[Zig]

So then, to matters central. I've also included detail of the church.

Canon, at 24mm, f9, 1/40s, ISO 100: centre:



Zeiss Distagon, 21mm, f11, 1/1000s, ISO 400: centre:




See?
Matters now are not quite as clearly defined. Is the crispness of 100 ISO really the Canon's only saving grace here? Textures, tones and detail are all well-represented. I can tell you that in the "real world", I'd have no problem whatsoever losing the outside few MP and cropping, in order to have a splendidly detailed shot. In fact, all one needs to do is to combine "feet-focusing" with rotating the zoom and getting a few versions of the same shot before the light changes. Given that the Canon has autofocus too, and our chances of a decent shot are optimised...our Zeiss is "restricted" to the one focal length, and I have to move about, refocus manually, do a double-take to ascertain hyperfocal length...and the light may have slipped past that decisive moment.
Another sobering point: from 18mm to 24mm, the Canon 16-35mm f2.8L hits its sweet range. At f11, between these 2 lengths, if you are prepared to lose a fraction of the peripheries of the frame, you are able to capture detail and texture with a very high degree of success. In fact, you do get used to the lens's quirks and compensate accordingly by allowing for the cropping process when you shoot. In other words, if you know you're going to get "mush" at the extremities, then make allowances for that. Having the equivalent of a couple of primes in the one lens(maybe not L primes though, yet it'll be pretty close in the centres!) is indeed an asset.
And that's not quite the end of the plaudits list for the Canon:
Though corners and edges at pretty much most apertures truly are PANTS from 16-18mm, I've found that between these 2 lengths the centre definition is even better than in the Canon shot above. In fact, I used to "see" a shot that cried out for, say, 21mm, yet I deliberately widened the zoom to 16 or 17mm. I did this firstly to further the hyperfocal distance but above all to be able to capture the shot with enough blindingly-crisp centre detail that the file coped effortlessly with the extra bit of uprezzing necessary later on.

However, we are not yet at the summarising stage. Here's some detail from the church building, which you remember is at "infinity", practically-speaking and top-centre of each shot.

Canon:



Zeiss:



It would initially seem that distortion mars the Zeiss shot, yet I am aware that in order to frame each shot, I needed to be at quite different heights and the Zeiss shot needed more tilt to actually make the shot work, it being an extra 3mm wider.
(I never thought I'd say that 3mm makes all the difference...! )
The Canon does well again, but bear in mind that it is "closer" to the eye at 24mm than the Zeiss at 21mm. And if you start having to ask about whether the ISO makes a difference, then again this argues well for the Canon.

SUMMARY:
So, given that I no longer have the Canon 16-35mm f2.8L, then why do I sing the praises of it and seemingly remain sotto voce about the Zeiss T* Distagon 21mm f2.8 ZE?
Partly because the Zeiss Distagon 21mm needs no defence or recommendation. The feel, build and balance all suggest but no do not prepare one for the unimpeachable quality of this lens. It is simply in a class of its own. Though I have yet no experience of Leica apart from drooling by stealth and reading many users' opinions, this lens surpasses the 19mm Elmarit in terms of its precision and otherworldy shimmer to its images.
To me, and under "normal conditions", a lens is a tool among a box of others that helps me deliver at the back end a representation of what I see and feel at the front end. To me, the Zeiss is not so much a lens as something that uncannily sets free the essence of that which I perceive in the visible spectrum. Though I cannot prove it, I can safely say that(despite vignetting) the centre of a shot at f2.8 will be just as detailed and precise as the edges and corners. Remarkable. This lens provides me with abnormal conditions, in other words. I read something at digilloyd(sorry if my spelling is incorrect) where, upon seeing the unprocessed results of the Distagon 21mm ZE with his Canon full-farme camera, his colleague opined, "heck, I can't even make my images do that afterwards in processing!"
For those who wish to see more shots with this lens, I'll be very shortly posting a gallery of them at my Pbase gallery space(see link in my sig below). There are of course a panoply of shots there with the Canon 16-35mm f2.8L MkII.
Yes, it is costly: the thick end of £1400 GBP at time of writing(July 2010); it is one focal length only(prime); it is manual-focus only(though full TTL and EXIF). It will also have the initial effect on you of sighing coldly at your Canon L primes(well, most of them)...until the confidence accrued from developing a relationship with the Zeiss galvanises you into squeezing every drop of excellence you can out of Canon optics.
A breathtakingly beautiful lens. If you are a landscape snapper, you will not ever be aware how much you do need this lens until the first time you try it. And then, as the guy says in Platoon as he introduces the rookie Charlie Sheen to cannabis via the barrel of a gun, "...then the worm has certainly turned for you, my friend..."
Any initial feelings of o-my-gawsh upon waving goodbye to the Canon and looking at the quirky shape of the Distagon were immediately replaced with a feeling of "how on earth have I managed to go so long without this lens?"
Nuff said.
The Canon 16-35mm f2.8L MkII then...and why do I still sound fond of it?
Seemples(tsk! )
Because anyone who blathers on tacky forums that "mushy edges" are a summation of this fine lens is misrepresenting its abilities and capabilities.
Yes indeed, Canon quality control is often found wanting: a supreme and largely unforgivable annoyance for anyone expecting that the "L"(for "luxury", is the irony) nomenclature should guarantee excellence. Many users have reported trying several copies of this lens before either they were satisfied or they gave up...or, as in my case, saying, "well, this is about as good as it gets, let's put up with it". The upside is of course, that when you finally DO get the Zeiss, you also do finally realise that excellence is possible rather than something that passed away 20 years ago.
You have a good chance of getting a lens that demonstrates even sharpness across the frame(I'm not referring to soft edges and corners as discussed above in the comparison) and you do have several "primes" in one: and you can quickly run through a plethora of angles and views before the light changes.
It is possible to use the weaknesses of a superwide zoom as advantages:
For example, the excellent centre sharpness from 16-24mm, from f4 to f8 is good enough to be able to visually "crop out" the peripheries of a shot during composition: centres will hold up well enough for even the 21MP of the 5DMkII(and you won't want to even try to rescue the edges). Of course, you are reducing the superwidth of your superwide...but be aware that zooms meanz compromisez, folks.

