Depth of field, depth of field – the distance along the optical axis of the lens between two planes in the space of objects, within which objects are displayed in the conjugate focal plane subjectively sharply. It directly depends on the most important characteristics of the optical system: the main focal length and relative aperture, as well as the focusing distance. In this case, only objects located in one plane of the subject space corresponding to the focusing distance are displayed absolutely sharply.
In everyday speech, the concept of depth of field is denoted by the shorter expression “depth of field”.
However, in optics, the latter denotes a different quantity, which is measured in image space. Its practical assessment is not performed by photographers and cameramen, but it plays an important role in applied fields. The depth of field can be assessed visually on the frosted glass of a direct-sight or SLR camera, as well as on the monitor of an electronic viewfinder or on the appropriate scale on the lens barrel and tables compiled when calculating the optical system.
Depth of Field Criteria
Depth of field is not an absolute value, since it is determined based on the lowest resolution of the lens, as well as on the observation conditions of the resulting image and the capabilities of human vision. The criterion for the depth of field is the scattering circle, which is larger than the diameter of the Airy disk of the objective, since the light scattering of the emulsion is taken into account, which reduces the resolution. In turn, the size of the scattering circles that form the image of the subject depends on the distance between it and the focusing plane. The greater the displacement from the aiming plane, the larger the diameter of such a circle and the lower the sharpness of the image. The points of objects located outside the plane of focus can be displayed subjectively sharply if the diameters of the corresponding scattering circles do not exceed the threshold value.
This value is chosen based on the consideration that when viewed from a distance of the best vision of 25 centimeters, the human eye perceives the image as sharp if the circle of dispersion is less than 0.1 mm. The diameter is accepted as the threshold for large format negatives intended for contact printing. Small-format photographic negatives intended for magnification allow a circle diameter of 0.03-0.05 mm, or 1/1000 of the frame diagonal. For medium format negatives 6 × 6 cm, the circle of scatter should not exceed 0.075 mm. This value is calculated for photo prints of average sizes 13 × 18 and 18 × 24 cm. At higher magnifications, objects located within the calculated depth of field may appear out of focus due to exceeding the threshold value that is invisible to the eye. However, this is compensated by the fact that large images are viewed from a distance.
For a 35-mm film negative, according to Soviet standards, a scattering circle of no more than 0.03 mm was allowed, and for a 16-mm one – 0.015 mm. In widescreen cinematography, the same scattering circle is considered to be calculated as on standard 35 mm film. Overseas, larger sizes of the scattering circle were adopted: in the United States they were 0.05 mm for 35 mm film, and 0.025 mm (0.001 inches) for 16 mm. All these values are also calculated based on the viewing conditions of the finished image, which depend on the size of the auditorium and standard screens.
Depth of Field Factors
Depth of field is inversely proportional to the focal length of the lens and is directly proportional to the f-number. The depth of field of zoom lenses changes simultaneously with the focal length. In addition, the depth of field is directly proportional to the distance the lens is focused. The maximum depth of field is attainable at infinity, which for most lenses starts from 15-20 meters. On the contrary, when aiming at closely spaced objects, a large depth of field is difficult to achieve. This is especially noticeable in macro photography, when the area of sharp images can be fractions of a millimeter even with strong aperture.
From the direct dependences of the depth of field on the focal length and focusing distance, another, indirect, follows: the depth of field is inversely proportional to the magnification of the image of the subject in the focal plane, that is, the scale with which it is displayed. Zooming in is achievable both by getting closer to the subject and by using a longer lens, which in both cases narrows the area of space that is displayed sharply. At the same time, small magnifications allow for a greater depth of field.
In practical photography and filming, the depth of field is often adjusted using an aperture diaphragm with a variable relative aperture. Aperture of the lens allows you to increase the depth of field, all other things being equal. Obtaining a shallow depth of field is possible at relatively short shooting distances using high-aperture optics with an open aperture. The ability to “separate” the subject from the background at large distances of 50-100 meters is provided only by high-aperture telephoto lenses specially produced for sports photography.
