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Depth of field
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In optics, particularly as it relates to film and photography, the depth of field (DOF) is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.
For some images, such as landscapes, a large DOF may be appropriate, while for others, such as portraits, a small DOF may be more effective.
The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number
(relative aperture).
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Encyclopedia
In optics, particularly as it relates to film and photography, the depth of field (DOF) is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions.
For some images, such as landscapes, a large DOF may be appropriate, while for others, such as portraits, a small DOF may be more effective.
The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number
(relative aperture). Except at close-up distances, DOF is approximately determined by the subject magnification and the lens f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.
When focus is set to the hyperfocal distance, the DOF extends from half the hyperfocal distance to infinity, and is the largest DOF possible for a given f-number.
The advent of digital technology in photography has provided additional means of controlling the extent of image sharpness; some methods allow DOF that would be impossible with traditional techniques, and some allow the DOF to be determined after the image is made.
Apparent sharpness
Precise focus is possible at only one distance; at that distance, a point object will produce a point image.
At any other distance, a point object is defocused, and will produce a blur spot shaped like the aperture, which for the purpose of analysis is usually assumed to be circular. When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as “acceptably sharp”. The diameter of the circle increases with distance from the point of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion, or informally, simply as the circle of confusion. The acceptable circle of confusion is influenced by visual acuity, viewing conditions, and the amount by which the image is enlarged (Ray 2000, 52–53). The increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp.
Several other factors, such as subject matter, movement, and the distance of the subject from the camera, also influence when a given defocus becomes noticeable.
For a 35 mm motion picture, the image area on the negative is roughly 22 mm by 16 mm (0.87 in by 0.63 in). The limit of tolerable error is usually set at 0.05 mm (0.002 in) diameter. For 16 mm film, where the image area is smaller, the tolerance is stricter, 0.025 mm (0.001 in). Standard depth-of-field tables are constructed on this basis, although generally 35 mm productions set it at 0.025 mm (0.001 in). Note that the acceptable circle of confusion values for these formats are different because of the relative amount of magnification each format will need in order to be projected on a full-sized movie screen.
(A table for 35 mm still photography would be somewhat different since more of the film is used for each image and the amount of enlargement is usually much less.)
The image format size also will affect the depth of field. The larger the format size, the longer a lens will need to be to capture the same framing as a smaller format. In motion pictures, for example, a frame with a 12 degree horizontal field of view will require a 50 mm lens on 16 mm film, a 100 mm lens on 35 mm film, and a 250 mm lens on 65 mm film. Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: a 50 mm lens has a horizontal field of view of 12 degrees on 16 mm film, 23.6 degrees on 35 mm film, and 55.6 degrees on 65 mm film. What this all means is that because the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. Therefore, compensations in exposure, framing, or subject distance need to be made in order to make one format look like it was filmed in another format.
| | | | | | | | | Above: DOF at various apertures | | —————————— | | | | | | Above: Selective focus |
Effect of lens aperture
For a given subject framing and camera position, the DOF is controlled by the lens aperture diameter, which is usually specified as the f-number, the ratio of lens focal length to aperture diameter. Reducing the aperture diameter (increasing the f-number) increases the DOF; however, it also reduces the amount of light transmitted, and increases diffraction, placing a practical limit on the extent to which DOF can be increased by reducing the aperture diameter.
Motion pictures make only limited use of this control; to produce a consistent image quality from shot to shot, cinematographers usually choose a single aperture setting for interiors and another for exteriors, and adjust exposure through the use of camera filters or light levels. Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects.
Obtaining maximum DOF
Lens DOF scales
Many lenses for small- and medium-format cameras
include scales that indicate the DOF for a given focus distance and
f-number; the 35 mm lens in the image above is
typical. That lens includes distance scales in feet and meters; when a
marked distance is set opposite the large white index mark, the focus is
set to that distance. The DOF scale below the distance scales includes
markings on either side of the index that correspond to
f-numbers; when the lens is set to a given f-number,
the DOF extends between the distances that align with the
f-number markings.
