Quote Originally Posted by Wouter from Brazil
. . . is it really an advantage to invest in the fast lenses (i.e. 35/1.4 and 75/1.4 in my case) ? How do they perform at this wide aperture ?
If the nature of your work requires the wider aperture - either for low-light situations or for subject isolation via shallow DOF - the 'Lux versions of the lenses are worth the extra cost. (The lens naming scheme assigns Summilux to the f/1.4s, Summicron to the f/2.0s, and Elmarit to the 2.8s.) Some of the newer 'Luxes also have aspheric elements, which adds to sharpness, designated as "ASPH". The 75mm 'Lux is somewhat of a special case, as it's an older design with a distinctive "signature". All of the f/1.4s perform well wide open, but do increase in sharpness even more when stopped down a bit.

If size and weight are more important to you, the Summicrons or Elmarits may be better choices, however. Weight specs are available on the Leica site.