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The 25s and the 28 are quite different though, so if lens character is important to you, they are not really interchangeable. The 25/2.8 is of course not as fast as the others, and its character is also that of a lens with a lot of depth of field, like the 21. Closer up, it needs stopping down at least once, but it is generally very sharp. The 25/2 I have not personally tried, but it seems to have some of the character of the 35/1.4, with subtle rendering and a more narrow depth of field, although of course a lot less than a 35. It has smudgy extreme corners until stopped down a bit, like the 50MP. The 28 is maybe the most different from the usual Zeiss look, with gentler colours and contrast (until stopped down somewhat), and it does have some CA (as does the 25/2, i think, but I don't think the 25/2.8 has much; someone else should confirm, it has been too long), but when you get the ratio between foreground and background just right, it has just gorgeous rendering. I own the 21 and 28 and they make a nice pair. Once stopped down to f/2.8 or f/4, the 28 also starts to have that sharp all-over Zeiss look, and it gets more punchy. IIRC, my current shot in Briantho's competition threads is with the 28/2.
By the way, when Zeiss lenses have weak corners, it generally means that there is strong curvature of field. Of these three, the 25/2.8 probably has least, wide open. Both the other two have significant amounts, but it can be used to advantage.