Hi Artie,
You said: "And the Canon 70-200 f/2.8 II L IS trumps the in general comment." to my comment that telephoto zooms are not as sharp as fixed focal length lenses. You are right, I usually say telephoto zooms above 200 mm are not as sharp and I forgot to add the above 200 mm part. Below and equal to 200 mm the 70-200 is an excellent example of sharp lens design (from more than just canon).
"How sharp do your images need to be..."?
Some of my image get enlarged quite a bit, 16x24, 20x30 and shown/sold in galleries, used in advertising, sold to private individuals/businesses and other uses. A 1DIV image is 4896 x 3264 and if printed at 20 x 30 inches is 245 pixels per inch. At that level, the image must be sharp at the pixel level or the print suffers. If the original image needed to be cropped, then it is an even more critical problem to have pixel level sharpness if one wants a large print. I agree that if one only wants web or magazine size (e.g. about 8x10 inches), requirements get relaxed and original pixel to pixel sharpness is not as much of a factor.
Regarding MTF charts, "they are created by the guys who make the cameras." Nah, they are created by the guys and gals who make design and make the lenses :w3.
Actually I find MTF charts quite accurate and they tell a lot about performance regarding image quality. From Canon's published MTF charts, I bet people will find the largest improvement in image quality with the new 500 in comparison to the old 500 and that the image quality difference from the other new versus old superteles will be less. The new 500 looks like a near perfect lens for astrophotography (the original reason I bought my 500 over a decade ago). Stars are the toughest test for any lens and if it does well on stars it will do well on any subject.
I also have a sigma 150-500 and find it sharper than my canon 100-400. My 100-400 can deliver an image that makes a nice 8x10 inch print, but not the level of detail the fixed focal length lenses can in larger prints. Compare the MTF charts for the 100-400 versus a 300 f/2.8 or the new 500 f/4 and we see a big difference:
http://www.usa.canon.com/cusa/consu...ef_lens_lineup/ef_100_400mm_f_4_5_5_6l_is_usm
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_300mm_f_2_8l_is_ii_usm
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_500mm_f_4l_is_ii_usm
Only look at the thin gray dashed and solid lines: that is the fine detail (but at the several pixel level; they don't show the pixel level as it is much worse). The closer the dashed and solid lines are together the better (otherwise stars look like little comets and find detail in bird feathers suffers) and the higher up these two lines are the better. Left to right on the graphs is center to the corner of a 35 mm frame. The 500 II MTF is well above 90% all across the frame (amazing--no other lens in the canon line-up is that good), compared to the 100-400 which drops down to 25% and separates the solid and dashed lines, make stars into little comets, and is about 10 to 15 times worse image quality at the edge. But whether it is important to anyone is up to them to decide.
Roger