Glossary |
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| ED glass | Nikon developed ED (Extra-low Dispersion) glass to enable the production of lenses that offer superior sharpness and color correction by minimizing chromatic aberration.
Put simply, chromatic aberration is a type of image and color dispersion that occurs when light rays of varying wavelengths pass through optical glass. In the past, correcting this problem for telephoto lenses required special optical elements that offer anomalous dispersion characteristics — specifically calcium fluoride crystals. However, fluorite easily cracks and is sensitive to temperature changes that can adversely affect focusing by altering the lens’ refractive index.
So Nikon designers and engineers put their heads together and came up with ED glass, which offers all the benefits, yet none of the drawbacks of calcium fluorite-based glass. With this innovation, Nikon developed several types of ED glass suitable for various lenses.
They deliver stunning sharpness and contrast even at their largest apertures. In this way, NIKKOR’s ED-series lenses exemplify Nikon’s preeminence in lens innovation and performance. |
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| EMD (Electromagnetic Diaphragm) | 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. |
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| Extension amount | 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. |
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| Extraordinary partial dispersion | 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. |
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| Eyesight, visual acuity | 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.) |
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