Depth of Field

Focus the lens

In pho­tog­ra­phy, space ahead of a lens is known as object space, while space behind is called the image space. In the­o­ry, rays of light from any point in object space should con­verge, or focus, at some point behind the lens. As the dis­tance between the lens and sub­ject changes, the dis­tance behind the lens at which the sub­ject is focused also changes. A sub­ject far­ther from the cam­era will focus clos­er behind the lens than a near­by sub­ject. [This is why macro lens­es are capa­ble of such a long exten­sion: to bring very close objects into focus.]

In prac­tice, pho­tog­ra­phy is a two-dimen­sion­al medi­um that projects light onto a flat image sen­sor for record­ing. The posi­tion of the image sensor’s sur­face deter­mines the focal plane. When rays of light from a sub­ject point con­verge to a point on the focal plane, they’re con­sid­ered in focus. A sub­ject point that’s in focus is sit­u­at­ed along an imag­i­nary two-dimen­sion­al plane, known as the plane of focus, which rep­re­sents the the­o­ret­i­cal plane of crit­i­cal focus. [The plane of focus is par­al­lel to the image sen­sor and per­pen­dic­u­lar to the opti­cal axis.] Focus­ing the lens adjusts its dis­tance to the image sen­sor and shifts the plane of focus either toward or away from the cam­era in object space.

Depth of field

Crit­i­cal focus may only be achieved at pre­cise­ly one plane of focus. All sub­ject points that align with this plane will also be in sharp focus (assum­ing your lens doesn’t exhib­it cur­va­ture of field); any devi­a­tion from this plane results in pro­gres­sive defo­cus­ing since the light rays no longer con­verge at the focal plane. Nev­er­the­less, in prac­tice, there’s an area just ahead of and behind the plane of focus that will be ren­dered as accept­ably sharp in the pho­to­graph because the devi­a­tions from absolute con­ver­gence are too small to notice. The depth of field describes the total region sur­round­ing the plane of focus in which objects are ren­dered as accept­ably sharp accord­ing to the sub­jec­tive stan­dards estab­lished for a par­tic­u­lar pho­to­graph.

Depth of field in practice: deep and shallow focus

Pho­tog­ra­phers exploit the depth of field all of the time to achieve effects such as deep or shal­low focus. Deep focus pho­tog­ra­phy relies on a con­sid­er­able depth of field to achieve accept­able sharp­ness in the fore­ground, mid­dle-ground, and back­ground of the pic­ture. This effect is often asso­ci­at­ed with land­scape pho­tog­ra­phy (where much of the image appears in sharp focus) and some forms of street pho­tog­ra­phy. Shal­low focus pho­tog­ra­phy fea­tures a nar­row or small depth of field, which is char­ac­ter­ized by a sharply focused sub­ject and an out of focus, or blurred, back­ground and fore­ground. This tech­nique is fre­quent­ly used by por­trait photographers—especially those work­ing on loca­tion as opposed to in studio—because it visu­al­ly sep­a­rates the sub­ject from the scene. Bokeh describes the aes­thet­ic qual­i­ty and char­ac­ter of how lens­es ren­der the out of focus ele­ments in a pic­ture.

Controlling the depth of field

The three main fac­tors that con­trol the depth of field are the aper­ture, lens focal length, and sub­ject dis­tance.

Aper­ture. An essen­tial prop­er­ty of all lens­es is that chang­ing the aperture’s diam­e­ter when adjust­ing expo­sure also affects the depth of field. Increas­ing the aper­ture diam­e­ter results in less depth of field and decreas­ing the aper­ture diam­e­ter results in more depth of field. Keep in mind that effects of dif­frac­tion still apply, and it may not be prac­ti­cal to use the small­est aper­ture diam­e­ter pos­si­ble in all sit­u­a­tions (see Reci­procity Law).

Focal length. Lens focal length is a sig­nif­i­cant fac­tor in man­ag­ing the depth of field. Short focal length lens­es pro­duce greater depth of field, while long focal length lens­es pro­duce shal­low depth of field.

Sub­ject dis­tance. As the sub­ject (on which you’re focused) moves pro­gres­sive­ly clos­er to the cam­era, the depth of field decreas­es. 

The depth of field is com­mon­ly expressed using units of length. The sub­ject dis­tance is mea­sured from the focal plane of the cam­era (whose posi­tion is indi­cat­ed on top of your cam­era with the focal plane indi­ca­tor, ɸ) to the point in object space on which the lens is focused. The total depth of field is the entire range of accept­able focus. it’s mea­sured from the near lim­it of accept­able focus, which lies between the cam­era and sub­ject, and the far lim­it of accept­able focus, which lies between the sub­ject and infin­i­ty.

