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Page 2 of 3
Distortion
The Zeiss Distagon 28mm f/2 produces a moderate degree (~1.8%) of barrel distortion.
The chart above has a real-world size of about 120x80cm.
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 ~2EV at f/2 - this will be visible in most scenes. The problem is still very pronounced at f/2.8 (@ ~1.3EV) but it's not overly significant anymore beyond f/4.
We're performing our vignetting analysis based on
(uncorrected) JPEGs straight from the camera. The JPG engine of the Nikon D3x features a rather flat
gradation curve, thus has a moderate contrast characteristic, resulting in comparatively low vignetting figures - the
corresponding Canon figures are roughly 40% higher due to the more
aggressive default contrast setting.
MTF (resolution)
The Zeiss was able to deliver very convincing results during our resolution tests. The performance is very good at all measurement spots at f/2 already. 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 in the center and very good borders and corners. Diffraction effects are starting to have an impact from f/8 onwards. However, the quality is still easily usable at f/11.
Please note that the MTF results are not directly comparable across the different systems!
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)
Lateral CAs (chromatic aberrations, color shadows at harsh contrast transitions) are moderate at around 1.5px on the average at the image borders. This may be visible at 100% magnification but it's not really an issue on prints anymore. In addition, it is possible to correct lateral CAs in most modern RAW converters. And many modern Nikon DSLRs already do this for you in-camera if you shoot JPG.
Bokeh
It is not one of the primary usage scenarios for a wide angle lens to seperate the main subject from the background. However, with a lens as fast and able to focus as close as the ZF 28 it is easily possible nonetheless. In such an image the quality of the bokeh (out-of-focus blur) can be of major significance.
The bokeh wide open is a bit nervous. This is not only true for the transition zone, were most lenses struggle, but also for subjects at some distance from the focal plane.
Background highlights retain a circular shape throughout most of the aperture range, except towards the image corners. Due to vignetting they are cut off on one side at f/2. There is a fair amount of outlining wide open as well as some traces of LoCAs (which are typical for this lens glass, see next section). Both can be reduced by stopping down. From f/5.6 onwards background highlights begin to develop straight edges. This is not shown here, but visible in one of the sample images on the next page (the one with the birch tree).
Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)
LoCAs (non-coinciding focal planes of the various colors) are a common issue with relatively fast glass. As you can
notice below, the halos have different colors - magenta (red + blue) in front the focus point
and green beyond. Truly "apochromatic" lenses don't show LoCAs but these lenses are very rare especially
below 100mm. Unlike lateral lateral CAs, LoCAs cannot easily be fixed in post processing.
Typical for most fast primes the ZF 28 shows some amount of LoCAs at large aperture settings, which can of course be reduced by stopping down. From f/4 onwards LoCAs are no longer field-relevant (except for scenes with very high contrast, which amplifies the issue).
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