Originally posted by mugund Fast lens is still valuable - Especially a fast prime in the speed f1.4 to f2.0 range. With these lenses you can stop down a bit and get exceptional quality.
This philosophy is not always true, though.
For example, the FA 50 1.4 is less sharp than the DA 40 2.8 at f 2.8. Obviously, with the DA 40 you are limited to f2.8, but if you really only shoot the FA 50 at 2.8, you should not expect improved image quality at that aperture.
Pentax is doing an odd thing - they are designing lenses for ASP-C digital, which makes it hard to compare their lenses to offerings from their brethren who offer more traditional lens choices. I will not argue that one is better than the other, but rather that the concept of speed is beginning to lose ground to the concept of quality in the Pentax realm.
I think there is no other explanation for the FA limited line, which are really not all that fast, but are quite exceptional when it comes to quality. The DA limited line are an even more extreme example of this, since they are very, very good wide open, but wide open is not extremely fast.
Instead of providing larger, heavier primes which need to be stopped down to f2 or 2.8 to reach really optimal levels, Pentax seems to prefer offering a lens that is quite good from wide open, in a lighter, more compact package.