Hi All,
There is very little I can add to what Wilson already expressed (above). My experiences with both a chrome and black one has been exactly the same, esepcially upon close inspection of the files and extensive focusing tests at varying distances and all apatures. I'm not convienced that a chrome vs. black lens and matericals used, is responsible for the differences (with many balcks optomised for 1.4 performance but suffer greatly upon stopping down due to focus shift)...but simply that Leica has changed how thay optimized the lens before leaving the factory. THat change might have occured at around the time production of chrome Summilux 35's had eneded. A systempatic study would need to be performed if there was any truth to this.
In any case lenses set up for front focuisng wide open, where the subject is at the very back edge of the depth of field and thereby looses some definition, can then be used at every other f-stop with confidence and perfomance that almost matches the Summicron at most f-stops. Slight difference in curvature and edge shapness can be seen at f2 and f2.8 between the 35mm f1.4 ASPH and 35mm f2 ASPH, but one has to examine the files very closely.
Conversely, if the lens is optimized for f1.4..its bitingly sharp at f1.4 BUT upon stopping down, large focus shifts quickly causes the lens to backfocus and subject is mostly out of depth of field till almost f5.6 and beyond.
So whether there really are two kinds of optimized 35mm ASPH, I don't know, but it seems many fall into these two catagories and their use (and desire to have one over the other) will be quite subjective. For me personally, I prefer those that are set up with front focusing wide open as described..as I can also focus slightly behind subject wide open at that one particular f-stop, if biting sharpness at that apature is required.
Lastly, one other advantage of the lens subject to slight front focus, wide open...and putting subject at the very rear of the depth of field is the incredably soft and dreamy bokeh just behind the subject....something thats not as prominant in either the Summicron ASPH nor Summilux's optimized for f1.4.....although those lenses have their own unique strengths.
Dave (D&A)