Glossary |
Terms that are on use on this site.
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| Aberration | he image formed by an ideal photographic lens would have the following characteristics:
1. A point would be formed as a point.
2. A plane (such as a wall) perpendicular to the optical axis would be formed as a plane.
3. The image formed by the lens would have the same shape as the subject.
Also, from the standpoint of image expression, a lens should exhibit true color reproduction. If only light rays entering the lens close to the optical axis are used and the light is monochromatic (one specific wavelength), it is possible to realize virtually ideal lens performance. With real photographic lenses, however, where a large aperture is used to obtain sufficient brightness and the lens must converge light not only from near the optical axis but from all areas of the image, it is extremely difficult to satisfy the above-mentioned ideal conditions due to the existence of the following obstructive factors:
* Since most lenses are constructed solely of lens elements with spherical surfaces, light rays from a single subject point are not formed in the image as a perfect point. (A problem unavoidable with spherical surfaces.)
* The focal point position differs for different types (i.e., different wavelengths) of light.
* There are many requirements related to changes in angle of view (especially with wide-angle, zoom and telephoto lenses).
The general term used to describe the difference between an ideal image and the actual image affected by the above factors is \"aberration.\" Thus, to design a high-performance lens, aberration must be extremely small, with the ultimate objective being to obtain an image as close as possible to the ideal image. Aberration can be broadly divided into two classifications: chromatic aberrations, which occur due to differences in wavelength, and monochromatic aberrations, which occur even for a single wavelength. |
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| AC adapter | Analogue-Digital Converter. Hardware that converts analogue information into digital data. (AD-Conversion) |
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| Achromat, achromatic lens | A lens which corrects chromatic aberration for two wavelengths of light. When referring to a photographic lens, the two corrected wavelengths are in the blue-violet range and yellow range. |
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| AF DC-NIKKOR lenses | AF DC-NIKKOR lenses feature exclusive Nikon Defocus-image Control technology. This allows photographers to control the degree of spherical aberration in the foreground or background by rotating the lens DC ring. This will create a rounded out-of-focus blur that is ideal for portrait photography. No other lenses in the world offer this special technique. |
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| AF Stop feature | Another feature unique to Canon\'s four Image Stabilized super-telephoto lenses. Four buttons appear on the outer barrel near the front of these lenses; pushing any one will temporarily lock AF if the camera is in the AI Servo AF mode. Custom Functions on many newer EOS bodies allow these buttons to assume a variety of additional functions. |
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| Air lens | The air spaces between the glass lens elements making up a photographic lens can be thought of as lenses made of glass having the same index of refraction as air (1.0). An air space designed from the beginning with this concept in mind is called an air lens. Since the refraction of an air lens is opposite that of a glass lens, a convex shape acts as a concave lens and a concave shape acts as a convex lens. This principle was first propounded in 1898 by a man named Emil von Hoegh working for the German company Goerz. |
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| Analogue | Opposite of digital. Analogue data merges continuously into each other without clearly defined steps. (E.g. the colours of a rainbow are not obviously separable from one another.) |
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| Angle of view | The area of a scene, expressed as an angle, which can be reproduced by the lens as a sharp image. The nominal diagonal angle of view is defined as the angle formed by imaginary lines connecting the lens\' second principal point with both ends of the image diagonal (43.2mm). Lens data for EF lenses generally includes the horizontal (36mm) angle of view and vertical (24mm) angle of view in addition to the diagonal angle of view. |
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| Angular aperture | The angle between the subject point on the optical axis and the diameter of the entrance pupil, or the angle between the image point on the optical axis and the diameter of the exit pupil. |
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| Aperture / effective aperture | The aperture of a lens is related to the diameter of the group of light rays passing through lens and determines the brightness of the subject image formed on the focal plane. The optical aperture (also called the effective aperture) differs from the real aperture of the lens in that it depends on the diameter of the group of light rays passing through the lens rather than the actual lens diameter. |
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| Aperture Priority AE | When using this mode, the user selects the aperture giving control over the
Depth of Field. A large aperture letting more light in gives a small depth of
field, meaning not much will be in focus. Whereas a small aperture, not letting
much light in, will give a greater depth of field or more will be in focus from
the front to back of the image
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| Aperture ratio | A value used to express image brightness, calculated by dividing the lens\' effective aperture (D) by its focal length (f). Since the value calculated from D/f is almost always a small decimal value less than 1 and therefore difficult to use practically, it is common to express the aperture ratio on the lens barrel as the ratio of the effective aperture to the focal length, with the effective aperture set equal to 1. (For example, the EF 85mm f/1.2L lens barrel is imprinted with 1:1.2, indicating that the focal length is 1.2 times the effective aperture when the effective aperture is equal to 1.) The brightness of an image produced by a lens is proportional to the square of the aperture ratio. In general, lens brightness is expressed as an F number, which is the inverse of the aperture ratio (f/D). |
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| APO | Definition for apochromatic corrected lenses (mostly made of fuorit-glas). Apochromatic corrected lenses have the property of breaking the beams of light so that the red, green and blue beams/waves exactly will meet on the same point and therefore will not be any chromatic abberation. |
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| Apochromat, apochromatic lens | A lens which corrects chromatic aberration for three wavelengths of light, with aberration reduced to a large degree particularly in the secondary spectrum. EF super-telephoto lenses are examples of apochromatic lenses. |
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| APS | Advanced Photo System. Developed together by five companies, this film system is distinguished by simple operation, a new picture format (16 x 30 mm) as well as a choice of three picture formats. Additional information (such as exposure, aperture and date) can be recorded on the magnetic strip of the APS film. However, APS is not digital photography. |
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| Archive | A collection of data in long term storage, usually the hard drive on your PC or
an external hard drive.
