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The lens shows moderate barrel distortion at around 1.3%. 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.
Fast lenses tend to suffer from high vignetting on full frame cameras and the AF-D 50 is no exception to this rule. At almost 1.9 EV the corners darken visibly wide open. There's a moderate level of vignetting left at f/2.0 which is reduced further to a negligible degree stopped down to f/2.8 and beyond.
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.
Standard primes have a reputation of delivering sharp images and the AF-D 50 does not disappoint in this regard.
In the image center the resolution is very good wide open already, reaching excellent figures at f/2.8. The borders and extreme corners start a little softer wide open (where the lens also lacks some contrast), but also improve by stopping down, reaching very good levels at f/4 and excellent sharpness at f/5.6 and f/8.
From f/2.8 onwards the extreme corners deliver slightly higher resolution figures than the borders.
The lens showed some focus shift when stopping down (residual spherical aberration).
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
Chromatic Aberrations (CAs)
Chromatic aberrations (color shadows at harsh contrast transitions) are in the range of roughly 0.9 to 1 pixels throughout the aperture range. 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.
The quality of the bokeh (out-of-focus blur) is a primary aspect for such a large aperture lens. Unfortunately at wide open aperture the AF-D 50 shows slightly nervous bokeh. Stopping down however helps to increase the overall bokeh quality. At f/2 already the bokeh smoothens considerably.
Background highlights are circular in the image center but cut off towards the borders due to mechanical vignetting. There is a fair amount of outlining at f/1.4 and f/2, from f/2.8 this issue is mostly resolved, though. However, starting at f/2.8 the non rounded aperture blades begin to shape highlights as "stop signs".
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 the focus point
and green beyond.
Typical for most fast primes the AF-D 50 shows a considerable amount of bokeh fringing at large aperture settings.
In addition, these shots also show the focus shift when stopping down and demonstrate the lack of contrast and sharpness wide open (the latter being emphasized on short subject distances).
Move the mouse cursor over the f-stop marks below to observe the respective bokeh fringing