The Zeiss Distagon T* 28mm ƒ/2 lens for Nikon F (ZF) or Pentax K mount (ZK) is one of the recent series of manual-focus SLR lenses designed by Carl Zeiss in Oberkochen and manufactured under Zeiss QC by Cosina in Japan. I recently took it for a brief spin in Pentax mount, on a K20D with a 1.2X viewfinder loupe (i.e., magnifying eyepiece).
Zeiss seems to prefer older-style lens designs—witness the resurgence of rangefinder Biogons—and this Distagon is in some ways a quintessential old-fashioned Zeiss. It shows evidence of many of the typical flaws of traditional spherical wide angles: there is noticeable linear distortion (barrel type); the corners are worse than the center at wider apertures, and would doubtless be worse still on 24x36mm cameras; optimal correction has obviously been chosen for a focus distance closer than infinity, because infinity resolution is not terribly good; and performance at the widest apertures is noticeably worse than at the center apertures (ƒ/5.6–11). Yet large-structure contrast is outstandingly good and color transmission is superior, making for pictures that are viscerally satisfying in ways that lesser lenses can't match.
Flare seems decently if not outstandingly well controlled, and out-of-depth-of-field blur (bokeh) is also better than average without being class-leading, meaning it shouldn't bother most photographers except in extreme cases. I could not induce purple fringing. That doesn't mean you never will, but at least it seems well controlled.
Unfortunately the lens's worst flaw also happens to be the quality that I'm most allergic to these days (golden-eyes syndrome...sigh): surprisingly pronounced curvature of field. Its other flaws are mild in comparison. Most photographers don't even know how to see curvature of field in pictures, so it might not bother many buyers. But it's an Achilles' heel for the Distagon 28mm, and if you know how to see this property and you don't like it, expect it to ruin the occasional unlucky shot for you when it shows up.
Applications: This won't be the best choice for the landscape photographer, and as an all-around snapshooting lens it will be better suited to someone who uses it a lot and bothers to get to know it well, because performance is variable across the frame, up and down the aperture range, and to a slight extent depending on focus distance. It doesn't match today's super lenses for consistency in all conditions. Rather, this lens will reward the careful user who learns how to get the most out of it and takes care to use it in sympathetic conditions. The widest apertures especially will come with pictorial effects that will need to be understood in order to be applied with intelligence.
Recommended? No. This lens is in no way, shape or form a "dog"; in fact it's quite an excellent lens, of which owners could rightly be proud. Most of the flaws I've mentioned are present in most wide-angles to some degree. However, of the thirty or so major constraints that face the lens designer, size and selling price are #1 and #2—although I wouldn't want to have to pick which is which—and both of these parameters are significantly relaxed with this lens, which is both large and expensive. I would simply expect a modern 10-element interchangeable SLR lens this big (93mm long), heavy (520g), and costly (US$1,033 at B&H Photo as I write this) to be better corrected technically—more like its stablemate, the Distagon 35mm ƒ/2. I would expect it to be more, well, modern.
Still, there are significant consolations, because it also shows the traditional Zeiss virtues in spades: at middle apertures and middling focusing distances (i.e., not stressed), it is very sharp over the greater part of its field, with superb macrocontrast and rich, deep, subtly variegated color that can be beautiful and pleasing. The best results from this lens will surpass the best you can get out of many lesser lenses in richness, vividness, and a subjective sense of clarity. And this doesn't require pixel-peeping or golden eyes to see: non-photographers could notice it, and it should be readily evident to you.
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Featured Comment by Carsten Bockermann: "Yes, the curvature of field is quite pronounced as can be seen in the example below (Nikon D700, ƒ/2). Looking at the ornaments on the wall in the lower third of the picture you will see that they are much sharper towards the lower left corner than in the center, although the distance is just the same. Apart from that I find the ZF 28/2 an excellent performer. The Nikkor AI 28/2 is also a joy to use. It has somewhat more even illumination across the field but less contrast, which is most evident around bright light sources in the photos."
1/28/09 Carsten adds: Sean Reid of Reid Reviews just published his review of the ZF 28/2.