing on extreme­ly close sub­jects, there­by ren­der­ing large repro­duc­tions. The mag­ni­fi­ca­tion ratio or mag­ni­fi­ca­tion fac­tor is the size of the sub­ject pro­ject­ed onto the image sen­sor in com­par­i­son to its actu­al size. A macro lens’ mag­ni­fi­ca­tion ratio is cal­cu­lat­ed at its clos­est focus­ing dis­tance. A true macro lens is capa­ble of achiev­ing a mag­ni­fi­ca­tion ratio of 1:1 or high­er. Lens­es with mag­ni­fi­ca­tion ratios from 2:1 to 10:1 are called super macro. Ratios over 10:1 cross over into the field of microscopy. When shop­ping for a macro lens, keep in mind that in the con­text of kit lens­es and point-and-shoot cam­eras, some man­u­fac­tur­ers use the macro moniker as mar­ket­ing short­hand for “close-up pho­tog­ra­phy.” These prod­ucts do not achieve 1:1 mag­ni­fi­ca­tion ratios. When in doubt, check the tech­ni­cal spec­i­fi­ca­tions.