The area in front of and behind a focused subject in which the photographed image appears sharp. In other words, the depth of sharpness to the front of sharpness to the front and rear of the subject where image blur in the film plane falls within the limits of the permissible circle of confusion. Depth of field varies according to the lens' focal length, aperture value and shooting distance, so if these values are known, a rough estimate of the depth of field can be calculated using the following formulas:

Front depth of field = d • F • a² / (f² + d • F • a)
Rear depth of field = d • F • a² / (f² – d • F • a)

f:  focal length
F:  F number
d:  minimum circle of confusion diameter
a:  subject distance (distance from 1st principal point to subject)

If the hyperfocal distance is known, the following formulas can also be used:

	Near Point limiting =	Hyperfocal distance X shooting distance
		Hyperfocal distance + shooting distance
	Far Point limiting =	Hyperfocal distance X shooting distance
		Hyperfocal distance – shooting distance
	(Shooting distance: Distance from film plane to subject)

In general photography, depth of field is characterized by the following attributes:
1.  Depth of field is deep at short focal lengths, shallow at long focal lengths.
2.  Depth of field is deep at small apertures, shallow at large apertures.
3.  Depth of field is deep at far shooting distances, shallow at close shooting distances.
4.  Front depth of field is shallower than rear depth of field.

The area in front of and behind the focal plane in which the image can be photographed as a sharp image. Depth of focus is the same on both sides of the image plane (film plane) and can be determined by multiplying the minimum circle of confusion by the F number, regardless of the lens focal length. With modern autofocus SLR cameras, focusing is performed by detecting the state of focus in the image plane (film plane) using a sensor which is both optically equivalent (1:1 magnification) and positioned out of the film plane, and automatically controlling the lens to bring the subject image within the depth of focus area.

A phenomenon in which light waves enter the shadow area of an object. With a photographic lens, exposure is often adjusted by varying the size of the lens' diaphragm (aperture) to adjust the amount of light passing through the lens. Diffraction in a photographic lens occurs at small apertures where the diaphragm edges obstruct the straight-line path of advancing light waves, causing light rays passing near the edge of the diaphragm to bend around the edges as they pass through the diaphragm.

Diffractive Optics, a revolutionary new lens optical technology that permits super-telephoto lenses that are significantly shorter and lighter than previously possible, while simultaneously improving optical performance by reducing chromatic aberrations and even spherical aberrations. More information

The degree to which the light ray bundles leaving the viewfinder converge or disperse. The standard diopter of all EOS cameras is set at — 1 dpt. This setting is designed to allow the finder image to appear to be seen from a distance of 1m. Thus, if a person cannot see the viewfinder image clearly, the person should attach to the camera's eyepiece a dioptric adjustment lens having a power which, when added to the viewfinder's standard diopter, makes it possible to easily see an object at one meter. The numerical values printed on EOS dioptric adjustment lenses indicate the total diopter obtained when the dioptric adjustment lens is attached to the camera.

A phenomenon whereby the optical properties of a medium vary according to the wavelength of light passing through the medium. When light enters a lens or prism, the dispersion characteristics of the lens or prism cause the index of refraction to vary depending on the wavelength thus dispersing the light. This is also sometimes referred to as color dispersion.

Distance from the optical axis of a parallel ray entering a lens.

Designed for use with the digital data transfer of the EOS system made possible by the fully electronic mount, every EF lens incorporates an EMD that electronically controls aperture diameter. The EMD is a diaphragm drive control actuator comprised of a deformation stepping motor and diaphragm blade unit. Features include the following. Because the system is digitally controlled, its level of precision is far higher than that of mechanical linkage systems. The small rotor blades help deliver excellent start/stop response and control. Elimination of linkage shock from mechanical levers makes the system extremely quiet. The fully electronic mount system makes it possible to close down the aperture and confirm the setting and depth of field at the touch of a button. The EMD mechanism delivers superior durability and reliability. Its diaphragm control components are integrated into a single compact unit. And, the electronic control system allows a high degree of freedom in designing unit layout.

With a lens that moves the entire optical system backward and forward when focusing, the amount of lens movement necessary to focus a subject at a limited distance from the infinity focus position.

The human eye can sense monochromatic light wavelengths within the range of 400nm (purple) to 700nm (red). Within this range, the difference in index of refraction between two different wavelengths is called partial dispersion. Most ordinary optical materials have similar partial dispersion characteristics. However, partial dispersion characteristics differ for some glass materials, such as glass that exhibits larger partial dispersion at short wavelengths, FK glass which features a small index of refraction and low dispersion characteristics, fluorite, and glass that exhibits larger partial dispersion at long wavelengths. These types of glass are classified as having extraordinary partial dispersion characteristics. Glass with this property is used in apochromatic lenses to compensate chromatic aberration.

The ability of the eye to distinguish details of an object's shape. Expressed as a numerical value which indicates the inverse of the minimum visual angle at which the eye can clearly distinguish two points or lines, i.e. the resolution of the eye in reference to a resolution of 1'. (Ratio with a resolution of 1' assumed as 1.)

The eye condition in which the image of an infinitely distant point is formed to the retina when the eye is in the accommodation rest state.

In 1856, a German named Seidel determined through analysis the existence of five lens aberations which occur with monochromatic (single wavelength) light. These are called the five aberrations of Seidel.

Distance from the camera's lens mount reference surface to the focal plane (film plane). In the EOS system, flange back is set at 44.00 mm on all cameras. Flange back is also referred to as flange-focal distance.

