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Distortion
The lens produces a small amount of barrel distortion at around 0.7%. This is already fairly low for a normal prime lens. In addition, software correction can be enabled in-camera, which brings the distortion down to around 0.2%.
Vignetting
Even though it is a bit larger than a typical nifty fifty, the lens is still fairly compact, which in return means that some vignetting is to be expected. Wide open, the amount of light fall-off towards the image borders exceeds 2 stops. As usual, stopping down significantly reduces vignetting. At f/2.8 it is already down to less than one stop and around half a stop at f/4. Stopping down further does not reduce the remaining amount of vignetting though.
Nikon offers in-camera correction for vignetting, but the situation is a bit more complicated than distortion correction, where it is basically just an on/off setting. Vignetting correction can be set to either 'off', 'low', 'normal' or 'high' in the camera menu, where the default setting is 'normal'. For JPGs, the chosen setting is applied to the final image. If you shoot RAW, the selected setting is stored in the meta data to be applied later by the processing software. In addition, Nikon embeds the full lens correction profile into the NEF files.
That correction profile seems to be of fairly high quality (and includes distortion correction data, too, btw.). If you shoot NEFs and use a RAW converter that allows full access to the embedded correction profile (like C1 does, for example, while Adobe products currently do not), vignetting can be completely eliminated. The initial idea was to include the results for fully applied correction in the chart below, however it would just have added empty space to the diagram, because at any aperture the software correction brought vignetting so close to zero that those bars would have been simply invisible in the chart.
Surprisingly, none of the in-camera correction settings Nikon offers makes full use of the correction profile in the same way. Even at the 'high' setting, vignetting is not fully corrected.
For our vignetting analysis of Z lenses, we do not use JPGs generated by the camera, as we usually do for other systems. Instead, we base the analysis on NEF files imported into CaptureOne, with vignetting correction applied to a level that resembles the 'normal' setting of the in-camera correction. This may seem like an unnecessarily complicated approach in the context of this review, but it gives us more flexibility in upcoming reviews of lenses with high distortion levels, where we want to address the effects of both kinds of software correction on vignetting combined and independently.
With that amount of 'normal' correction applied, more than one stop of vignetting remains at f/1.8, while from f/2.8 onwards it's reduced to a level that should not be noticeable anymore with most subjects.
MTF (resolution)
The Nikkor shows impressive performance in the lab. There is excellent resolution in the image center and near center straight from the maximum aperture.
The borders and corners follow a bit behind with very good resolution straight from f/1.8, with the borders even reaching excellent levels stopped down to f/5.6.
The lens showed a small amount of focus shifting 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 very well controlled by the lens, with values of just below half a pixel at the image borders wide open, slowly decreasing to below 0.3 pixels stopped down to f/11.
Bokeh
One of the main reasons to use a fast prime over any kind of slower zoom is its ability to separate the main subject from the background. For such images, the quality of the background blur is of major importance.
The Nikkor delivers very smooth and pleasing image blur in these kinds of shots.
Out-of-focus highlights are evenly filled with virtually no outlining. Thanks to the 9 rounded aperture blades, highlights retain their circular shape even when the lens is stopped down.
The shape of the highlights deteriorates a bit towards the image borders due to mechanical vignetting at large apertures. However, stopping down solves that issue.
In the focus transition zone, the lens shows smooth image blur behind the focal plane (to the left below), but a bit more nervous bokeh due to mild double images in the foreground (to the right below).
Bokeh Fringing / LoCA
Bokeh fringing (non-coinciding focal planes of the various colors, also referred to as longitudinal chromatic aberration, or LoCA for short) is an axial color fringing effect and a common issue with relatively fast glass. The halos typically have different colors - magenta (red + blue) in front of the focus point and green beyond. Unlike lateral CAs, bokeh fringing can not easily be fixed in post processing.
The Nikkor is not free of of bokeh fringing, but shows a fairly low amount wide open for its lens class. As usual, stopping down the lens reduces the amount of bokeh fringing further.
In addition, these shots also illustrate the small amount of focus shift when stopping down that was mentioned in the MTF section. It's worth mentioning though that down to f/5.6, Z cameras focus with the lens stopped down (unless the lens is even slower than f/5.6, of course), so the focus shift can only become an issue if the lens is focussed manually and the aperture changed later (that's actually what we do for the images below).
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