Considering the Zeiss 21

[Axilrod]

I have the 21mm, as well as the 50mm f/2 and 100mm f/2, and I prefer all of them to their Canon counterparts.  They are super sharp across the frame and I love the way they render color.  Also built like tanks and the focus rings are smooth as butter.  The 21mm is definitely way sharper than my 16-35mm, especially in the corners.  Excellent lens, but I'd buy it used as it's pretty damn expensive new.

[noisejammer]

I own several ZE lenses and the 17 TS-E. I used to own the 17-40L.
My take is that the ZE 21/2.8 is a good lens, but I don't rate it as highly as several others in the Zeiss stable.
As for the ZE 18/3.5. You may need to be aware that it's very difficult to focus through the eyepiece (live view is fine.) It also suffers from a lot of edge fall off so that it only really reaches sensible levels at f/8 or f/11.
My suggestion would be to go for the 17-40 and exploit it's versatility. Maybe you can get a TS lens when it's appropriate. (Personally, I'd suggest the TS-E 17/4L because you can add a 1.4x tc and it works just fine at f/8.)

[wayno]

Just to close this out, some may find my findings useful... I came close to purchasing a Zeiss 18 but on close scrutiny I didn't see enough of an improvement on the 17-40. It was a good price but not good enough for a few nanowidgets of corner sharpness. The edges were nowhere as impressive as those of the Samyang 14 which are ludicrously bang-on.

So I'll keep my hard earneds for now. The 24-70ii is just so impressive, it makes other lenses in the wide range look average!!!

[Canon 14-24]

I am curious to those that mention the 17-40 and 16-35 II are not sharp in the corners compared to the Zeiss 21mm, what f-stop were you using to compare to make the statement? Was it wide open or stopped down to around f/8-f/11? From what I've seen, the Zeiss only handles chromatic aberration and slightly less distortion than the Canon equivalents in the corner, but in regards of detail/sharpness they are pretty much similar stopped down to around f/8 (which I would assume most would use the lens for in landscapes - with the exceptions of astrophotography, low light events, or portraits).

In my personal usage, I have passed on owning the Zeiss 15mm and 21mm as they were not weather sealed like the Canon equivalents. Taking the lens out to destinations right up close to the ocean shoreline, dealing with mist from waterfalls, in rain, snow, etc. didn't seem practical in the long-term for usage.

For the price of a new Zeiss 15mm + $300+ filter, I decided on a used 16-35mm II + 17mm TS-e to cover my wide angle uses (16-35 II when I need filters & versatility of a zoom for more outdoor landscapes and the 17mm for urban/interiors).

By the way the 17mm TS-E can use filters with the compromise of slight vignetting at certain degrees of shift. This link can provide the DIY details:
http://www.google.com/translate?hl=en&sl=de&tl=en&u=http%3A%2F%2Fwww.linsenschuss.de%2Findex.php%2Fblog%2F79-canon-ts-e-17mm-f4l-filterhalter

[wayno]

To my eye the 17-40 appeared a bit sharper in the centre with only marginal corner improvements found in the Zeiss 18. All my tests were at f8. I rarely shoot UWAs wider.

(And colour, contrast etc were not much different to my eyes).

As for the reference to corner sharpness in the 16-35 and by recognized extension, the 17-40, there is some general sponginess at F8 in the corners/edges vs the results I've seen from the TS lenses and the Zeiss 21.