EOS Documentation Project

Glossary of Terms

by Jeff Conrad

AE

Automatic exposure.

AF

Autofocus.

aperture, relative aperture

The opening of a lens stop. Also, the diameter of the aperture. Strictly speaking, aperture diameter has a linear dimension, but “aperture” commonly is used to mean a dimensionless relative aperture expressed as the ratio of the lens focal length to the f-number (e.g., f/2.8).

One properly speaks of a relative aperture of f/2.8 but an f-number of 2.8. A numerically greater aperture corresponds to a larger opening and results in more exposure; however, a numerically greater aperture has a numerically smaller denominator, e.g., an aperture of f/2.8 is larger and admits more light than an aperture of f/4.

The amount of light admitted by a lens stop depends upon the area of the stop opening; the diameter of the opening is proportional to the square root of the area, so in the standard progression each stop diameter differs from the preceding or following value by a factor of the square root of two, e.g.,

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, ...

Actual nominal values progress by exactly the square root of two; the values in the series above derive from tradition. Many cameras allow the lens opening to be set 1/2-step increments, and some allow it to be set in 1/3-step increments.

Camera and lens markings and displays usually omit the “f/” and show only the f-number. Common practice is to indicate the maximum aperture of a lens in the form 1:N, where N is the f-number; a lens with a maximum aperture of f/2.8 might be marked 1:2.8.

Av

On Canon EOS cameras, aperture-priority automatic exposure mode.

camera movements, movements

See view camera

circle of confusion (CoC)

The image of a point that is not on the plane of focus. Commonly used to mean “permissible circle of confusion,” the largest circle that is indistinguishable from a point under specified conditions of reproduction and viewing. Canon use a value of 0.035 mm for the CoC.

The diameter of the “permissible” circle of confusion for the captured image (i.e., film or electronic sensor) depends upon three factors:

1. The distance at which the final image is viewed. The commonly assumed value is 250 mm, the approximate distance at which human visual acuity is maximum. A comfortable viewing distance is one at which the field of view is approximately 60 degrees, so that an especially small or large final image is likely to be viewed at a distance other than the standard 250 mm.
2. Human visual acuity. Under ideal conditions, the eye can just distinguish a point that subtends one minute of arc (1/60 degree), equivalent to 0.073 mm at the normal viewing distance of 250 mm. Under normal conditions, a more realistic value is about 0.1 mm; the spatial resolution threshold is twice this value, or 0.2 mm. This is a common value for final-image CoC, although 0.25 mm also is sometimes used. In angular terms, with the value of 0.2 mm at a distance of 250 mm, the final-image CoC would subtend 2.75 minutes of arc.
3. The enlargement of the captured image required to produce the final image. If a full-frame 35 mm image is enlarged to fit the short dimension of an 8" x 10" final image, the enlargement is approximately 8x, and the CoC for the captured image is then 1/8 of that for the final image.

Other criteria are sometimes used to determine CoC. One approach is to relate the CoC to the format diagonal; Canon apparently use this approach, and choose CoC equal to 1/1250 of the 35 mm format diagonal to arrive at the CoC of 0.035 mm (see the end of the first paragraph under “Permissible circle of confusion” on page 193 of Canon’s Lens Work II).

DEP, DEP mode

Depth-of-field automatic exposure mode on Canon EOS cameras; the camera sets a focus distance and aperture such that all parts of a scene between user-specified near and far distances are rendered acceptably sharp. The focus distance selected is approximately 7/17 of the distance between the specified near and far distances; otherwise, the result is nearly the same as that achieved with zone focusing.

depth of field (DoF)

A zone in a scene for which all objects are rendered acceptably sharp in the image. A camera can precisely focus on only one plane; a point object in another plane is imaged as a circle rather than a point. The farther the plane from the plane of focus, the larger the circle. If the circle is sufficiently small, however, it is perceived as sharp, so that a zone of acceptable sharpness exists between two planes on either side of the plane of focus. The closest plane in the scene is the near limit of the DoF, the farthest plane the far limit of the DoF. The diameter of a “sufficiently small” circle is known as the permissible circle of confusion, or simply as the circle of confusion.

