Sigma 50mm f/1.4 EX DG HSM - £370/$500
Compared with the Nikon 50mm f/1.4G, which has the same maximum aperture, Sigma's equivalent lens is bigger and 225g heavier. The filter thread is also relatively large at 77mm, compared with the Nikon's 58mm. On the plus side, the lens feels robust and well built, although it lacks the Nikon's rubber sealing ring around the mounting plate.
High-end features include a ring-type HSM (Hyper-Sonic Motor), which comes complete with full-time manual override. As with the ring-type Nikon lenses, autofocus is practically silent. Another similarity is that the overall length of the lens doesn't increase at short focus distances - despite the lens not having a true internal-focus design, the front element only extends within the boundary of the outer barrel.
There's plenty of sharpness and contrast between f/4 and f/16, but between f/1.4 and f/2.8 performance is disappointing. The rounded, nine-blade diaphragm helps create a beautiful bokeh, but no better than that formed with the Nikon 50mm f/1.4. At £80/$40 more, the Sigma is relatively poor value.
The lack of sharpness at large apertures is disappointing, although results are good at f/4 and smaller.
Sharpness at f/2.8: 656
Sharpness at f/8: 1886
Sharpness at f/16: 1779
Lab scores for colour fringing are poor for a prime lens of this price. The cheaper Nikon 50mm f/1.4G fares much better.
Fringing at f/2.8: 0.32
Fringing at f/8: 0.28
Fringing at f/16: 0.42
There's some barrel distortion but it's not noticeable in most pictures, as is the case with the equivalent Nikon lens.
Image test verdict
Let down by a real lack of sharpness and contrast at large apertures, and the image quality isn't a match for that from cheaper Nikon lenses.