Glossary |
Terms that are on use on this site.
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| Calibration | The act of adjusting the colour of one device to match that of another. For
example when you match the calibration of your screen to that of your printer to
ensure what you see is what you print. It is also used in the film SLR's Canon
EOS-3 and EOS 5 which have eye-controlled focussing. You calibrate the cameras
focussing to where your eye is looking in the viewfinder. (Some fighter planes
also have this. The missile follows the trajectory of the pilot's eye).
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| Camera shake | A major cause of unclear pictures, this unwanted movement is caused by involuntary hand and body tremors jarring the camera. |
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| Card Reader | Used for transferring data from your flash memory card to your PC. A better way
of transferring your image files than connecting the camera to your PC.
Sometimes the cameras circuitry can become corrupt. Better to fry a memory card
than your camera
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| CCD | (Charged Coupled Device). This is a light sensitive chip used in your
digital camera for image gathering. The CCD Pixels gather the colour from the
light and pass it to the shift register for storage. CCD's are analogue
sensors, the digitising occurs when the electrons are passed through the A to D
converter. This "Analogue to Digital" converter converts the analogue signal to
a digital file or signal
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| Chromatic aberration | When white light (light containing many colors uniformly mixed so that the eye does not sense any particular color and thus perceives the light as white) such as sunlight is passed through a prism, a rainbow spectrum can be observed. This phenomenon occurs because the prism\'s index of refraction (and rate of dispersion) varies depending on the wavelength (short wavelengths are more strongly refracted than long wavelengths). While most visible in a prism, this phenomenon also occurs in photographic lenses, and since it occurs at different wavelengths is called chromatic aberration.
There are two types of chromatic aberration: \"axial chromatic aberration,\" where the focal point position on the optical axis varies according to the wavelength, and \"chromatic difference of magnification,\" where the image magnification in peripheral areas varies according to the wavelength. In actual photographs, axial chromatic aberration appears as color blur or flare, and chromatic difference of magnification appears as color fringing (where edges show color along their borders). Chromatic aberration in a photographic lens is corrected by combining different types of optical glass having different refraction and dispersion characteristics. Since the effect of chromatic aberration increases at longer focal lengths, precise chromatic aberration correction is particularly important in super-telephoto lenses for good image sharpness.
Although there is a limit to the degree of correction possible with optical glass, significant performance improvements can be achieved using man-made crystal such as fluorite or UD glass. Axial chromatic aberration is also sometimes referred to as \"longitudinal chromatic aberration\" (since it occurs longitudinally with respect to the optical axis), and chromatic difference of magnification can be referred to as \"lateral chromatic aberration\" (since it occurs laterally with respect to the optical axis).
Note: While chromatic aberration is most noticeable when using color film, it affects black-and-white images as well, appearing as a reduction in sharpness. |
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| Circle of confusion | Since all lenses contain a certain amount of spherical aberration and astigmatism, they cannot perfectly converge rays from a subject point to form a true image point (i.e., an infinitely small dot with zero area). In other words, images are formed from a composite of dots (not points) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called \"circles of confusion.\" Thus, one way of indicating the quality of a lens is by the smallest dot it can form, or its \"minimum circle of confusion.\" The maximum allowable dot size in an image is called the \"permissible circle of confusion.\" |
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| Circular aperture | Certain Canon lenses feature a new Circular Aperture diaphragm unit, which uses curved aperture blades to provide for a more rounded opening as the lens is stopped down. It\'s especially effective at rendering out of focus background highlights as natural rounded shapes. In lenses such as the EF 70-200mm f/2.8L IS lens, the lens opening is virtually circular from f/2.8 to f/5.6. These lenses retain all the benefits previously available with Canon\'s Electromagnetic Diaphragm smooth and consistent stop-down operation (even at up to 10fps with the EOS-1v), near-silent aperture control, and total absence of mechanical levers or switches in the lens mount. |
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| Circular polarizing filter | A circular polarizing filter is functionally the same as a linear polarizing filter as it only passes light vibrating in a certain direction. However, the light passing through a circular polarizing filter differs from light passing through a linear polarizing filter in that the vibrational locus rotates in a spiral pattern as it propagates. Thus, the effect of the filter does not interfere with the effect of half-mirrors: allowing normal operation of TTL-AE and AF functions. When using a polarizing filter with an EOS camera, be sure to always use a circular polarizing filter. The effectiveness of a circular polarizing filter in eliminating reflected light is the same as that of a linear polarizing filter. |
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| Close-Range Correction system | The Close-Range Correction (CRC) system is one of Nikon’s most important focusing innovations, for it provides superior picture quality at close focusing distances and increases the focusing range.