The DoF depends upon the lens focal length, the object distance, and the lens f-number. The lens focal length and object distance determine the magnification, so that DoF depends on magnification and f-number; lesser magnification and greater f-number (smaller lens opening) give a greater DoF.

EMD

Electromagnetic diaphragm.

exposure step, step

A discrete change in the amount of exposure. The standard photographic exposure scale uses a change of two as the basic step. The terms “step” and “stop” are often used interchangeably, but “stop” properly refers to a lens stop. The progressions of aperture, shutter speed, and film speed correspond to standard exposure steps.

exposure value (EV)

A variable in the Additive Photographic EXposure system (APEX) representing either a combination of camera settings or a light level. Developed in Germany in the early 1960s when cameras lacked exposure meters and many people relied on exposure tables, the APEX system used a logarithmic scale in attempt to simplify determining camera settings. With a step of two, the standard (arithmetic) exposure scale progresses in the sequence

1, 2, 4, 8, 16, 32, 64, 128, 256, ...

A base 2 logarithmic scale corresponding to the same exposure step progresses in the sequence

0, 1, 2, 3, 4, 5, 6, 7, 8, ...

and the resulting exposure equation

E_v = A_v + T_v = L_v + S_v

requires only simple addition to calculate camera settings. The APEX system never became popular, probably because lens and shutter markings on most cameras corresponded to the arithmetic exposure scale. Moreover, soon after the APEX system was introduced, cameras began to include built-in exposure meters, making reference to exposure tables unnecessary.

The concept of exposure value survives, however, and in the current arithmetic scale is defined by the equation

2^EV = A²/T = LS_x/K

Where

EV	= Exposure value
A	= Lens f-number
T	= Shutter time (“shutter speed”)
L	= Scene luminance (“brightness”)
Sx	= Arithmetic ISO film speed
K	= Calibration constant

When the scene luminance L is measured in cd/m², the calibration constant K has values between about 10.7 and 14; Canon use a value of 12.5.

An exposure value can be regarded either as a combination of aperture and shutter speed, or as a luminance level. With the first interpretation, the value EV0 corresponds to an aperture of f/1.0 and a shutter time of 1 s (or any equivalent combination). The basic daylight exposure given by the “sunny 16” rule (f/16 @ 1/ISO film speed) for a camera using ISO 100 film corresponds to approximately EV14.6.

Exposure value as a luminance level often is used by camera manufacturers to specify the sensitivity of autofocus or metering systems. To have meaning in such a situation, the film speed and the calibration constant must be known. Most manufacturers use a film speed of ISO 100^*.

The labels Av and Tv that indicate aperture-priority and shutter-priority AE modes on Canon cameras are remnants of the APEX system, even though apertures and shutter speeds on Canon cameras correspond to the arithmetic progressions.

^*The exact speed is 101.594, an integral multiple of the cube root of two.

f-number

The ratio of lens focal length to aperture diameter (e.g., 2.8). One properly speaks of an f-number of 2.8 but a relative aperture of f/2.8. A numerically greater f-number corresponds to a smaller lens opening and results in less exposure, e.g., an f-number of 4 admits less light than an f-number of 2.8.

f stop

See lens stop.

film speed

A standard indicator of a film’s sensitivity to light; a numerically greater film speed indicates greater sensitivity. Standard arithmetic film speeds are in a progression of a factor of two, e.g.,

32, 64, 125, 250, ...

Actual nominal values progress by exactly a factor of two, with speeds of 32, 64, 128, etc., exact; the values in the sequence above derive from tradition. Films often are available in speeds with 1/3-step increments.

ground glass

A flat piece of glass with a fine texture on one surface. The texture provides a translucent surface suitable for viewing a projected image from a lens, allowing the image to be assessed and the correct focus to be determined.

hyperfocal distance

The focus distance for which the far limit of the depth of field is at infinity. When focus is set to the hyperfocal distance, the near limit of the depth of field is half the hyperfocal distance. The hyperfocal distance is given by

u_h = f ²/Nc + f

Where

c	= Permissible circle of confusion
f	= Lens focal length
N	= Lens f-number
uh	= Hyperfocal distance, measured from the front nodal plane of the lens

An alternative definition for hyperfocal distance is the near limit of the depth of field when the lens is focused on infinity; the hyperfocal distance is then given by

u_h = f ²/Nc

The difference between the two definitions is negligible for nearly all practical purposes.