Zone focusing
When the 35 mm lens above is set to f/11 and
focused at approximately 1.4 m, the DOF (a “zone” of acceptable sharpness)
extends from 1 m to 2 m. Conversely, the required focus and f-number
can be determined from the desired DOF limits by locating the near and far DOF
limits on the lens distance scale and setting focus so that the index mark
is centered between the near and far distances; the required
f-number is determined by finding the markings on the DOF scale
that are closest to the near and far distances. For the 35 mm
lens above, if it were desired for the DOF to extend from 1 m to 2 m,
focus would be set to approximately 1.4 m and the aperture set to f/11.
The DOF limits can be determined from a scene by focusing on the farthest object to be
within the DOF and noting the distance on the lens distance scale, and repeating the process
for the nearest object to be within the DOF.
If the near and far distances fall outside the largest f-number markings on the DOF
scale, the desired DOF cannot be obtained; for example, with the 35 mm lens above,
it is not possible to have the DOF extend from 0.7 m to infinity.
Some distance scales have
markings for only a few distances; for example, the 35 mm lens above
shows only 3 ft and 5 ft on its upper scale. Using other distances
for DOF limits requires visual interpolation between marked distances; because the
distance scale is nonlinear, accurate interpolation can be difficult. In most cases,
English and metric distance markings are not coincident, so using both scales to note
focused distances can sometimes lessen the need for interpolation.
Many autofocus lenses have smaller distance and DOF scales and fewer
markings than do comparable manual-focus lenses, so that determining focus
and f-number from the scales on an autofocus lens may be more
difficult than with a comparable manual-focus lens. In most cases, using the lens DOF
scales on an autofocus lens requires that the lens or camera body be set to manual focus.
On a view camera, the focus and f-number can be obtained by
measuring the focus spread
and performing simple calculations; the procedure is described in more detail in the section
Focus and f-number from DOF limits.
Some view cameras include DOF calculators that indicate focus and f-number without
the need for any calculations by the photographer (Ray 2002, 230–231).
Hyperfocal distance
The hyperfocal distance is the nearest focus distance at which the DOF extends to infinity; focusing the camera at the hyperfocal distance results in the largest possible depth of field for a given f-number (Ray 2000, 55). Focusing beyond the hyperfocal distance does not increase the far DOF (which already extends to infinity), but it does decrease the DOF in front of the subject, decreasing the total DOF. Some photographers consider this wasting DOF; however, see The object field method below for a rationale for doing so. If the lens includes a DOF scale, the hyperfocal distance can be set by aligning the infinity mark on the distance scale with the mark on the DOF scale corresponding to the f-number to which the lens is set. For example, with the 35 mm lens shown above set to f/11, aligning the infinity mark with the ‘11’ to the left of the index mark on the DOF scale would set the focus to the hyperfocal distance. Focusing on the hyperfocal distance is a special case of zone focusing in which the far limit of DOF is at infinity.
The object field method
Traditional depth-of-field formulas and tables assume equal circles of
confusion for near and far objects. Some authors, such as
Merklinger (1992),
have suggested that distant objects often need to be much sharper to be
clearly recognizable, whereas closer objects, being larger on the film, do
not need to be so sharp. The loss of detail in distant objects may be
particularly noticeable with extreme enlargements. Achieving this additional
sharpness in distant objects usually requires focusing beyond the
hyperfocal distance, sometimes almost at infinity. For example, if
photographing a cityscape with a traffic bollard in the foreground, this
approach, termed the object field method by Merklinger, would recommend
focusing very close to infinity, and stopping down to make the bollard
sharp enough. With this approach, foreground objects cannot always be made
perfectly sharp, but the loss of sharpness in near objects may be
acceptable if recognizability of distant objects is paramount.
Moritz von Rohr also used an object field method, but unlike Merklinger, he
used the conventional criterion of a maximum circle of confusion diameter in
the image plane, leading to unequal front and rear depths of field.
Limited DOF: selective focus
Depth of field can be anywhere from a fraction of a millimeter to virtually infinite.
In some cases, such as landscapes, it may be desirable to have the entire image in focus,
and a large DOF is appropriate. In other cases, artistic considerations may dictate that only
a part of the image be in focus, emphasizing the subject whilst de-emphasizing the background,
perhaps giving only a suggestion of the environment (Langford 1973, 81).
