When shopping for a new lens, sooner or later you’re going to run into the terms “ED” (extra-low dispersion), “LD” (low dispersion), “SLD” (special low dispersion), “ELD” (extraordinary low dispersion), and “ULD” (ultra-low dispersion),
and each of these variants represents a standard above the norm when it comes to image quality.
The key reason LD, ED, ELD, ULD and SLD glass are important in lens design is because of their ability to reduce levels of chromatic aberration, or color fringing, an optical phenomenon in which the colors that make up the image come into focus at slightly differing planes (similar to the way a prism or rainbow breaks white light into individual color channels) as they strike the surface of your camera’s imaging sensor. The net effect is a loss of contrast, color tonality and a perceptual loss of image sharpness.
The chemical makeup of ED glass compresses the distance between each color’s plane of focus, resulting in greater color saturation, contrast and image detail. ED-type lenses also perform better in terms of light transmission, which makes for quicker focusing times and brighter images in your viewfinder.
Though fluorite based, low-dispersion glass technologies go back to the 1920s and ’30s, their composition proved difficult to form and polish, and the elements were prone to fracturing if not manufactured to exacting standards. Current generation ED-type optical components are made with rare-earth elements including zirconium dioxide, calcium fluoride, titanium dioxide and other earthly exotica that are easier to form and polish, though they do sound alarmingly like some of the more suspect ingredients of hot dogs.
Nikon is considered a modern-day pioneer in ED glass technologies in its quest to reduce green and magenta color fringing that’s common in 300mm and longer lenses. Canon uses similar technologies in the production of its premium calcium fluorite-based ‘L’-series lenses. Apochromatic (APO) lenses, which are formulated to correct both chromatic and spherical aberrations, rely heavily on ED-type elements in order to achieve the levels of accuracy for which these pricey optics are recognized.
Most commonly used in the design of telephoto lenses, LD, ED, ELD, ULD and SLD glass elements are usually in groups called “achromatic doublets,” in which a positive element made of low-dispersion glass is bonded to a complementary negative element of higher-dispersion glass, creating a single, higher-performance element group that greatly narrows the distance between each of the image’s color layers as they strike the surface of the imaging sensor. What you get for your troubles are sharper, contrastier and more lush-looking image files.
Another optical term that’s common to an increasing number of camera lenses is “aspheric.” Aspherical (ASPH) surfaces are lens surfaces that, unlike spherical concave and convex surfaces common to most lens elements, have counter-curves toward their edges that help maintain higher levels of sharpness toward the edges of the frame, which is particularly beneficial in the case of wide-angle lenses. Aspheric elements are also incorporated into the design of wider-aperture lenses to reduce coma, or comatic aberration, which manifests itself in the form of comet-tail-like blurring of point light sources. Higher-quality aspheric elements, the type used in premium wide angle and telephoto lenses, are usually hand ground and polished, while aspheric lenses used in less expensive optics and point-and-shoot cameras are molded from optical resin.
Regardless of what they’re made of, aspheric elements enable lens manufacturers to produce sharper, not to mention smaller and lighter optics that require fewer elements and lens groups. Aspheric lens surfaces are also crucial in the design of telescopes, scientific instruments and rear-projection TVs, and missile-guidance systems.