As important as the shape of the individual elements are, the materials those lens elements are made from is essential, particularly when it comes to chromatic aberation.
I don't have alot of lenses, but I do have alot of telescope eyepieces--some of the simplest have a 4 element design--two groups (seperated by air) of two lenses (typically different shapes and materials) bonded together--and some have as many as 13 elements, with some of the elements made from glass that has been doped with pretty exotic materials such as Lanthanum and Flourite.
A lens labelled "apochromatic" is pretty much guarunteed to have one or more exotic elements in it, as they are used to correct chromatic abberation, and exotic materials add significantly to the cost of the lens as they are usually very rare materials and often make the resulting glass much more difficult to machine--making it softer, or more brittle, etc.
In general, a quality lens will have the non-exotic elements made from ED glass, or extra-low dispersion glass. This is simply glass that is "more transparent" than normal--all glass scatters some light going through it, ED glass just scatters less.
Additionally, there is the matter of controlling reflection--every lens element in the camera is a source of reflection, and light that is reflected doesn't make it through the lens resulting in a loss of resolving ability and contrast. This is controlled by coating the lenses, usually with magnesium flouride. If you shine a light into your lens and see reflections that are blue or magenta, it is probably a single coating of magnesium flouride. If the reflections are faint and dark green, it has multiple thin coats of magnesium flouride. A fully multi-coated lens (meaning all the lens-air surfaces throughout the lens have multi-coatings) will let through more than 99% of the light that enters it, whereas an uncoated lens can loose perhaps 10% or more of that light through reflections.
One last difference between a cheap lens and a quality lens--a quality lens will have the interior of the barrel coated with a non-reflective paint, and may even include baffles outside of the direct light path to minimize any possible chance of stray reflections within the lens. In a newtonian telescope, the simple act of painting the inside of the tube black and adding a few baffles will make a small, but noticable, improvement in contrast at the eyepiece.
Here is a website that has many articles on designing and making telescopes (including grinding lenses) some of which may be insightful to you:
http://www.atmsite.org/author.html
Here is a link to a site that has a freeware program called "Newt" used to design the optical path for a newtonian telescope--although not directly related, it is about optics, at least, and fun to fiddle with besides:
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm
Hope this helps some!
I have been wondering how it is they make these lenses that are in
digicams--I read that the best 35mm lenses resolve to about 5
microns (a red blood cell is about 7) and yet, the 4/3 system will
need to resolve to about one micron. This is in aerospace
territory and it looks like the cost would be enormous unless they
have some very high tech to do it. Getting the QC for this seems
equally near impossible. I have tried to find explanations on how
it is done but can't. Anyone know where to look?
--
John Ellis