For example, a common technique in melodramas and horror films is a closeup of a person's face,
with someone just behind that person visible but out of focus. A portrait or
closeup still photograph might use a small DOF to isolate the subject
from a distracting background. The use of limited DOF to emphasize one part of an image is known
as selective focus or differential focus.
Although a small DOF implies that other parts of the image will be unsharp, it does not, by itself,
determine how unsharp those parts will be. The amount of background (or foreground) blur depends
on the distance from the plane of focus, so if a background is close to the subject, it may be difficult
to blur sufficiently even with a small DOF. In practice, the lens f-number is usually adjusted
until the background or foreground is acceptably blurred, often without direct concern for the DOF.
Sometimes, however, it is desirable to have the entire subject sharp while ensuring that the background
is sufficiently unsharp. When the distance between subject and background is fixed, as is the case with
many scenes, the DOF and the amount of background blur are not independent. Although it is not always
possible to achieve both the desired subject sharpness and the desired background unsharpness, several
techniques can be used to increase the separation of subject and background.
For a given scene and subject magnification, the background blur increases with lens focal length.
If it is not important that background objects be unrecognizable, background de-emphasis can be
increased by using a lens of longer focal length and increasing the subject distance to maintain
the same magnification. This technique requires that sufficient space in front of the subject
be available; moreover, the perspective of the scene changes because of the different camera position,
and this may or may not be acceptable.
The situation is not as simple if it is important that a background object, such as a sign, be unrecognizable.
The magnification of background objects also increases with focal length, so with the technique just described,
there is little change in the recognizability of background objects.
However, a lens of longer focal length
may still be of some help; because of the narrower angle of view, a slight change of camera position may suffice
to eliminate the distracting object from the field of view.
Although tilt and swing are normally used to maximize the part of the
image that is within the DOF, they also can be used, in combination with a
small f-number, to give selective focus to a plane that isn't
perpendicular to the lens axis. With this technique, it is possible to
have objects at greatly different distances from the camera in sharp focus
and yet have a very shallow DOF. The effect can be interesting because it
differs from what most viewers are accustomed to seeing.
Near:far distribution
The DOF beyond the subject is always greater than the DOF in front of the
subject. When the subject is at the hyperfocal distance or beyond, the far
DOF is infinite; as the subject distance decreases, near:far DOF ratio
increases, approaching unity at high magnification. The oft-cited
“rule” that 1/3 of the DOF is in front of the subject and 2/3
is beyond is true only when the subject distance is 1/3 the hyperfocal
distance.
DOF vs. format size
To a first approximation, DOF is inversely proportional to format size.
More precisely, if photographs with the same final-image size are taken in
two different camera formats at the same subject distance with the same
field of view and f-number, the DOF is, to a first
approximation, inversely proportional to the format size. Strictly
speaking, this is true only when the subject distance is large in
comparison with the focal length and small in comparison with the
hyperfocal distance, for both formats, but it nonetheless is generally
useful for comparing results obtained from different formats.
To maintain the same field of view, the lens focal lengths must be in
proportion to the format sizes. Assuming, for purposes of comparison, that
the 4×5 format is four times the size of 35 mm format, if a
4×5 camera used a 300 mm lens, a 35 mm camera would need a
75 mm lens for the same field of view. For the same
f-number, the image made with the 35 mm camera would have
four times the DOF of the image made with the 4×5 camera.
In many cases, the DOF is fixed by the requirements of the desired image.
For a given DOF and field of view, the required f-number is
proportional to the format size. For example, if a 35 mm camera
required 11, a 4×5 camera would require 45 to give the
same DOF. For the same ISO speed, the exposure time on the 4×5 would
be sixteen times as long; if the 35 camera required 1/250 second, the
4×5 camera would require 1/15 second. The longer
exposure time with the larger camera might result in motion blur, especially with windy conditions, a moving subject, or an unsteady camera.
Adjusting the f-number to the camera format is equivalent to
maintaining the same absolute aperture diameter.