Three prin­ci­ples gov­ern the depth of field.

  1. The depth of field dou­bles when the aper­ture f-num­ber dou­bles.1It’s impor­tant not to con­fuse this with idea of dou­bling as it relates to pho­to­graph­ic stops! For exam­ple, stop­ping down from ƒ/2 to ƒ/4, or from ƒ/8 to ƒ/16, will dou­ble the depth of field. Sim­i­lar­ly, the depth of field decreas­es by half when halv­ing the f-num­ber.
  2. Dou­bling the focal length will reduce the depth of field to one quar­ter; reduc­ing the focal length by one-half will increase the depth of field by four. The depth of field is pro­por­tion­al to the square of the change in focal length.
  3. Dou­bling the sub­ject dis­tance increas­es the depth of field by four times while tripling it will increase the depth of field by nine times. The depth of field is pro­por­tion­al to the square of the change in sub­ject dis­tance.

Acceptable sharpness and circles of confusion

Light from any point in object space emerges from the rear ele­ment of a lens as a cone. When a sub­ject point is in focus, the apex of its light cone coin­cides with the focal plane, which forms an image point in the pho­to­graph. If the sub­ject point doesn’t come into per­fect focus on the image sen­sor, it cre­ates a small blurred cir­cle called a cir­cle of con­fu­sion. The three fac­tors that con­trol the depth of field—the aper­ture, focal length, and sub­ject distance—do so by vary­ing the size of the blur cir­cles. The diam­e­ter of the cir­cle of con­fu­sion with the res­o­lu­tion of the image sen­sor is used to cal­cu­late the depth of field.

It’s impor­tant to under­stand that the depth of field is a the­o­ret­i­cal cal­cu­la­tion that doesn’t take into account lens aber­ra­tions, light dif­frac­tion, and post-cap­ture manip­u­la­tions such as sharp­en­ing and crop­ping.

Camera resolution and display size

There’s no tru­ly objec­tive mea­sure for what qual­i­fies as an accept­able degree of sharp­ness con­cern­ing the depth of field. A pho­to­graph that looks ade­quate­ly sharp when enlarged to fit a 15-inch note­book dis­play may appear slight­ly unsharp when expand­ed to a 30-inch desk­top dis­play. A 24×36 inch print may look sharp from across the room, less sharp from a com­fort­able read­ing dis­tance, and down­right blur­ry from the tip of your nose.

Depth of field scale

Some lens­es have a depth of field scale print­ed direct­ly on their bar­rels or under a trans­par­ent plas­tic win­dow. The depth of field scale con­sists of sev­er­al pairs of num­bers on either side of the dis­tance index, with each pair rep­re­sent­ing an f-stop of cor­re­spond­ing val­ue. When the aper­ture is set to one of the f-stops indi­cat­ed on the scale, the range on the dis­tance scale that lies between this pair is con­sid­ered the depth of field. The f-stop lines on the far side of the focus index rep­re­sent the far lim­its of accept­able focus and the lines on the near side of the focus index rep­re­sent the near lim­its of accept­able focus.

Hyperfocal distance

The hyper­fo­cal dis­tance is the clos­est focus dis­tance at which the depth of  field’s far lim­it of accept­able sharp­ness aligns with infin­i­ty. When a lens is focused to the hyper­fo­cal dis­tance, its near lim­it of accept­able sharp­ness will reside at half that dis­tance to the cam­era. If your lens has the depth of field scale, the sim­plest method for focus­ing to the hyper­fo­cal dis­tance is by rotat­ing the focus ring until the line cor­re­spond­ing to your f-stop’s far lim­it of accept­able sharp­ness aligns with the infin­i­ty mark.   

For any giv­en cam­era, the fac­tors in deter­min­ing hyper­fo­cal dis­tance are the lens focal length and aper­ture size. Adjust­ments to the aper­ture will change the hyper­fo­cal dis­tance: a larg­er aper­ture diam­e­ter will pro­duce a hyper­fo­cal dis­tance that is far­ther out and a small­er aper­ture diam­e­ter will move the hyper­fo­cal dis­tance clos­er to the cam­era. Sim­i­lar­ly, a longer focal length will increase your hyper­fo­cal dis­tance while a short­er focal length will bring it clos­er. Since the hyper­fo­cal dis­tance describes the dis­tance to which your lens must be focused, sub­ject dis­tance isn’t a fac­tor.