Light reflected from lens surfaces, the inside of the lens barrel and the inner walls of the camera's mirror box can reach the film and fog part or all of the image area, degrading image sharpness. These harmful reflections are called flare. Although flare can be reduced to a large extent by coating the lens surfaces and using anti-reflection measures in the lens barrel and camera, flare cannot be completely eliminated for all subject conditions. It is therefore desirable to use an appropriate lens hood whenever possible. The term "flare" is also used when referring to the effects of blurring and halo caused by spherical and comatic aberration.

General photographic lenses are designed to achieve an optimum balance of aberration compensation at only one commonly-used shooting distance. Thus, although aberrations are well compensated at the reference shooting distance, aberrations increase at other shooting distances (especially at close shooting distances) and cause image degradation. To prevent this from happening, a floating system is used which varies the interval between certain lens elements in accordance with the extension amount. This method is also referred to as a close-distance aberration compensation mechanism.

Fluorite has extremely low indexes of refraction and dispersion compared to optical glass and features special partial dispersion characteristics (extraordinary partial dispersion), enabling virtually ideal correction of chromatic aberrations when combined with optical glass. This fact has long been known, and in 1880 natural fluorite was already in practical use in the apochromatic objective lenses of microscopes. However, since natural fluorite exists only in small pieces, it cannot be used practically in photographic lenses. In answer to this problem, Canon in 1968 succeeded in establishing production technology for manufacturing large artificial crystals. Thus opening the door for fluorite use in photographic lenses.

When parallel light rays enter the lens parallel to the optical axis, the distance along the optical axis from the lens' second principal point (rear principal point) to the focal point is called the focal length. In simpler terms, the focal length of a lens is the distance along the optical axis from the lens' second principal point to the film plane when the lens is focused at infinity.

When light rays enter a convex lens parallel to the optical axis, an ideal lens will converge all the light rays to a single point from which the rays again fan out in a cone shape. This point at which all rays converge is called the focal point. A familiar example of this is when a magnifying glass is used to focus the rays of the sun to a small circle on a piece of paper or other surface; the point at which the circle is smallest is the focal point. In optical terminology, a focal point is further classified as being the rear or image-side focal point if it is the point at which light rays from the subject converge on the film plane side of the lens. It is the front or object-side focal point if it is the point at which light rays entering the lens parallel to the optical axis from the film plane side converge on the object side of the lens.

A feature on the Image Stabilized super-telephoto EF lenses. The photographer can focus upon a subject and memorize that focus setting, and later return instantly to it with a brief turn of the metal "playback" ring on the lens' barrel.

Absorption lines discovered in 1814 by a German physicist named Fraunhofer (1787-1826), comprising the absorption spectrum present in the continuous spectrum of light emitted from the sun created by the effect of gases in the sun's and earth's atmospheres. Since each line is located at a fixed wavelength, the lines are used for reference in regard to the color (wavelength) characteristics of optical glass. The index of refraction of optical glass is measured based on nine wavelengths selected from among Fraunhofer's lines. In lens design, calculations for correcting chromatic aberrations are also based on these wavelengths.

A type of converging lens, formed by finely dividing the convex surface of a flat convex lens into many concentric circle-shaped ring lenses and combining them to extremely reduce the thickness of the lens while retaining its function as convex lens. In an SLR, to efficiently direct peripheral diffused light to the eyepiece, the side opposite the matte surface of the focusing screen is formed as a fresnel lens with a 0.05mm pitch. Fresnel lenses are also commonly used in flash units, indicated by the concentric circular lines visible on the white diffusion screen covering the flash tube. The projection lens used to project light from a lighthouse is an example of a giant fresnel lens.

The rear group remains fixed and only the front group moves straight backward and forward during focusing. Examples of front group linear extension lenses include the EF 50mm f/2.5 Compact Macro and the EF 85mm f/1.2L USM.

The lens barrel section holding the front lens group rotates to move the front group backward and forward during focusing. This type of focusing is used only in zoom lenses and is not found in single focal length lenses. Representative examples of lenses using this method are the EF 35-80mm f/4-5.6 USM and
EF 100-300mm f/5.6L. Since the filter attachment ring and hood rotate with the lens during focusing, care must be taken when shooting through a glass window to make sure the end of the lens does not contact the glass.

A system that allows the photographer to turn the lens' manual focusing ring and instantly override autofocus – while the lens' AF/MF switch is still in the autofocus mode. More than half of Canon's EF lenses with Ultrasonic Motors have this feature.

Development of the EOS system began with Canon's own "body range-finding and in-lens motor drive system" and "fully electronic mount system", technologies that were developed in 1985 to quickly respond to the trend towards full-fledged autofocusing SLR cameras. The EOS system centers on the camera body and consists of various components including Canon's full line of EF lenses, Speedlite flash units, and interchangeable backs. The three main features of the EOS system are as follows.

1.	Multi-processor system control A high-speed processor in the camera body interfaces with processors in the lens and the flash units, (for high-speed data processing, calculation and communications), to carry out high level systems control.
2.	Multi-actuator system The ideal actuator for each drive unit is located near the drive unit to form a multi-actuator system that realizes high-level automation, high efficiency, and high performance.
3.	Fully electronic interface All data transfer between the camera body, lens, flash and interchangeable back is handled electronically. This not only increases the functionality of the current system, but also creates a network ready to accept future system developments.