The greater DOF with the smaller format can be either an advantage or a
disadvantage, depending on the desired effect. For the same amount of
foreground and background blur, a small-format camera requires a smaller
f-number and allows a shorter exposure time than a large-format
camera; however, many point-and-shoot digital cameras cannot provide a very shallow
DOF. For example, a point-and-shoot digital camera with a 1/1.8″
sensor (7.18 mm × 5.32 mm) at a normal focal
length and 2.8 has the same DOF as a 35 mm camera with a normal
lens at 13.
Camera movements and DOF
When the lens axis is perpendicular to the image plane, as is normally the
case, the plane of focus (POF) is parallel to the image plane, and the DOF
extends between parallel planes on either side of the POF. When the lens
axis is not perpendicular to the image plane, the POF is no longer parallel
to the image plane; the ability to rotate the POF is
known as the Scheimpflug principle. Rotation of the POF is
accomplished with camera movements
(tilt, a rotation of the lens about a horizontal axis, or swing, a rotation
about a vertical axis). Tilt and swing are available on most view cameras, and
are also available with specific lenses on some small- and medium-format
cameras.
When the POF is rotated, the near and far limits of DOF are no longer
parallel; the DOF becomes wedge-shaped, with the apex of the wedge nearest
the camera (Merklinger 1993, 31–32). With tilt,
the height of the DOF increases with distance
from the camera; with swing, the width of the DOF increases with distance.
In some cases, rotating the POF can better fit the DOF to the
scene, and achieve the required sharpness at a smaller f-number.
Alternatively, rotating the POF, in combination with a small f-number,
can minimize the part of an image that is within the DOF.
Depth of field formulas
The basis of these formulas is given in the section
Derivation of the DOF formulas;
refer to the diagram in that section for illustration of the quantities discussed below.
Hyperfocal Distance
Let be the lens focal length,
be the lens f-number, and be the
circle of confusion for a given image format. The
hyperfocal distance is given by
Moderate-to-large distances
Let be the distance at which the camera is focused (the
“subject distance”). When is large in comparison with the
lens focal length, the distance from the
camera to the near limit of DOF and the distance
from the camera to the far limit of DOF are
and
When the subject distance is the hyperfocal distance,
and
The depth of field is
For , the far limit of DOF is at infinity and the DOF
is infinite; of course, only objects at or beyond the near limit of DOF
will be recorded with acceptable sharpness.
Substituting for and rearranging, DOF can be expressed as
Thus, for a given image format, depth of field is determined
by three factors: the focal length of the lens, the f-number of the
lens opening (the aperture), and the camera-to-subject distance.
Close-up
When the subject distance approaches the focal length, using
the formulas given above can result in significant errors. For close-up
work, the hyperfocal distance has little applicability, and it usually is
more convenient to express DOF in terms of image magnification. Let
be the magnification; when the subject distance is small in
comparison with the hyperfocal distance,
so that for a given magnification, DOF is independent of focal length.
Stated otherwise, for the same subject magnification, all focal lengths
give approximately the same DOF. This statement is true only when
the subject distance is small in comparison with the hyperfocal distance,
however.
The discussion thus far has assumed a symmetrical lens for which the
entrance and exit pupils coincide with the front and rear
nodal planes, and for which the pupil magnification
(the ratio of exit pupil diameter to that of the
entrance pupil) is unity.
Although this assumption usually is reasonable for large-format lenses, it
often is invalid for medium- and small-format lenses.
When , the DOF for an asymmetrical lens is
where is the pupil magnification. When the
pupil magnification is unity, this equation reduces to that for a
symmetrical lens.
Except for close-up and macro photography, the effect of lens asymmetry is
minimal. At unity magnification, however, the errors from neglecting the
pupil magnification can be significant. Consider a telephoto lens with
and a retrofocus wide-angle lens with , at . The asymmetrical-lens formula gives
and ,
respectively. The symmetrical-lens formula gives in either case. The errors are −33% and 33%, respectively.