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| ASIC chip | A chip designed for a specific application. They are used by cameras to quickly process the captured image data. |
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| Aspect Ratio | The ration of horizontal to vertical dimensions of an image. For example, 35mm
slide film = 3:2, TV = 4:3, HDTV = 16:9, 4x5 Film = 5:4.
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| Aspherical lens | Photographic lenses are generally constructed of several single lens elements, all of which, unless otherwise specified, have spherical surfaces. Because all surfaces are spherical, it becomes especially difficult to correct spherical aberration in large-aperture lenses and distortion in super-wide-angle lenses. A special lens element with a surface curved with the ideal shape to correct these aberrations, i.e., a lens having a free-curved surface which is not spherical, is called an aspherical lens. The theory and usefulness of aspherical lenses have been known since the early days of lens making, but due to the extreme difficulty of actually processing and accurately measuring aspherical surfaces, practical aspherical lens manufacturing methods were not realized until fairly recently. The first SLR photographic lens to incorporate an aspherical lens was Canon\'s FD 55mm f/1.2AL released in March 1971. (Leica offered the 50mm f/1.2 Noctilux lens with aspherical surfaces for its rangefinder cameras many years before 1971.)
Due to revolutionary advances in production technology since that time, Canon\'s current EF lens group makes abundant use of various aspherical lens types such as ground and polished glass aspherical lens elements, ultra-precision glass molded (GMo) aspherical lens elements, composite aspherical lens elements and replica aspherical lens elements. |
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| Aspherical lens elements | Nikon introduced the first photographic lens with aspherical lens elements in 1968. What sets them apart? Aspherical lenses virtually eliminate the problem of coma and other types of lens aberration even when used at the widest aperture. They are particularly useful in correcting the distortion in wideangle lenses. In addition, use of aspherical lenses contributes to a lighter and smaller lens design.
Nikon employs three types of aspherical lens elements. Precision-ground aspherical lens elements are the finest expression of lens-crafting art, demanding extremely rigorous production standards. Hybrid lenses are made of a special plastic molded onto optical glass. Molded glass aspherical lenses are manufactured by molding a unique type of optical glass using a special metal die technique. |
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| Auto bracketing | Using this mode, a series of shots each adjusted to a different exposure value is taken in succession. This is very useful in tricky lighting conditions where it is difficult to assess the settings. After all shots have been taken, the best may be selected and the others deleted. |
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| Auto flash | One of the flash modes. When the flash mode is set to [AUTO], the camera sets off the flash according to the ambient light conditions. When the flash goes off, the shutter speed will be fixed at a value that is less prone to camera movement blur. |
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| Autofocus illuminator | Some cameras are equipped with an AF illuminator which assists the normal autofocus in poor lighting conditions by illuminating the subject. In this way, the regular passive AF system (e.g. contrast detecting / phase differential method) can determine the correct focus settings even in dark surroundings. |
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| Automatic Exposure | The camera sets the shutter speed and aperture for the correct exposure
according to the light.
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| Automatic Focus | The lens on the camera focuses automatically when the shutter is half pressed.
The viewfinder normally has focussing points shown to assist the user in knowing
what will be in focus.
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| Automatic Program | Within a programm automat the diaphragm and the shutterspeed according to the lightconditions are automatically set. On most cameras indicated with the symbol \"P\". |
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| AVI | Audio Video Interleave. Standard file format from Microsoft (and therefore for Windows computers). It is used for saving video sequences with or without sound. |
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| AWB | Automatic White Balance. Most digital cameras have this feature where the camera
sets the white balance. Override is available in most DSLR's.
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