Doug wrote:
> Pentax, according to the info I've read, doesn't use plastic
> "elements" in their lenses. What they have done, in efforts to
> achieve Aspheric shapes, is to mould a =surface=, which is then
> cemented to a regular glass element...[snip some other good info]
Doug,
Indeed. There are three types of aspheric elements: hand-ground and
polished, glass-moulded, and hybrid aspherics, which is what you're talking
about--a think layer of acrylic is bonded to a regular spherical element to
change the shape of its surface. It's the least accurate but also the
cheapest method of manufacturing an aspheric element, most often found in
"consumer grade" zooms.
Hand-grinding and polishing has always been, and remains, prohibitively
expensive. (It's also the reason why "aspheric" is now a term equating to
luxurious high quality in lenses: because the first aspherics were all hand
ground and polished, they were also very expensive, which never fails to
impress those who equate high price with prestige.) Glass
moulding--essentially, press-forming--was initially limited to elements of
small diameter (I believe the technique was originally developed for
microscope objectives, although I'm not sure of that), which meant that
early glass-moulded aspherics had to use the aspheric element in one of the
small innermost elements. However, Canon developed a much better technology
for glass-moulding aspheric elements (I don't know when--it came online
mid-'90s maybe?), so now it's much cheaper and more common.
Asphericism essentially corrects one thing--spherical aberration at high
aperture in fast wide-angles. It allows lens designers considerable freedom
in making such lenses smaller and with fewer elements, especially zooms.
However, there is no inherent reason why a lens using an aspheric element
"has" to be of higher quality than a spherical lens. As usual, the quality
of a lens depends more on other factors--chiefly, the cost and size
parameters that the designer had to work with, and quality control during
manufacturing.
There's also no longer any reason why an aspheric lens "has" to be costly.
The higher cost of the aspheric element may be offset by the savings in the
size and the number of the rest of the elements. For instance, one of the
best aspheric lenses (IMNSHO) is found in the Olympus Stylus Epic, which has
a tiny four-element 35mm f/2.8 lens of quite extraordinary quality for its
size and cost--which has two aspheric surfaces. You can buy the whole camera
for $100, so, as you can see, that lens itself is not particularly dear
(much of its cost went into its development, and economies of scale make a
big difference too).
The new SMC-FA 35mm f/2 is an aspheric lens. So is the 43mm Limited. Both
almost certainly use glass-moulded aspherics.
Consumer camera lenses are at a very high state of technical development, if
you consider cost efficiency especially. SLR lenses are already basically
better than they need to be (as is shown by the public's mass defection
since the '80s to the "point and shoot" genre of camera, which have much
worse lenses that nevertheless are considered acceptable).
--Mike
P.S. In answer to the question in this thread title, I'd say "both" <g>.
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