Focus and f-number from DOF limits
For given near
and far DOF limits and ,
the required f-number is smallest when focus is set to
When the subject distance is large in comparison with the lens focal
length, the required f-number is
When the far limit of DOF is at infinity,
and
In practice, these settings usually are determined on the image side of the
lens, using measurements on the bed or rail with a view camera, or using
lens DOF scales on manual-focus lenses for small- and medium-format
cameras. If and
are the image distances that correspond to the near and far limits of DOF,
the required f-number is minimized when the image distance
is
In practical terms, focus is set to halfway between the near and far
image distances. The required f-number is
The image distances are measured from the camera's image plane to the
lens's image nodal plane, which is not always easy to locate. In most
cases, focus and f-number can be determined with sufficient
accuracy using the approximate formulas above, which require only the
difference between the near and far image distances;
view camera users often refer to the difference
as the focus spread.
Most lens DOF scales are based on the same concept.
Foreground and background blur
If a subject is at distance and the foreground or background is at distance
, let the distance between the subject and the foreground or background be
indicated by
The blur disk diameter of a detail at distance
from the subject can be expressed as a function of the focal length , subject magnification , and
f-number according to
The minus sign applies to a foreground object, and the plus sign applies to a background object.
The blur increases with the distance from the subject; when , the detail
is within the depth of field, and the blur is imperceptible. If the detail is only slightly
outside the DOF, the blur may be only barely perceptible.
For a given subject magnification, f-number, and distance from the subject
of the foreground or background detail, the degree of detail blur varies with the lens focal length.
For a background detail, the blur increases with focal length; for a foreground detail,
the blur decreases with focal length. For a given scene, the positions of the subject,
foreground, and background usually are fixed, and the distance between subject and the
foreground or background remains constant regardless of the camera position; however, to maintain
constant magnification, the subject distance must vary if the focal length is changed.
For small distance between the foreground or background detail, the effect of focal length is small;
for large distance, the effect can be significant. For a reasonably distant background detail,
the blur disk diameter is
depending only on focal length.
The blur diameter of foreground details is very large if the details are close to the lens.
The ratio is independent of camera format; the blur then is in terms of
circles of confusion.
The magnification of the detail also varies with focal length; for a given detail,
the ratio of the blur disk diameter to imaged size of the detail is independent of focal length,
depending only on the detail size and its distance from the subject. This ratio can be useful
when it is important that the background be recognizable (as usually is the case in evidence or
surveillance photography), or unrecognizable (as might be the case for a pictorial photographer
using selective focus to isolate the subject from a distracting background). As a general rule,
an object is recognizable if the blur disk diameter is one-tenth to one-fifth the size of the object
or smaller (Williams 1990, 205),
and unrecognizable when the blur disk diameter is the object size or greater.
The effect of focal length on background blur is illustrated in van Walree's article on
.
Practical complications
The distance scales on most medium- and small-format lenses indicate
distance from the camera's image plane. Most DOF
formulas, including those in this article, use the object distance
from the lens's object nodal plane, which often is not easy to
locate. Moreover, for many zoom lenses and internal-focusing non-zoom
lenses, the location of the object nodal plane, as well as focal length,
changes with subject distance. When the subject distance is large in
comparison with the lens focal length, the exact location of the object
nodal plane is not critical; the distance is essentially the same whether
measured from the front of the lens, the image plane, or the actual nodal
plane. The same is not true for close-up photography; at unity
magnification, a slight error in the location of the object nodal plane can
result in a DOF error greater than the errors from any approximations in
the DOF equations.
The asymmetrical lens formulas require knowledge of the
pupil magnification, which usually is not specified for medium- and
small-format lenses. The pupil magnification can be estimated by looking
into the front and rear of the lens and measuring the diameters of the
apparent apertures, and computing the ratio (rear diameter divided by front
diameter).
However, for many zoom lenses and internal-focusing non-zoom lenses, the
pupil magnification changes with subject distance, and several measurements
may be required.
Limitations
Most DOF formulas, including those discussed in this article, employ
several simplifications:
- Paraxial (Gaussian) optics is assumed, and technically, the formulas are valid only for rays that are infinitesimally close to the lens axis. However, Gaussian optics usually is more than adequate for determining DOF, and non-paraxial formulas are sufficiently complex that requiring their use would make determination of DOF impractical in most cases.