The distance scales on most hand-camera lenses are calibrated to indicate object-to-image plane distance (shooting distance in Lens Work II), so it often is difficult to exactly correlate calculated hyperfocal distances with marked distances.

hyperfocal focusing

Setting the focus distance to the hyperfocal distance so that the far limit of the depth of field is at infinity; the near limit of the depth of field is at half the hyperfocal distance. “Hyperfocal focusing” often is loosely used to mean zone focusing.

image controls, controls

See view camera

image distance

The distance between the rear nodal plane of a lens and the rear focal plane. Loosely, the distance between the lens and the image.

image space

The space between the rear nodal plane of a lens and the rear focal plane. Loosely, the space between the lens and the image.

IS

Image stabilizer.

lens stop, stop

A device, usually an adjustable iris diaphragm, that controls the amount of light admitted by a lens. The amount of light reaching the image depends upon the lens focal length as well as the size of the stop. The ratio of lens focal length to stop diameter is the f-number; all lenses with the same f-number admit the same amount of light^*. When the diameter of a stop is related to the focal length of a lens, the stop is known as an f stop.

^*This isn’t quite true. The f-number is a purely geometric ratio; the actual amount of light also depends on the transmittance of the lens elements. A lens with many elements usually transmits slightly less light than a lens with only a few elements. With modern coated lenses, the difference in most cases is negligible for still photography.

nodal distance, internodal distance

The distance between the front and rear nodal planes of a lens. The value can be positive, negative, or zero.

nodal space

The space between the front and rear nodal planes of a lens.

magnification, image magnification, lateral magnification

The ratio of image size to object size, usually expressed as decimal value. A multiplication symbol (“x”) is sometimes appended, e.g., 0.25x, with the implication of “times life-size.” An alternative form for expressing magnification is the reproduction ratio, which frequently is included on distance scales of macro lenses.

object distance

The distance between the front nodal plane of a lens and the object. Loosely, the distance between the lens and the subject. Called subject distance in Lens Work II.

object space

The space between the front nodal plane of a lens and the object. Loosely, the space between the lens and the subject.

program mode

An exposure mode on many automatic-exposure cameras in which the camera selects both the aperture and shutter speed. The selections are made in accordance with a “program” that attempts to use the “best” combination for the lighting conditions. At low light levels, the program usually attempts to minimize the effects of camera shake by selecting the lens’s largest aperture to allow use of the fastest shutter speed that will give the correct exposure. At higher light levels that allow a shutter speed sufficiently fast to prevent camera motion blur, aperture is reduced as shutter speed is more gradually increased. On cameras for which the program takes into account the lens focal length, a “sufficiently fast” shutter speed often is approximately the reciprocal of the lens focal length. Some cameras offer the choice of more than one program; some cameras also include program shift that allows the user to override the program.

program shift

A feature on some automatic-exposure cameras that allows the user to use the camera’s program mode and then override the program and select among a series of aperture/shutter speed combinations that give the same exposure. For example, if the camera program selected settings of f/8 @ 1/400 second, program shift would allow the user to “shift” to the equivalent combination of f/11 @ 1/200 second.

reproduction ratio, scale of reproduction

Magnification expressed in the form i:o, where i is the image size and o the object size. When the magnification is less than unity, the image size commonly is set to unity (e.g., 1:5 for a magnification of 0.2). When the magnification is greater than unity, the object size commonly is set to unity (e.g., 2:1 for a magnification of 2).

shooting distance

The distance between the object and the rear focal plane. Loosely, subject-to-image plane distance. The sum of object distance, nodal distance, and image distance.

shutter speed, shutter time

The amount of time that a shutter is open; properly, “shutter time.” Exposure depends directly on the shutter time, so in the standard progression, each shutter time differs from the preceding or following value by a factor of two, e.g.,

1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, ...

Actual nominal values progress by exactly a factor of two; the values in the sequence above derive from tradition. Many cameras allow the shutter time to be set 1/2-step increments, and some allow it to be set in 1/3-step increments.

