Nothing much to add to the excellent replies so far. But here's my take:

I personally choose my lenses based on 2 criteria, spacing and characteristics.

I don't know much about the ZE line, but if I was a landscape/cityscape shooter, I would choose the 21 for corner to corner sharpness and ideal AOV. If I was more of a reportage/documentary shooter, then the more modern look and nice bokeh (for a lens of this focal lens) as well as the wide aperture of the 25 would appeal more to me. I do believe these 2 lenses were designed for different purposes.

As for spacing, I find these combinations to have a good spacing: 15-21-28-50 or 18-25-35-50. I chose the latter. But since I use ZM, the lenses do not have the same characteristics of the ZE. My 25 was chosen exactly because it is the superlative optical equivalent of the ZE 21.

So basically each lens has a purpose for me, 18 for interiors and dramatic architecture shots, 25 for landscapes and less dramatic architecture, 35 and 50 for documentary/portraits... Etc.

I have the zeiss 25f2, Ts17, and zeiss 35 f1.4 and canon 85f1.2. I will be getting the TS24.

My view is f2 is better than f2.8 when you need speed. Twice the light permits northern lights, milky way, etc. I considered the 24 1.4 but concluded the zeiss f2 was sharper at f2 than the canon 24 1.4 was at f2 and the 24 1.4 was difficult to focus in the dark without the hard stop at infinity.

My other view is I am not willing to put up with wa distortion of the Zeiss 21 f2.8 when I have the TS lens available. At 21 the wa become significant with any tilt. I like my pictures to be less distorted. The 24mm will do 21mm with a horizontal shift when needed with far more versatility on shift to remove wa distortion.

Which takes me to that 17ts, 24ts, and 25f2 and zeiss 35 f1.4 and 85 f1.2 are the best combination for me. Fast for night shooting and shift for long duration wa shots.

The zeiss 25f2 is a very hight quality lens - the only complaint I have hear about it is that infinity is focussed on edges rather than centre. I really enjoy using it but have never used the 21f2.8 because I want a faster lens.

In summary, I think the biggest difference between the two lens is that 25f2 is twice as fast but not as wide -

Late to the party ... but since getting my 24L TS-E II I essentially quit shooting my 21mm or 20mm glass, the jump just isn't enough in most instances to even bother switching lenses ... or so it seems ... unless I need to put on the 18 or 17.

That being said, the 15, 25, 35 lineup makes a lot of sense to me on FL distribution alone. The other attributes convolute the decision somewhat into personal preferences, but the 25/2 would likely be my replacement for the 24L TS-E II if it didn't exist. If money wasn't an issue, I'd probably even pair the TS-E with the 25/2 rather than the 21/2.8 even though the 24 & 25 are essentially the same FL for the 25/2 speed and drawing style.

I should note that my widest Zeiss is the C/Y 28/2.8, so I've not used either the 25/2 or the 21/2.8, but the 25/2 is on my radar much more than the 21/2.8.

RustyBug wrote:
Late to the party ... but since getting my 24L TS-E II I essentially quit shooting my 21mm or 20mm glass, the jump just isn't enough in most instances to even bother switching lenses ...

I bought my ZE 21 on the same order/shippment from B&H as my 24 TS-E II. An expensive order to be sure!

So for me, poetically speaking, they were born on the same day, as twins, with only hopes and no particular advantage one over the other.

In less than a year, I'd come to the same conclusion as you Rusty, there wasn't enough FOV difference between them. The slightly wider 21 lost its angle by cropping and straightening, whereas the 24 retained its "angle of integrity" through shifting. I also preferred the zero distortion of the TS lens compared to the ZE's moustache, that again needed cropping. If anything, the TS was sharper from edge to edge, and definitely had considerable less light fall-off/vignetting that the Zeiss.

All these factors, and a few less significant, made the choice to get rid of the ZE 21 relatively painless. But not completely without sense of loss or discouragement. Like a brilliant child who never grows into their potential and the dreams you surround them with.

I miss the focal length for its possibilities, and wish I had a 20mm Shift-E that took filters -- that would be awesome! In lieu of that, the 24 TS-E works beautifully for me.

Looks as if I am faced with the same conundrum as a few before me, although I have less lenses in my arsenal (and I shoot Nikon).