- Lens aberrations are ignored. Including the effects of aberrations is nearly impossible, because doing so requires knowledge of the specific lens design. Moreover, in well-designed lenses, most aberrations are well corrected, and at least near the optical axis, often are almost negligible when the lens is stopped down 2–3 steps from maximum aperture. Because lenses usually are stopped down at least to this point when DOF is of interest, ignoring aberrations usually is reasonable. Not all aberrations are reduced by stopping down, however, so actual sharpness may be slightly less than predicted by DOF formulas.
- Diffraction is ignored. DOF formulas imply that any arbitrary DOF can be achieved by using a sufficiently large f-number. Because of diffraction, however, this isn't quite true. Once a lens is stopped down to where most aberrations are well corrected, stopping down further will decrease sharpness in the center of the field. At the DOF limits, however, further stopping down decreases the size of the defocus blur spot, and the overall sharpness may increase. Consequently, choosing an f-number sometimes involves a tradeoff between center and edge sharpness, although viewers typically prefer uniform sharpness to slightly greater center sharpness. The choice, of course, is subjective, and may depend upon the particular image. Eventually, the defocus blur spot becomes negligibly small, and further stopping down serves only to decrease sharpness even at DOF limits. Typically, diffraction at DOF limits becomes significant only at fairly large f-numbers; because large f-numbers typically require long exposure times, motion blur often causes greater loss of sharpness than does diffraction. Combined defocus and diffraction is discussed in Hansma (1996), Conrad's (PDF), and Jacobson's .
- Post-capture manipulation of the image is ignored. Sharpening via techniques such as deconvolution or unsharp mask can increase the DOF in the final image, particularly when the original image has a large DOF. Conversely, image noise reduction can reduce the DOF.
- For digital capture with color filter array sensors, demosaicing is ignored. Demosaicing alone would normally reduce the DOF, but the demosaicing algorithm used might also include sharpening.
The lens designer cannot restrict analysis to Gaussian optics and cannot
ignore lens aberrations. However, the requirements of practical
photography are less demanding than those of lens design, and despite the
simplifications employed in development of most DOF formulas, these
formulas have proven useful in determining camera settings that result in
acceptably sharp pictures. It should be recognized that DOF limits are not
hard boundaries between sharp and unsharp, and that there is little point
in determining DOF limits to a precision of many significant figures.
Photolithography
In semiconductor photolithography applications, depth of field is extremely important as integrated circuit layout features must be printed with high accuracy at extremely small size. The difficulty is that the wafer surface is not perfectly flat, but may vary by several micrometres. Even this small variation causes some distortion in the projected image, and results in unwanted variations in the resulting pattern. Thus photolithography engineers take extreme measures to maximize the optical depth of field of the photolithography equipment. To minimize this distortion further, chip makers like IBM are forced to use chemical mechanical polishing machines to make the wafer surface even flatter before lithographic patterning.
Ophthalmology and optometry
A person may sometimes experience better vision in daylight than at night because of an increased depth of field due to constriction of the pupil (i.e., miosis).
Digital techniques for increasing DOF
Focus stacking
Focus stacking is a digital image processing technique which combines multiple images taken at different focus distances to give a resulting image with a greater depth of field than any of the individual source images. Available programs for multi-shot DOF enhancement include Syncroscopy AutoMontage, PhotoAcute Studio, Helicon Focus and CombineZM.
Getting sufficient depth of field can be particularly challenging in macro photography. The images to the right illustrate the increase in DOF that can be achieved by combining multiple exposures.
Wavefront coding
Wavefront coding is a method that convolves rays in such a way to provide an image where fields are in focus simultaneously with all planes out of focus by a constant amount.
Plenoptic cameras
A plenoptic camera uses a microlens array to capture 4D light field information about a scene.