Camera markings and displays usually omit the numerator, so that a shutter time of 1/8 second is indicated as 8, a time of 1/125 second as 125, and so forth. On some cameras an s is appended to indicate shutter times longer than one second (e.g., 2s). EOS camera displays indicate shutter times longer than one second with the second mark (e.g., 3"), and, when necessary, indicate decimal shutter times using the second mark as the decimal mark (e.g., 0"7).

subject distance

Object distance. The distance between the front nodal plane of a lens and the object. Loosely, the distance between the lens and the subject.

sunny 16 rule

An empirical formula for determining exposure without a meter. The “sunny 16 ” rule states that, for an average frontlighted outdoor scene on a clear day, the correct exposure is f/16 @ 1/ISO film speed (e.g. f/16 @ 1/100 second for ISO 100 film) or an equivalent setting (e.g., f/8 @ 1/400 second for ISO 100 film). The “sunny 16” rule applies when the sun is approximately 20° or more above the horizon, which at moderate geographic latitudes is roughly equivalent to times between 2 hours after sunrise and 2 hours before sunset.

Tv

On Canon EOS cameras, shutter-priority automatic exposure mode.

UD

Ultra-low dispersion.

USM

Ultrasonic motor.

view camera

A camera that allows direct viewing of the image on a ground glass prior to exposure. Exposure is made by replacing the ground glass with a film holder that places the film in the same position that was occupied by the ground glass.

A basic view camera consists of a front standard that holds the lens, a rear standard that holds the ground glass (or the film), and a light-tight flexible bellows that connects the standards. The standards can be supported by a flat bed, as in the case of technical or field cameras, or by a monorail that functions as an optical bench. With either means of support, provision for focusing usually is provided by a rack-and-pinion mechanism that adjusts the distance between the standards.

In addition to focusing, a view camera usually provides for movements that allow other adjustments of the standards relative to each other. Monorail cameras usually offer a greater range of movements, while flat-bed cameras usually are lighter and more compact.

Two types of movements usually are possible: displacement of the lens parallel to the image plane, and rotation of the lens axis relative to the image plane.

Parallel displacement allows the line of sight to be varied while controlling the relationship between parallel lines. When the motion is vertical, it is called rise or fall (or sometimes, rise or drop); when the motion is horizontal, it is called shift or cross.

Rotation of the lens axis allows control of the part of the scene that is acceptably sharp. Rotation about a vertical axis is called a swing; rotation about a horizontal axis is called a tilt.

Because they allow control of the image shape and the zone of sharpness, camera movements are also known as image controls, or sometimes, simply controls.

zone focusing

Setting a focus distance and aperture such that all parts of a scene between specified near and far distances are within a “zone” of acceptable sharpness. On a manual-focus hand camera, this usually is accomplished by using the lens depth-of-field scales; some view cameras include a calculator that performs the same function. When the far limit of the depth of field is infinity, the focus distance is the hyperfocal distance.

The focus distance in object space is given by

u = 2u_nu_f / (u_n + u_f )

The f-number is given approximately by

N = (f ²/c) (u_f –u_n ) / (2u_f u_n )

Setting a greater f-number is equivalent to using a smaller circle of confusion; the focus distance is independent of the f-number, so refocusing isn’t required.

The relationships in image space are somewhat simpler; the focus distance in image space is given approximately by

v = (v_n + v_f ) / 2

and the f-number is given approximately by

N = (v_n –v_f ) / 2c

Where

c	= Permissible circle of confusion
f	= Lens focal length
N	= Lens f-number
u	= Focus distance in object space
uf	= Far limit of the depth of field, in object space
un	= Near limit of the depth of field, in object space
v	= Focus distance in image space
vf	= Image distance for the far limit of the depth of field.
vn	= Image distance for the near limit of the depth of field.

The latter two approximations are the basis of many depth-of-field scales and calculators; they give acceptable results at normal object distances, but the error increases as object distance decreases, so they often aren’t suitable for extreme close-ups.

The DEP mode on EOS cameras achieves much the same result as depth-of-field scales, and does it much faster. As with most depth-of-field scales, DEP mode is optimized for moderate distances rather than for extreme close-ups.

Revision History

v1.2b 2002Sep04

jul: minor reformat

Version 1.2  3 Feb 2002

Revised description of circle of confusion; expanded description of camera controls

Version 1.1 31 Jan 2002

Initial release

Comments

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