With CRC, the lens elements are configured in a “floating element” design wherein each lens group moves independently to achieve focusing. This ensures superior lens performance even when shooting at close distances.
The CRC system is used in fisheye, wideangle, Micro, and selected medium telephoto NIKKOR lenses. |
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| Coating | When light enters and exits a lens, approximately 5% of the light is reflected back at each lens-air boundary due to the difference in index of refraction. This not only reduces the amount of light passing through the lens but can also lead to repeating reflections which can cause unwanted flare or ghost images. To prevent this reflection, lenses are processed with a special coating. Basically this is carried out using vacuum vapor deposition to coat the lens with a thin film having a thickness l/4 the wavelength of the light to be affected, with the film made of a substance (such as magnesium fluoride) which has an index of refraction of n, where n is the index of refraction of the lens glass. Instead of a single coating affecting only a single wavelength, however, EF lenses feature a superior multi layer coating (multiple layers of vapor deposited film reducing the reflection rate to 0.2-0.3%) which effectively prevents reflections of all wavelengths in the visible light range. Lens coating is carried out not only to prevent reflections, however. By coating the various lens elements with appropriate substances having different properties, coating plays an important role in providing the overall lens system with optimum color balance characteristics. |
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| Color balance | The color reproduction fidelity of a photo taken through a lens compared to the original subject. Color balance in all EF lenses is based on ISO recommended reference values and maintained within a strict tolerance range that is tighter than ISO\'s CCI allowable value range. |
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| Coma, comatic aberration | Coma, or comatic aberration, is a phenomenon visible in the periphery of an image produced by a lens which has been corrected for spherical aberration, and causes light rays entering the edge of the lens at an angle to converge in the form of a comet instead of the desired point, hence the name. The comet shape is oriented radially with the tail pointing either toward or away from the center of the image. The resulting blur near the edges of the image is called comatic flare. Coma, which can occur even in lenses which correctly reproduce a point as a point on the optical axis, is caused by a difference in refraction between light rays from an off-axis point passing through the edge of the lens and the principal light ray from the same point passing through the lens center. Coma increases as the angle of the principal ray increases, and causes a decrease in contrast near the edges of the image. A certain degree of improvement is possible by stopping down the lens. Coma can also cause blurred areas of an image to flare, resulting in an unpleasing effect. The elimination of both spherical aberration and coma for a subject at a certain shooting distance is called aplanatism, and a lens corrected as such is called an aplanat. |
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| Contrast | The degree of distinction between areas of different brightness levels in a photograph, i.e., the difference in brightness between light and dark areas. For example, when the reproduction ratio between white and black is clear, contrast is said to be high, and when unclear, contrast is said to be low. In general, quality lenses producing high quality images have both high resolution and high contrast. |
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| Cos4 law | States that light fall-off in peripheral areas of the image increases as the angle of view increases, even if the lens is completely free of vignetting. The peripheral image is formed by groups of light rays entering the lens at a certain angle with respect to the optical axis, and the amount of light fall-off is proportional to the cosine of that angle raised to the fourth power. As this is a law of physics, it cannot be avoided. However, with wide-angle lenses having a large angle of view, decreases in peripheral illumination can be prevented by increasing the lens\' aperture efficiency (ratio of the area of the on-axis entrance pupil to the area of the off-axis entrance pupil). |
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| Curvature of field | Curvature of field is a phenomenon which causes the image formation plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat subject. When the center of the image is in focus, the periphery is out of focus, and when the periphery is in focus, the center is out of focus. The degree of curvature of field is largely affected by the method used for correcting astigmatism. Since the image plane falls between the sagittal and meridional image surfaces, good correction of astigmatism results in small curvature of field. Since curvature of field cannot be improved very much by stopping down the lens, lens designers reduce it as much as possible using various methods such as changing the shapes of the various single lens elements making up the lens and changing the position of the aperture. In doing this, one necessary condition that must be satisfied to simultaneously correct astigmatism and curvature of field is Petzval\'s Condition (1843). Petzval\'s Condition states that a lens element is good if a result of zero is obtained when the inverse of the product of the index of refraction and focal length of that lens element is added to the total number of lens elements making up the lens. This sum is called Petzval\'s Sum. |
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