"The camera begins and ends with the lens" - The Four Thirds Organisation. They are not wrong. Transforming quality glass for into quality lens for quality images is one of last true forms of alchemy.
Good lenses exhibit both optical and mechanical quality and, for me at least, should exhibit four main characteristics: optical rendering, speed, mobility and robustness. Then, of course, there is the question of affordability.
As ever these characteristics can conflict, for example speed with quality may call for compromises on weight and size, hence on mobility. This can be mitigated by the use of multiple exotic glass elements and specially shaped multiple aspherical elements and so on, all of which come at a price of course.
For example my Olympus M.Zuiko 17mm f1.2 Pro has
- 1 x ED-DSA lens elements
- 1 x SED lens elements
- 3 x ED lens elements
- 1 x Super HR elements
- 1 x Aspherical glass elements
SED = Super Extra-Low Dispersion; ED = Extra-Low Dispersion; ED-DSA = Extra-Low Dispersion, Dual Super-Aspherical; SHR = Super High Refractive Index; ASPH = Aspherical; EDA = Extra-Low Dispersion Aspherical.
This is a technological masterpiece which, despite its super speed, is really compact and optically outstanding.
Lens alchemy all boils down in the end to years of knowledge and experience, balanced designs, cutting edge technologies, plus ultra precise manufacturing, assembly, testing and measurement. Which brings us to an important point. Always buy the best quality lenses you can afford, without exception, because if you buy a high end camera and put a cheap lens on it you are going to be very disappointed.