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The lens shows a moderate amount of barrel distortion which can be visible in critical shots especially with straight lines near the image borders. However, the distortion is symmetrical and easy to correct in post processing.
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-S 24/1.4 is no exception to this rule. In fact, with almost 2.2 EV wide open the amount of vignetting is exceptional (in a negative sense) and of course clearly visible in the final image (see sample image section for examples). Stopping down helps to reduce vignetting, but even at f/2 and f/2.8 the amount of light falloff remains rather high. From f/5.6 onwards vignetting is no longer field relevant, except for really critical scenes.
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.
In our lab tests the AF-S 24 produced very good results for a lens in this class. The center resolution is very good wide open already, increasing to excellent figures from f/2.0 onwards.
Typical for such lenses the borders and corners however are somewhat soft wide open. Stopping down to f/4 increases the resolution to very good figures, though. Surprisingly, the corners show slightly more sharpness than the borders from f/2.8 onwards.
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) reach around 1.7 pixels on the average at the image borders throughout the aperture range. This might be visible in very large prints (or crops). However, for typical print sizes the problem isn't really field relevant for most subjects. In addition, CAs can easily be corrected in software or by the camera itself (JPEGs).
One of the primary usage scenarios for a large aperture lens is to seperate the main subject from the background. In such an image the quality of the bokeh (out-of-focus blur) is of major significance.
Bad news first - the bokeh in front of the focal plane can be somewhat nervous. However, for a wide angle lens, this is hardly a field-relevant issue. Behind the focal plane, the lens produces very smooth and pleasant blur, including the transistion zones.
Thanks to 9 rounded aperture blades, background highlights retain their circular shape throughout the whole aperture range. Highlights are evenly filled, there is almost no outlining, but some traces of LoCAs (which are typical for this lens class, see next section). For a wide angle lens, this is a very impressive performance.
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, especially ultra fast ones, the AF-S 24 shows some amount of LoCAs 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.
Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
It does not come as a surprise that a lens as complex as the AF-S 24 produces some amount of flare in backlight conditions. As you can see in the f/8 sample below, there's a single spot and a bunch of rainbow coloured shades opposite to the source of the light. However, thanks to Nano Coating the total amount is rather low and the contrast remains on a very high level. This (high contrast) is also true for large apertures, where the flare spot is larger but also more transparent and only single coloured and the rainbow flares are reduced to a slight haze.
These two images are also available in full resolution in the samples section on the next page