Originally posted by Chaos_Realm I understand that that Aperture is = (FL/AD) where FL = Focal length and AD = Aperture diameter.
I'm stepping it late, but I'm an optical designer so I'll help.
The aperture diameter you refer to is not the diameter of the front element of the lens, it is the MINIMUM aperture in the optical path (more accurately named the aperture stop). Usually, but not always, that's the iris.
Originally posted by Chaos_Realm Why can't they design a 200mm F/1.8 or faster? which would have AD=111.11mm. Is it that the lens barrel would be to large, and thus the front element being HUGE?.
Is it just not cost effective to do so or is there physical design restraints?
Any light that could be shed on the subject to calm my curiosity would be great
You can design anything. The constraints in your particular case are:
-cost : such a lens would be very expensive if only because of the amount of glass used, so there might not be a good market case
-size : it would need to be huge, bordering on the unusable for many, again working against the business case
-design cost and compromises : it would cost a lot of money to design and develop such a lens. There would be many optical compromises necessary, and it would probably
NOT be amazing optically, especially in the corners. Aberrations would probably be quite ugly wide open. You would need to employ, among other things, lots of exotic (expensive) glass to mitigate that, increasing the cost even more
There are other challenges involved, these are the main ones. Starting with a slower lens design, I could make you a 200mm f1,8 lens in half an hour. To make a GOOD lens would take me a lot of time, and I'm not sure I could do it (photographic lens design is not my particular specialization).
Edit : added the "not" in bold. I meant the opposite of what I wrote...