Originally posted by ManuH A couple of points:
FF lenses are not necessarily bigger to get the same performance as APS-C. An f/4 lens on a FF camera will perform better than a f/2.8 lens on an APS-C camera! Because of the 1.3 stop advantage over APS-C. 1.3 stop better noise performance, 1.3 stop less DOF means that an f/4 lens is like a f/2.4 lens on APS-C... Not to mention it's easier to design a good f/4 lens than a f/2.8 one.
Yes they do, and you are only looking at the advantages from one direction. Look at the performance of the Sigma 50mm f/1.4 on a FF 5D and then on a APS-C (SLRgear.com). By stopping the 50mm Sigma down to improve overall IQ I have to give up any speed advantage I gain by using the larger sensor. Since the APS-C is only using the "sweet spot" of the lens you get very good performance from f/1.4 up. If you use the center AF point and then recompose to using the rule of thirds you have positioned the subject in the soft area of the lens on a FF, but on an APS-C your subject would still be sharp.
Yes, it is easier to design an F/4 lens than an F/2.8, but there is no market for for lenses that slow at normal focal lengths. F/2.8 is slow enough as it is. The only advantage of making f/4 glass is cost and size/weight.
You are also assuming the 1.3 stop advantage is a hard rule, and it is not. My 5D has an easy 2 stop advantage over the K-7, but the K-x is probably less than a stop behind my 5D. The K-x has a 1+ stop advantage on the K-7 so we know that even among sensors of the same size there is significant variation. The only place the theoretical 1.3 stop advantage exists is in a lab or computer model where there are no other variables.