Aspherical elements are meant to reduce the
level of distortions and to improve the
sharpness at the image borders. Basically aspherical elements can optimize
the focus of edge and center light rays (see picture below). It is very
popular to name a lens e.g. "28-80 aspherical" in these days but the word
"aspherical" does not reveal the effectiveness and quality of the aspherical
There're three types of aspherical elements:
Hybrid asphericals are quite cheap to produce
and widely used for cheap consumer grade zoom lenses. Basically it's a
spherical glass lens with an additional plastic surface which forms the
Ground aspherical lenses consist of glass
only (obviously). The grinding process of a spherical element and the following
polishing process is very expensive so you only find them in Pro-class
lenses (e.g. Canon 17-35L, Nikkor 20-35 etc.) and a few older lenses.
Molded aspericals are produced by directly
molding glass in a molding machine by incorporating an aspherical metal
die. These elements are often used in good amateur class lenses.
The gain of quality by using hybrid lenses
seems to be quite limited while lenses with true ground aspherical lenses
often produce outstanding results, molded aspericals are in between quality-wise.
Apart from the sheer material and production
aspects the positioning is also very important. The best wide-angle lenses
use a large diameter aspherical element in the front group (probably for
a better correction of light rays with steep entry angle ?), rear aspherical
elements (=smaller and therefore cheaper) are less effective.
However it should also be mentioned that
the maybe very best wide-angle zoom, the Carl-Zeiss T* 28-85/3.3-4, doesn't
feature any aspherical elements at all so the game plays also in the quality
of the general lens construction (that should be obvious anyway).
In general asperical elements are used
in ultra-wide and wide-angle fixfocals & zooms as well as standard
and so-called super zooms.
"APO" elements (UD, SUD, CaF2,
LD, SLD, ED etc.) improve contrast and sharpness by reducing chromatic
aberration (color defects) that usually occur in tele lenses. These elements
are able to focus different wave lengths of one light ray in one point
(see picture below). These elements are quite expensive and usually not
used for cheaper lenses. The problem is however that the quality of these
special elements varies heavily so the effect is often downgraded to a
marketing gag - this is especially true for some third-party manufacturers!
As a rule-of-thumbs a good long tele lens
will always feature two or more of these special elements. Recently the
first ultra-wide and wide-angle lenses emerged using APO elements besides
aspericals in order to reduce problems with lateral color shifts.