Glossary |
Terms that are on use on this site.
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| Far-sightedness | 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. |
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| Five aberrations of Seidel | 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. |
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| Flange back | 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. |
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| Floating system | 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. |
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| Fluorite | 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. |
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| Focal length | 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. |
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| Focal point, focus | 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. |
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| Focus Preset | 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. |
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| Fraunhofer\'s lines | 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. |
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| Fresnel lens | 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. |
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| Front group linear extension | 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. |
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| Front group rotational extension | 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. |
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| Full-time manual focusing | 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. |
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| Fully electronic mount system | 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. |
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