Carl Zeiss Distagon T* 25mm F2.8 ZF (Nikon AiS)

To sell you this lens, I first have to sell you the focal length. 25 mm is somewhere between moderately wide and really wide. It is definitely an in-betweener, perhaps even an oddball, bringing together the expansive scope of, say, a 20 or 21 mm with the relative ease-of-use of a 28 mm. I find it a natural focal length for both indoor and outdoor shots, especially bigger landscapes with lots of sky or conversely tight spaces indoors. Unlike a 20 mm you don't have to get too contrived with the foreground composition to fill the frame.

It doesn't take much research to discover that the Zeiss 25/2.8 ZF is not especially well-regarded optically.  Soft corners and vignetting wide open are found to be particularly dire. Well, okay, but this is normal behavior for a lens of this focal length. Look at it this way: a lens doesn't have to be good at everything all the time. This Zeiss is meant to be used at f/8 for maximum edge-to-edge sharpness, while the center remains sharp all the way down to f/2.8. The key point is unlike, say, the Nikkor AF or AiS 24/2.8, CA is very low at all apertures. Distortion is also relatively well controlled.

So while I can't say I prefer this lens to the 35/2 Distagon, it's a good, useful lens that performs very well when used at the aperture appropriate for the scene. Physically it is a little smaller than the 35 mm, and has a "secret weapon": close focusing down to 17 cm. It is the cheapest of the ZF lenses after the 50/1.4, and can sometimes be found at bargain prices on the used market. Mine was just $350.

Carl Zeiss Distagon T* 25mm F2.8

Note: don't bother with this lens as a 35 mm equivalent on crop format cameras. The barrel distortion which has a handlebar shape on full-frame is distractingly curved on APS-C, and even the central sharpness does not impress at the higher pixel densities found in most crop sensors. It's a lot of money for little return.

Sigma High-Speed Wide II 28mm F1.8 (Nikon AF)

This wide angle Sigma lens offers cheap AF thrills. Like many other Sigma pre-ART-era prime lenses, it is designed to meet an astonishing headline spec. - f/1.8 maximum aperture at 28 mm - while still undercutting the name-brand alternatives from Canon and Nikon at retail. Performance in other areas is therefore limited to achieving passable center sharpness. The rest is expected to naturally sort itself out as the lens is stopped down.

Sigma High-Speed Wide 28mm 1:1.8 II

With a lens of this type expectations have to be commensurate with its design goals. It's not a high-end lens. It may, however, do what you want.

The High Speed Wide 28/1.8 is the model Sigma made before the macro 28/1.8 EX. That lens is much larger, heavier, and more expensive than this one, so if you don't need the macro (close focus) function the older lens has a lot to recommend it just from a convenience and value standpoint. Indeed the main competitor, assuming a moderate budget and that the requirements are for a compact 28mm prime with autofocus, is the Nikkor 28/2.8 AFD. That lens is little over a stop slower, nothing to write home about optically, and typically costs twice as much as the Sigma. If you want a decent performing lens at this price point, I suggest an old Nikkor Ai manual-focus 28/3.5.

The biggest optical problem with the Sigma is the soft/dark corners, which are astoundingly bad wide open and only gradually improve down through f/8. This will be far less in evidence if you use the lens on a crop-format body of course. It is not a sharp lens, anywhere, by any measure.

While we up listing the negatives, the Sigma is finished in a soft, rubbery coating that does not age well. It is easily damaged and has a sticky feel to it.

Ok, so lets look at why you might want this lens. First, the lens is both quick and quiet to focus and index. Focus is accurate (on my D600). On the optical side the lens has two things going for it in addition to brightness. First, there is minimal chromatic aberration. This, I have found, seems to be typical of these simple, uncorrected lens designs, Second, the barrel distortion is very well controlled. So no nasty color fringing, and straight lines are straight. For the rest, the bokeh is nice and smooth as long as point sources are not being imaged. They tend to have bright outlines. Flare and ghosting will happen, but the lens flare is pretty cool if you are into that kind of thing.

So in summary I wouldn't peg this lens as a classic, but I do think it is under-rated. If you are willing to sacrifice corner sharpness this is an interesting value alternative to the Nikkor 28mm F1.8 AFS. Just make sure your camera supports AF screw drive.