I previously owned the Nikon 24mm f/1.4 and currently own the Zeiss 21mm f/2.8, but I can't decide if the focal length is right for me. I live in a major city, so I use the Zeiss 21mm for cityscapes and street shots every now and then, but I feel that it gives certain shots an unnatural perspective/field of view. Thus, I am renting the Zeiss 25mm f/2.0 this weekend to see if I like it more than the Zeiss 21mm. Specifically, I will be looking at the focal length and to see how soft the corners really are.

Only time will tell . . .

I was in the same situation a few weeks ago. I have the Nikon 14-24, Zeiss 21, 35, 100, Leica 50 cron.

Wanted to get the Zeiss 25/2 but after a lot of thought, reading and discussions with people opted for the Leica Elmarit 28 f/2.8 v2. So glad I did.. Pure leica colors, very good distortion control and keeps up well with the D800e's demanding sensor. At that focal length, I wanted something I could use for environmental people shots, architecture and landscapes. Food for thought.

And no, I wouldn't sell the zeiss 21.

In my book a 25 (or 28) is always more useful than a 21 and a lot more useful than a 15mm. How many shots a year need the 15mm treatment? A 25mm is a great wide angle, not too wide to provide undue edge distortion, but still demonstrably gets a lot more in than a 35 or 50mm. I think selling the 21 for a faster 25 makes a lot of sense. I agree with the others a 15 and a 21 are not similar at all. Why not split the difference: sell the 15 and 21 and get the 18mm Zeiss and the 25? A much more harmonious pair.

Robin Smith wrote:
In my book a 25 (or 28) is always more useful than a 21 and a lot more useful than a 15mm. How many shots a year need the 15mm treatment? A 25mm is a great wide angle, not too wide to provide undue edge distortion, but still demonstrably gets a lot more in than a 35 or 50mm. I think selling the 21 for a faster 25 makes a lot of sense. I agree with the others a 15 and a 21 are not similar at all. Why not split the difference: sell the 15 and 21 and get the 18mm Zeiss and the 25? A much more harmonious pair.

I wish I owned the 15mm, but I currently own only the 21mm. In fact, it's the only wide-angle lens in my stable. That's why I am so conflicted.

'Like a brilliant child who never grows into their potential and the dreams you surround them with.'

"In 1992, Karl-Heinz Schuster at Carl Zeiss developed the Distagon T* 2,8/21 for the Contax/Yashica system, a retrofocus superwide-angle lens which was at least as good as the best symmetric types with respect to image sharpness.

Already at aperture f/4, the Distagon T* 2,8/21 achieved superb image quality; thus it is no wonder that its price on the pre-owned market often exceeded the original price after it was no longer produced.

I like wild children. The better ones grow up to create their own dreams, their own way. In an age of numbing and bland conformity, it makes for a pleasant change. No one ever accused the 21mm of lacking photographic impact, despite its faults. An auditor would never have approved its production ;-)

Can't afford to keep both right now. The decision, for me, will boil down to this: will the Zeiss 25mm f/2 take sharp enough photos at f/8 and f/11 such that it'll perform 95% as well as the Zeiss 21mm f/21.8 at those apertures? If so, I think I'll be switching because I think the 25mm focal length is a bit more natural.

answer, for city work, I believe is 'yes', and it works much better at f2 (!); it is also a more versatile lens with a modern 'look'.

Zeiss wrote quite a lot about it when they penned the paper I quoted above. I tend to listen to people who can do what they do, you might find it useful from pages 6-8:

http://blogs.zeiss.com/photo/en/wp-content/uploads/2011/12/en_CLB41_Nasse_LensNames_Distagon.pdf

I mention the quote above for the future ;-) I am processing lots from the CY 21mm right now, and it is just magical at times, as in SOOC near perfection, I have never seen anything quite like it. There is a long (and sometimes angsty) thread on the 25/2 here as well.

That paper also outlines very neatly the challenges Sony (in particular) will face when developing the FF mirrorless, the most important still camera in their career thus far. best, philip.

There is more to consider than just the difference in focal length with these two.

Well, I've pitted those two very recently and quite extensively in the most demanding photographic territory in short space I know: the Père-Lachaise cemetery in Paris.
That means a *very wide* gamut of textures of stones / foliages / flowers ... etc

Verdict : very embarrassing for the 21, I've kept only the ones where the larger FL is the only plus ... 80 % photos were dumped and the remaining need to be slightly processed

Whereas the 25 ones don't need to be processed or dumped : more accurate to the reality

Most important factor of all to me : I didn't smile a lot to say the very least while reviewing the 21 shots while I was constantly grinning with the 25 ones