The larger the format of the negative (sensor), the more difficult it is to achieve a greater depth of field at the same image scale, since you have to use a longer lens. Large-format cameras require a strong aperture to get a portrait that sharply shows the entire head at the same time, while on a small-format negative this is achievable even at medium apertures. Camcorders with a miniature CCD sensor provide a huge depth of field, even when shooting close-ups. The phenomenon is explained by the dependence of the focal length required to obtain an image with a certain angle of field of view on the size of the frame window. Reducing the frame size to fill it with the same subject allows a shorter throw lens to be used.
Therefore, two images of the same object, taken with cameras of different formats at the same scale from the same distance, with the same relative aperture of the lenses, have different depth of field. A camera with a smaller frame size gives a wider depth of field as a shorter focus lens is used to achieve the same scale.
Influence of camera movement
The described principles of the depth of field dependence are valid only if the optical axis of the lens is strictly perpendicular to the plane of the photographic material or matrix. The tilt of the axis as a result of the movements changes the picture of the sharpness distribution due to the mismatch of the plane of the sharp image with the frame window. This can be used both to expand the area of the image displayed sharply, and to artificially narrow it.
The ability to control the depth of field using slides is typical for gimbal cameras and cameras equipped with a shift lens with the ability to tilt. Compliance with the Scheimpflug principle allows you to display sharply objects located at different distances without aperture of the lens. However, this does not increase the depth of field, but moves the area of the space displayed sharply. Objects outside this area appear out of focus, even if they are at the same distance as sharp ones. The tilt of the optical axis gives the effect of a shallow depth of field in distant landscapes, usually sharp across the entire frame. As a result, large subjects appear subjectively miniature, similar to a model or a toy.
Features of digital photography
The depth-of-field scales, printed on the frames of most interchangeable photo lenses, are designed for photographic film, the emulsion of which has light scattering, which reduces the sharpness of the image. Photo matrices affect the resolution to a much lesser extent, allowing more full use of the capabilities of the same optics used with modern digital SLR cameras. The standards for the latest DSLR lenses are 1.5 times stricter, and are based on the size of the circle of confusion, which is 1/1500 of the diagonal of the full-frame sensor, that is, 28 micrometers. The depth of field, determined by such scales, is quite consistent with the most popular format of a photo print of 10 × 15 cm. For larger photographs and images on a computer monitor, it turns out to be overestimated, since modern sensors provide a higher resolution than film. To an even greater extent, the discrepancy between such scales is manifested when using reduced-size APS-C and Nikon DX sensors. To take into account modern technical capabilities, alternative depth-of-field calculators can be used, calculated based on the matrix pixel size.
Existing digital photography technologies can also significantly increase the depth of field by combining multiple photographs taken at different lens focusing distances (focus bracketing). Special computer applications are currently available that allow you to stitch pictures with variable focus. This technique, which received the name, has become widespread in applied scientific photography, mainly in macro and microphotography, since it is suitable only for shooting stationary objects.
The latest technology of the camera of the light field allows you to adjust the focusing distance and depth of field of the image after shooting using software methods.
The latest models of Nokia smartphones since 2013 have been equipped with a built-in camera with depth-of-field control, which has received the trade name “Refocus”. In this case, the focus can be changed after shooting, which is especially effective for scenes extended in depth.
The front and back boundaries of the sharpened area can be determined by the formulas:
– distance to the front edge of the sharply depicted area;
– focusing distance;
– distance to the back edge of the sharpened space;
– rear main focal length of the lens in meters;
– denominator of the geometric relative aperture of the lens or f-number;
– the diameter of the circle of confusion or the allowed circle of scattering, for negatives with a format of 24 × 36 mm equal to 0.03-0.05 mm (the value in meters is substituted into the formula).
