Okay, I know this is somewhat off topic -- but where better to ask an optics question than on a photography forum?
Question: Why are lenses made of glass always sharper than lenses made of other substances, such as various optical plastics? Is there a substance other than glass that could make a sharper lens than a glass lens?
I have a very personal reason for asking -- my glasses. I've had glass lenses "forever" and my glasses weigh a ton. With my current prescription, my optician has been unable to find a lab willing to make it in glass. The lab that made my last three or four prescriptions says they've tried to make the prescription in glass several times and the lenses keep breaking when mounted.
I'm currently trying out high-index plastic lenses (ones with an index of refraction very close to glass), and I very much like the weight of the glasses, but the focus is very soft and beyond about a hundred feet, focus degrades rapidly. I've tried plastic lenses before, and with every previous try, I've also had an issue with sharpness and a loss of focus at distance which does not occur with glass lenses of the same prescription.
So, back to my questions:
1) What is it about plastic lenses that make them less sharp than glass? I've found the closer to glass the index of refraction is on plastic lenses, the sharper they are -- but why is that?
2) Why the loss of depth of field in plastic lenses relative to glass lenses?
3) Is there any substances that give a sharper focus than glass? If so, what about that substance makes the focus sharper?
Thanks in advance for any technical explanations anyone can provide.
I know this is somewhat off topic, but I'm certain that the same theoretical questions about vision glasses also apply to camera lenses! :-)