Derivation of the DOF formulas
DOF limits
A symmetrical lens is illustrated at right. The subject, at distance
, is in focus at image distance . Point objects
at distances and would be
in focus at image distances and , respectively; at image distance , they are imaged
as blur spots. The depth of field is controlled by the aperture stop
diameter ; when the blur spot diameter is equal to the
acceptable circle of confusion , the near and far limits
of DOF are at and . From
similar triangles,
and
It usually is more convenient to work with the lens f-number
than the aperture diameter; the f-number is
related to the lens focal length and the aperture diameter
by
substituting into the previous equations and rearranging gives
and
The image distance is related to an object distance
by the thin lens equation
substituting into the two previous equations and rearranging gives the
near and far limits of DOF:
and
Hyperfocal distance
Setting the far limit of DOF to infinity and
solving for the focus distance gives
where is the hyperfocal distance. Setting the subject
distance to the hyperfocal distance and solving for the near limit of DOF
gives
For any practical value of , the focal length is negligible
in comparison, so that
Substituting the approximate expression for hyperfocal distance into the
formulas for the near and far limits of DOF gives
and
Combining, the depth of field is
Moderate-to-large distances
When the subject distance is large in comparison with the lens focal length,
and
so that
For , the far limit of DOF is at infinity and the DOF
is infinite; of course, only objects at or beyond the near limit of DOF
will be recorded with acceptable sharpness.
Close-up
When the subject distance approaches the lens focal length,
the focal length no longer is negligible, and the approximate formulas
above cannot be used without introducing significant error. At close
distances, the hyperfocal distance has little applicability, and it usually
is more convenient to express DOF in terms of magnification. Substituting
and
into the formula for DOF and rearranging gives
after Larmore (1965). At the hyperfocal distance,
the terms in the denominator are equal, and
the DOF is infinite. As the subject distance decreases, so does the second
term in the denominator; when , the second term becomes
small in comparison with the first, and
so that for a given magnification, DOF is independent of focal length.
Stated otherwise, for the same subject magnification, all focal lengths for
a given image format give approximately the same DOF. This
statement is true only when the subject distance is small in comparison
with the hyperfocal distance, however. Multiplying the numerator and
denominator of the exact formula by
gives
Decreasing the focal length increases the second term in the
denominator, decreasing the denominator and increasing the value of the
right-hand side, so that a shorter focal length gives greater DOF. The
effect of focal length is greatest near the hyperfocal distance, and
decreases as subject distance is decreased. However, the near/far
perspective will differ for different focal lengths, so the difference in
DOF may not be readily apparent. When the subject distance is small in
comparison with the hyperfocal distance, the effect of focal length is
negligible, and, as noted above, the DOF essentially is independent of
focal length.
Near:far DOF ratio
From the “exact” equations for near and far limits of DOF, the DOF in front of the subject is
and the DOF beyond the subject is
The near:far DOF ratio is
This ratio is always less than unity; at moderate-to-large subject distances, , and
When the subject is at the hyperfocal distance or beyond, the far DOF is infinite, and the near:far ratio is zero. It’s commonly stated that approximately 1/3 of the DOF is in front of the subject and approximately 2/3 is beyond; however, this is true only when .
At closer subject distances, it’s often more convenient to express the DOF ratio in terms of the magnification
substitution into the “exact” equation for DOF ratio gives
As magnification increases, the near:far ratio approaches a limiting value of unity.
Focus and f-number from DOF limits
The equations for
the DOF limits can be combined to eliminate and solve for
the subject distance. For given near and far DOF limits
and , the
subject distance is
the harmonic mean of the near and far distances. The equations for DOF limits also can be combined to eliminate
and solve for the required f-number, giving
When the subject distance is large in comparison with the lens focal
length, this simplifies to
When the far limit of DOF is at infinity, the equations for and give indeterminate results. But if all terms in the numerator and denominator on the right-hand side of the equation for are divided by , it is seen that when is at infinity,
Similarly, if all terms in the numerator and denominator on the right-hand side of the equation for are divided by , it is seen that when is at infinity,
Most discussions of DOF concentrate on the object side of the lens,
but the formulas are simpler and the measurements usually easier to make on the
image side. If the basic image-side equations
and
are combined and solved for the image distance , the result is
the harmonic mean of the near and far image distances. The basic image-side equations can also be combined and solved for , giving
The image distances are measured from the camera's image plane to the
lens's image nodal plane, which is not always easy to locate. The harmonic mean is always less than the arithmentic mean, but when the difference between the near and far image distances is reasonably small, the two means are close to equal, and focus can be set with sufficient accuracy using
This formula requires only the difference
between the near and far image distances.
