Sorry, but these charts looks very much a marketing trick.
Eh, Canon's MTF charts have a well-deserved reputation. Bad individual copies of lenses excepted, I don't think anybody's ever come up with a lens that didn't perform similarly to their published charts.
What's the point of comparing lenses at maximum aperture, if it's not the same?
Because you want to know how well the lens performs at its maximum aperture. What would be the point of an MTF chart for the 400 f/2.8 that only had lines for f/5.6?
The f/8 lines are there for the apples-to-apples comparison you're hinting that you want.
Not surprising that the pancake is sharper @f/2.8 than the fifty @f/1.8. Let's compare them both @f/2.8 and let's see. Even @f/1.8 the fifty is not far behind, I would not be surprised to see it catch up by f/2.8.
Considering how much better the 40 is at f/8, I rather doubt either 50 will be as sharp at f/2.8 as the 40. It could happen, but I doubt it.
The lines @f/8 might well be a measurement of maximum sharpness, but honestly, who cares? Who's buying these lenses to shoot @f/8?
Are you kidding? Or are you not a photographer?
Seriously, wide-open shooting is the exception in the world of photography, not the rule. Sure, there are lots of types of photography that call for big apertures, but there're many more that require the entire subject to be sharp. And f/8 is the perfect aperture for that -- it's close to if not at every lens's optimum performance; it's small enough for a generous depth of field; and it's not too small for diffraction to muddy things up. There's a reason why the saying is, "f/8 and be there," and not "wide open and be there."
I remain of the same idea, unless you shoot a lot of videos this pancake is a waste of money, as little as it could be.
Again, missing the point.
Sure, the video crowd is going to eat this up with the silent autofocussing and what-not.
But I'm sure you'll find an awful lot of pros getting these as replacement body caps, and then using them in place of digicams. It's the perfect pro's party camera, for example, especially with that one-foot minimum focus distance. Imagine a wedding photographer getting out on the dance floor, in the middle of the Conga line,
with one of these for what I mean.