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Distortion
The lens shows moderate barrel distortion at around 1.1%. This is more than you'd expect from a fix focal lens. However, this is actually a typical amount of distortion for fast standard primes and unless you shoot subjects with straight lines near the image borders it's usually not field-relevant.
The chart above has a real-world size of about 120x80cm.
Vignetting
Typical for most fast primes, the Nikkor shows some visible vignetting wide open with a light falloff towards the corners of around 1.5 EV. As usual, stopping down reduces the corner darkening considerably. From f/2.8 onwards it's no longer an issue except for really critical subjects.
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 center resolution is very good wide open already (but lacks some contrast here) and reaches excellent values at f/2.8 and beyond, except at f/11, where diffraction reduces the resolution again.
The border sharpness is very good at almost all tested apertures and excellent at f/5.6. The extreme corners follow a slight bit behind with good sharpness wide open and very good resolution at f/2.8 and beyond.
The global performance peak is reached at f/5.6.
The lens showed some focus shift when stopping down (residual spherical aberration).
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
Chromatic Aberrations (CAs)
Chromatic aberrations (color shadows at harsh contrast transitions) are moderate and in the range of roughly 1 to almost 1.4 pixels, achieving the lowest values wide open and slowly increasing with smaller apertures. This is a good result in the full format scope and not an issue in the field for most subjects. In addition, CAs can easily be corrected in software or by the camera itself (if you shoot JPEGs and own a current Nikon DSLR).
Bokeh
One of the primary usage scenarios for a large aperture lens is to separate the main subject from the background. In such an image the quality of the bokeh (out-of-focus blur) is of major importance.
Image blur shows traces of double contures and as such a bit nervous characteristic behind the plane of focus. Slightly stopped down (not shown here) the general impression is a bit more pleasing.
Background highlights are not perfectly circular wide open towards the borders due to mechnical vignetting. Stopped down, the shape of the aperture shapes, which are only slightly rounded, comes through and creates polygons instead of circles. The amount of outlining is quite low, though.
Bokeh Fringing
Bokeh fringing is a common issue with relatively fast glass. It's visible as halos of different colors in out-of-focus areas - magenta (red + blue) in front of the focus point
and green beyond.
Typical for most fast primes, the Nikkor shows noticeable bokeh fringing at large aperture settings, which can of course be reduced by stopping down.
In addition, these shots also show the focus shift when stopping down that was mentioned in the MTF section, as well as the reduced contrast wide open. The latter issue is emphasized here due to the close focus distance.
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