What's the Big Deal Over ASPH, ED, LD, ELD, LD, SLD and ULD Photo Glass
While shopping for that new camera lens, sooner than later you’ll running into various terms for lens glass, such as 'ASPH', (Aspherical) 'ED', 'ELD[ (extraordinary low dispersion), (extra-low dispersion), 'LD' (low dispersion), 'SLD' (special low dispersion), and 'ULD' (ultra-low dispersion). Each and every one of these variants characterizes a higher than normal standard when related to image quality.
An optical label that’s become increasingly common to many camera lenses is 'Aspherical' (ASPH) lens surfaces are dissimilar to spherical convex and concave surfaces which are common to a majority of lens elements, feature counter-curves in the direction of their edges which assist in maintaining higher sharpness levels near the frame edges, which is especially beneficial when used with wide-angle lenses.
Aspheric elements have been integrated into wider-aperture lens designs to reduce
chromatic aberration and coma, which occurs with the appearance of comet-tail-appearing blurring of point sources of light. Higher-grade aspheric elements, the kind used in premium telephoto and wide angle lenses, are usually ground and polished by hand, while aspheric lenses installed in less costly optics and point & shoot cameras are made from molded optical resin. Read more about ASPH glass here
The primary purpose for LD, ED, ELD, SLD and ULD glass in designing lenses is their ability to diminish color fringing and high chromatic aberration levels, an optical incident in which colors that are part of the image move toward focus at somewhat differing planes (not unlike the way a rainbow or prism breaks up white light into unique individual channels of color) as they strike your camera's imaging sensor surface, resulting in a perceptual image sharpness loss, loss of contrast and color tonality.
The chemical elements of ED glass reduces the space between each color focus plane, resulting in increased color saturation, image detail. and contrast. ED lenses also function superior in terms of transmitting light, which provides for faster focusing times plus brighter viewfinder mages.
Although low-dispersion glass engineering with a fluorite base had their beginnings in the 1920s and the’30s, their make-up proved to be arduous to form and subsequently polish, while the elements were subject to fracturing if they were not built to precise standards. Present generation ED-style optical components are manufactured using rare-earth elements which include titanium dioxide, calcium fluoride, zirconium dioxide and other exotic earthly elements that are must simpler to form and therefore polish, however they also alarmingly sound similar some suspect hot dogs ingredients.
Nikon thought to be a present-day pioneer in using ED glass engineering in its pursuit to diminish magenta and green color fringing that is typically common among 300mm and longer telephoto lenses. Canon uses similar engineering to produce of its up-scale ‘L’-series lenses. which feature calcium fluorite-based APO (Apochromatic) lenses, which are devise for the correction of both spherical and chromatic aberrations rely heavily upon ED-style elements as a way to acquire the high accuracy levels for which these costly optics are known for.
Typically used in the construction of telephoto lenses, ED, ELD, LD, SLD and ULD glass elements are most often in groups known as "achromatic doublets," where a positive element created with low-dispersion glass has been bonded with a corresponding negative element using higher-dispersion glass, thus creating a solitary, higher-performing element group which greatly reduces the space between each one of the color layers of the image as they reach the imaging sensor's surface. What results with all this effort are contrastier,
sharper and more lush-appearing image files.
No matter what they are made up of, aspheric elements allow lens manufacturers to create smaller, lighter and sharper optics that uses less lens groups and elements. Aspheric lens elements are also critical in designing scientific instruments, telescopes and rear-projection TV sets, plus missile-guidance systems