Values ,, are measured from the focal plane of the camera (where the photographic material or the sensor is located). The depth of field is determined by the difference between the back and front edges of the field:
The distance at which the lens is focused when the rear edge of the field of focus lies at “infinity” for a given geometric aperture is called “hyperfocal”. The concept of hyperfocal distance is important in practical photography and filming because it provides the greatest possible depth of field, ranging from infinity to half the focusing distance.
In landscape photography with short-focus optics, the best sharpness is achieved when the lens is focused not at “infinity”, but at the hyperfocal distance. In a simplified way, this is achieved by combining the “infinity” symbol of the focusing scale with the division of the depth of field scale corresponding to the current aperture. Then the front edge of the sharply depicted space will be at a distance equal to half the aiming distance. When the objects are located no closer than this distance, the entire displayed space in the photograph will be practically sharp, taking into account the size of the scattering circle. Most wide-angle lenses for small format cameras and 35mm cinema cameras, when focusing at the hyperfocal distance, display sharp objects at almost any distance. Before the advent of effective autofocus systems, this phenomenon was used in reportage and sports photography, when there was not enough time for accurate focusing.
Compact devices with a small frame size and short-throw lens, such as webcams, action cameras, camera phones and CCTV cameras, often do not require focusing due to the fixed installation of a prime-focus lens at the hyperfocal distance. The same applies to the simplest cameras and movie cameras.
The hyperfocal distance for each lens is individual and depends on the current f-number. Calculated by the formula:
– focal length;
– the denominator of the relative hole;
– diameter of the scatter circle;
– hyperfocal distance.
For practical calculations, you can use a simplified formula:
When photographing infinity, the use of hyperfocal distance simplifies the formulas for calculating the boundaries of sharpness:
– the front edge of the sharpened space;
is the distance to focus on;
– the back edge of the sharpened space;
– hyperfocal distance at a given relative aperture.
From the formulas it follows that the area of focus is longer from the focusing plane to the rear edge of focus than from the focusing plane to the front edge of focus.
To determine the aiming plane at the given front and rear boundaries of sharpness, use the formula:
Practical importance of depth of field
The large depth of field required to accurately display detail is not always considered a good shot. Sharpening the main subject in art photography and cinematography is traditionally used as an expressive means, along with tonal and linear perspective.
For classic photo and film cameras with a large frame size, a shallow depth of field is characteristic, which makes it possible to effectively use this technique. Full-frame digital SLR cameras and digital cinema cameras of the Super-35 format are especially convenient in this respect. Special portrait lenses belong to the telephoto group and have a shallow depth of field. On the contrary, the miniaturization of technology and the spread of mobileography are characterized by a tendency to increase the depth of field, easily achievable at short focal lengths. This allows most of these devices to do without focusing, but affects the aesthetics of the image devoid of volume.
Depth of field imitation is often used in 3D graphics and computer games to give an image a true “optical” look. It also helps focus the player’s attention on the main subject or character. On specialized sites, this effect is usually called the English analogue of the term “depth of field” – Depth of Field, DOF.
At the same time, modern cinematography, which is developing in the direction of increasing entertainment due to the widespread distribution of 3D technologies, tends to abandon such an expressive means as sharpening at its shallow depth. Volume transfer is achieved in stereoscopic cinema in other ways that do not require “classical” expressive means. This approach makes it difficult to stage complex scenes, for example, when filming the movie “Stalingrad” using the latest IMAX 3D technologies, when the image was shot with the expectation of achieving the maximum depth of field for the entire frame. The image of the fantastic “Avatar” was created in the same way. The modern cinematography school proceeds from the premise that a large depth of field allows you to fully exploit the advantages of a three-dimensional image and increase the effect of presence.
In traditional “flat” cinematography, cinematographers prefer to use relatively long focal length cine lenses, which allow them to sharpen their subject. Compact camcorders with a small sensor can use the entire frame of such optics using DOF adapters with an intermediate image.