View camera users often refer to this difference as the focus spread;
it usually is measured on the bed or focusing rail.
Focus is simply set to halfway between the near and far image distances.
Substituting into the equation for and rearranging gives
One variant of the thin-lens equation is , where is the magnification; substituting this into the equation for gives
At moderate-to-large subject distances, is small compared to unity, and the
f-number can often be determined with sufficient accuracy using
For close-up photography, the magnification cannot be ignored, and the f-number should be determined using the first approximate formula.
As with the approximate formula for , the approximate formulas for
require only the focus spread
rather than the absolute image distances.
When the far limit of DOF is at infinity, .
On manual-focus small- and medium-format lenses, the focus and f-number
usually are determined using the lens DOF scales, which
often are based on the approximate equations above.
Foreground and background blur
If the equation for the far limit of DOF is solved for , and the far distance
replaced by an arbitrary distance , the blur disk diameter
at that distance is
When the background is at the far limit of DOF, the blur disk diameter is equal to the circle
of confusion , and the blur is just imperceptible. The diameter of the background
blur disk increases with the distance to the background. A similar relationship holds for the
foreground; the general expression for a defocused object at distance is
For a given scene, the distance between the subject and a foreground or background object is usually
fixed; let that distance be represented by
then
or, in terms of subject distance,
with the minus sign used for foreground objects and the plus sign used for background objects.
For a relatively distant background object,
In terms of subject magnification, the subject distance is
so that, for a given f-number and subject magnification,
Differentiating with respect to gives
With the plus sign, the derivative is everywhere positive,
so that for a background object, the blur disk size increases with focal length.
With the minus sign, the derivative is everywhere negative,
so that for a foreground object, the blur disk size decreases with focal length.
The magnification of the defocused object also varies with focal length; the magnification of the
defocused object is
where is the image distance of the subject. For a defocused object
with some characteristic dimension , the imaged size of that object is
The ratio of the blur disk size to the imaged size of that object then is
so for a given defocused object, the ratio of the blur disk diameter to object size
is independent of focal length, and depends only on the object size and its distance from the subject.
Asymmetrical lenses
The discussion thus far has assumed a symmetrical lens for which the
entrance and exit pupils coincide with the object and image
nodal planes, and for which the pupil magnification is unity.
Although this assumption usually is reasonable for large-format lenses, it
often is invalid for medium- and small-format lenses.
For an asymmetrical lens, the DOF ahead of the subject distance and the
DOF beyond the subject distance are given by
and
where is the pupil magnification.
Combining gives the total DOF:
When , the second term in the denominator becomes
small in comparison with the first, and
When the pupil magnification is unity, the equations for asymmetrical
lenses reduce to those given earlier for symmetrical lenses.
Effect of lens asymmetry
Except for close-up and macro photography, the effect of lens asymmetry is
minimal. A slight rearrangement of the last equation gives
As magnification decreases, the term becomes smaller in
comparison with the term, and eventually the effect of
pupil magnification becomes negligible.
Further reading
- Hummel, Rob (editor). 2001. American Cinematographer Manual. 8th ed. Hollywood: ASC Press. ISBN 0-935578-15-3
See also
External links
- Luminous Landscape when f-number and subject image size are maintained
- only under certain conditions
- Jeff Conrad’s (PDF). Includes derivations of most DoF formulas
- Joe Englander’s (PDF). Alternative criteria for circle of confusion
- David Jacobson’s
- Rik Littlefield’s
- Justin Snodgrass’s .
- Paul van Walree’s .
- Paul van Walree’s . Includes derivation
- —proprietary software for combining differently focused photos to produce an image with extended DoF.
- —free software for combining differently focused photos to produce an image with extended DoF.
- —proprietary software for combining differently focused photos to produce an image with extended DoF.
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