the following is the result of a lot of investigation I have done recently.
I've owned the ZM only, so I will keep the following to only what you can observe from measurements and then provide my analysis / commentary on them. that means I will not discuss relative build quality, ergonomics, etc. which I have no insight on. well, other than to say I like the ZM and having a recent example I am hoping that the newer grease formula that they supposedly began using will mean my lens will last through the next 5 years with light use.
Anyway, on to the technical bits.
So first of all between the V4/5 50 Summicron, the 50 ZM Planar and the M-Hexanon, in a perfect world the Summicron is in fact the sharpest. Looking at the relative MTFs, I would guess the Zeiss would edge out the M-Hexanon for terminal resolution, too.
This is assuming a lot. Firstly, that focus is equivalent. Secondly, that the sensor / film / bench is of sufficient resolution to tell a difference. Finally, that we are looking at a representative example of each and not one in the top / bottom range of performance. Those are pretty big assumptions.
What it boils down to for me is this: the finer the structure you are looking at the more the Summicron differentiates itself from the ZM Planar. At MTF50, for example, the Summicron decimates EVERY current Zeiss 50 at f2 and f2.8 and the Summilux ASPH is better even than the Summicron (AT THIS MEASUREMENT). Please reference the LensRentals 50mm shootout for the measurements.
What effect, then, does this have on us end users? Almost none. In the regions that actually matter for reasonably sized enlargements all of these lenses are so close that it makes no difference unless you shoot subjects with exceptionally fine textures. In fact, if you look at a portrait by the Summicron and the Planar that are somehow identically focused at f2.8 you might think the Planar is sharper because the eyelashes and the details of the eye will look really sharp and the summicron is going to show you similar "detail" in the skin but it's going to look harsher because skin is such a fine texture. This is my experience, which I think is supported by the graphs.
It just so happens that I really like to shoot textures, so for me the V4 summicron would be ideal. But it's too big of a price difference for me to go from my ZM Planar to a V4 cron at this time. The Planar can also be shot into the sun and IMO handles OoF a lot better. So, for now I will wait.
Now, as far as the Hexanon goes, when I look at the MTFs I see a similar situation, which suggests that the Hexanon is generally speaking about the same as the Summicron and the Planar for the vast majority of things (as in, pick one of these three at random and you are unlikely to see much difference). Pick between the ZM and Hexanon on ergonomics and which one you can get a good price on, IMO.
Now, before you say that this was an overly technical explanation without much value, let me just say that lens choice is personal and the lens with the highest resolution is not always going to be the best choice for everyone. But this information has proven useful to me and I hope it will to someone else, as well.