The other thing I've not mentioned here is the subject of "optimisation". The Canon is reportedly optimised for use at wider apertures...and for close subjects. As a matter of fact(and by way of contrast) the Zeiss Distagon 21mm is optimised in terms of visual performance for subjects that are further away.
This means, that though the Canon has "only" a close-focusing distance of 0.28m compared to the Distagon's 0.22m, it can do "wide-macro" work very well indeed, and produce excellent results at wide apertures. Consequently, jamming your Canon hand-held up against a bunch of wild crocuses at f3.5, with continuous autofocus locked on, produces some lovely and interesting work, with equally nice bokeh. And given the short depth of field, who needs to worry about edges and corner resolution then?
By the same token, the Canon performs nominally poorly from 30-35mm: by 35mm even the centres have long since packed up, gone home and are tucking the kids in. And yet here we have a most remarkable portrait lens: 35mm at f2.8= fast, handheld, a pleasing vignette, all of which combine for quite ethereal female portraiture.
And you'll find many such oddities as you flick through the combinations of aperture and focal length: distortion in, say, architectural shots, is of course pronounced...but it is by and large constant and quite linear, thus easily enough corrected for in software. The Zeiss Distagon, however, has its distortion just about corrected, which is a remarkable feat of engineering given the edge sharpness. I say just about, as it displays its trademark distortion signature, which is complex and not too easily correctable(though PTLens have done an excellent job to do so). This distortion has been referred to on tacky forums as "moustache distortion" so frequently that you could be forgiven for thinking it's an optical term. As I mentioned earlier, this phenomenon is often referred to, usually in those tones of forced nonchalance that belie that exquisite combo of little intelligence coupled with barely-covered jealousy. It is, when you look through the Zeiss and actually take a picture, negligible. Or if you can't make it negligible, then chances are you know jack about taking piccies anyway.

So. There we just about have it.
Zeiss=sharper. By far. But, dear reader, horses for courses. The Zeiss Distagon T* 21mm f2.8 is excellent and consistent at being the Zeiss Distagon. I've yet to hear of a "bad copy" straight from the box...not something that is shared with its wider stablemate, also in Canon(ZE) fit, the 18mm, which even ocassionaly displays uneven sharpness across the frame. Optical quality, even with no aperture ring and an extra lens element, is consistent with the Contax Zeiss of old. Landscapers: you were made for this lens. If architectural shots are your bag...well, stop reading this artcicle; move along please, there's nothing to see: spend a few hundred pounds more and get the astonishingly good Canon 17mm f4 TSE. Then wow the landscapers with the Distagons by doing cracking unshifted shots as well.
The Canon 16-35mm f2.8L Mk2= jack of many trades..and master of several, as long as you are its master and not its servant. Spend time getting underneath its skin, be inventive, avoid polarisers, shoot handheld...make yourself do macros, architecture, portraits and of course landscapes...even try going out with its zoom taped at one focal length then rely on foot-focusing and aperture-testing. Register the fact that if you research it on the forums, the yammering cry will be "mushy edges and corners", and ignore them. They are right of course...but there are rafts of possibility with this lens. Know its apertures and lengths, and it will reward you with many pleasant surprises. If, though, you are a corner-and-edges peeper and never actually go outside with the camera, I have to say that this lens is not for you. Nor is any, to be honest

I hope that whereas this "comparison" has not been the most disciplined and objective, it still has enabled the reader to move beyond mere "sharpness" as justification of a lens's worth and existence.
This is an open forum, so of course feel free to add any thoughts of your own.
If you should wish to see more examples of my work with these and other lenses, I'd love you to visit my galleries at the link below.
Shots with the Zeiss Distagon T* 21mm f2.8 ZE: click here folks: Shaun Reeder Zeiss Distagon 21mm f2.8
By all means PM or email me if you wish.
Blessings in a barn dance,
Shaun R (aka Zig)

Addendum: Of course, of far more relevance to those who want a few more "equal" comparisons, are the numerous head-to-heads of primes out there. Here's a thread from another forum I'd missed in which is compared the Zeiss to one of the Canon prime 24mms: here

[Toad]

A Tour de Force in 3 parts, Zig!

Amazing comparison review of these 2 great lenses. In the detail comparison at the end - I like the look of the Zeiss, but in the first set of center crops, the Canon sings.

I can't say that if I were trying to decide which of these lenses to buy based on your review, I would have my answer. Your tests prove them both extremely capable with slightly different strengths and no obvious weaknesses. Perhaps, it would come down to usability in the end - AF zoom versus MF prime. Its a style thing, I suppose.

[Toad]

Hmm. The first time that I read this thread (and commented), your analysis ended with your question about singing the Zeiss's praises in sotto voice. Now, I can see that there are several hundred more words. Some sort of rip n the matrix of time and space, I suppose. Anyway, well done. Certainly, this analysis tells readers anything they need to know about these lenses - detailed enough for the dedicated pixel peeper - but paying equal attention to the *experience* of the lenses.

Have you noticed that Shuttertalk has been posting some pretty fine reviews and analyses lately?

[matthew]

Quite the write-up, Zig - a tremendous amount of material there. Quite the comparison.

The Zeiss Distagon T* 21mm f2.8 is excellent and consistent at being the Zeiss Distagon.

Love that.