 BetterPhoto Member |
Lens Terms What do these terms related to lenses mean - aspherical, APO, mirror? What are their advantages over ordinary lenses?
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Jon Close |
Conventional lenses have surfaces that follow the curve of a sphere. Light rays from the outer portion of the lens will focus to a different point than rays from the center portion, leading to "spherical aberations." These aberations are controlled by using several lens elements in groups. An aspherical lens has a non-sherical surface contour to focus all rays to the same point. Aspherical elements can be used to reduce the number of elements used in a lens, saving weight and fewer surfaces help to better control flare (light reflecting off the surface of a lens element rather than passing through it). APO is short for apochromatic. Conventional glass does not focus all wavelengths of light (colors) to the same focus point. This chromatic aberation can also be controlled with complex groups of elements, or with elements made of special superlow dispersion glass or flourite crystal. The manufacturers will designate the use of these special elements with the terms APO (Minolta, Sigma), ED (Nikon's Extralow Dispersion), LD (Tamron's Low Dispersion), UD or L (Canon's Ultralow Dispersion or Flourite crystal). A mirror lens (aka Catadioptric) is a compact way to build a telephoto lens (and telescopes) by using mirrors to form a "folded" light path. They are inexpensive to make and don't introduce the as much chromatic and spherical aberration as conventional lens elements. But mirror lenses are limited to a single fixed aperture and have the unique and (sometimes) annoying side effect of out-of-focus highlights are rendered as doughnuts.
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