Zeiss Distagon T* 28mm f/2 ZF (ZE) (on Canon EOS) - Lab Test / Review - Analysis
Lens Reviews - Canon EOS (Full Format)
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Introduction
Analysis

Distortion

The Zeiss Distagon 28mm f/2 produces a moderate degree (~1.8%) of barrel distortion. Interestingly this is the 5th Zeiss Z-series lens with this result (plus/minus a fraction).

Vignetting

The vignetting characteristic seems to be a real weakness of Zeiss Z-series lenses when used on full format DSLRs. The Distagon shows a fairly extreme deterioration of ~2.4EV at f/2 - this will be visible in most scenes. The problem is still very pronounced at f/2.8 (@ ~1.6EV) but it's not overly significant anymore beyond f/4.

MTF (resolution)

The Zeiss was able to deliver very convincing results during our resolution tests. The center performance is great straight from f/2 onwards. The borders/corners follow on a very good level. However, the lens suffers from field curvature at this setting (see next chapter). The peak performance is reached around f/5.6 with an excellent quality (just) across the frame. Diffraction effects are starting to have an impact from f/8 onwards. However, the quality is still easily usable at f/11.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Field Curvature

We were asked to elaborate a little bit more on the field curvature characteristic of the Zeiss lens. "Field curvature" refers to the shape of the focus field. Normally we assume that it's flat, a plane, but it's never (rarely) like that. Most lenses have some degree of field curvature. The Zeiss 28mm f/2 has a bigger issue here specifically at large aperture settings. The focus field remains relatively flat for most of the image field but the extreme corners bend quite a bit away from the "perfect plane". The Zeiss lens is surely not a lens to take images of flat objects - say a building or a ...test chart - at f/2 or f/2.8 because the corners will run out-of-focus. From a user perspective this is a blurred image portion - no more, no less. The field curvature diminishes when stopping down and it's not really field relevant from f/5.6 onwards.

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are comparatively low at around 1px on the average at the image borders. This may be visible at 100% magnification but it's not really an issue on prints anymore.

Verdict

We weren't really impressed by the T* 25mm f/2.8 but the The Zeiss Distagon T* 28mm f/2 plays in a very different (better) quality league - it's even slightly superior to the T* 35mm f/2 which is certainly no slouch. The lens is sharp straight from f/2 and it reaches excellent quality levels around f/5.6. There's some field curvature at f/2 and f/2.8 which may be a problem in certain scenes (out-of-focus corners) though. Lateral CAs are comparatively low and nothing to worry about either. The moderate degree of distortions is about average for a prime lens in this class. A real weakness is the annoyingly high amount of vignetting at large aperture settings.

Just like the rest of its family the Zeiss lens is built to the highest standards. Some users may complain about the lack of AF but this isn't really a significant flaw for a wide-angle lens. The focus confirmation is available in the viewfinder (ZE version) and in very critical (close focus) scenes Live-View can give you the needed guidance. That said, it remains a bit of an anachronism these days.

The price level is quite steep but the performance level is accordingly impressive.

Optical Quality: (downrated due to field curvature issue)    
Mechanical Quality:
Price/Performance:
      
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