Studio Tests - 35mm full frame
The Sigma puts in a decent performance on full frame, especially when compared to older designs such as the Canon EF 50mm F1.4 USM or the Nikon AF-Nikkor 50mm F1.4 D. Here the most striking improvement is in central sharpness at wider apertures, where the Sigma performs much better across a wider area of the frame than its counterparts.
|Sharpness||Central sharpness is impressively high even wide open, however edges and corners are nothing to write home about (not unusual for a fast 50mm prime on full frame). Sharpness improves progressively on stopping down, and the lens produces exceptional results in the centre of the frame from F2.8 through F8 - impressive indeed. However the corners lag behind somewhat, reaching a maximum at about F7.1-F8.|
|Chromatic Aberration||Lateral chromatic aberration is kept well under control under full-frame, although it's still slightly higher than similar lenses. However again the most striking result is the effective elimination of spherochromatism, which considerably improves perceived image quality at wide apertures.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. Vignetting is distinctly low for a fast 50mm, measuring 2 stops wide open (approximately 2/3 stop better than the Canon or Nikon), and falling to 1 stop at F2.2.|
|Distortion||Distortion on full frame measures as about 1% barrel, which will be just about noticeable in some shots; however again this is significantly better than either the Canon or Nikon equivalent.|
Full-frame compared to APS-C
Eagle-eyed viewers will no doubt have noticed that the MTF50 sharpness data at any particular focal length/aperture combination is distinctly higher on full-frame when compared to APS-C. This may at first sight appear unexpected, but in fact is an inevitable consequence of our presentation of the sharpness data in terms of line pairs per picture height (and thus independent of format size).
Quite simply, at any given focal length and aperture, the lens will have a fixed MTF50 profile when expressed in terms of line pairs per millimeter. In order to convert to lp/ph, we have to multiply by the sensor height (in mm); as the full-frame sensor is 1.6x larger, MTF50 should therefore be 1.6x higher.
In practice this is an oversimplification; our tests measure system MTF rather than purely lens MTF, and at higher frequencies the camera's anti-aliasing filter will have a significant effect in attenuating the measured MTF50. In addition, our testing procedure involves shooting a chart of fixed size, which therefore requires a closer shooting distance on full frame, and this will also have some influence on the MTF50 data.
Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. Our test sample of the lens was in Canon EF mount, and we tested it using on both APS-C and full-frame bodies (mainly the EOS 450D and EOS 5D).
The Sigma 50mm F1.4, with its large front element, might be expected to be somewhat susceptible to flare, but we found it actually performed very well in everyday use, aided no doubt by the fact that the front element is well-recessed behind the filter thread at longer focus distances (as well as by that generous hood).
With the sun placed in the top corner of the frame, no significant loss of contrast or flare patterns are seen even with the lens stopped right down to F16. The optics are fazed only when a bright light source is placed just outside the frame, giving a loss of contrast and red glow on the opposite side. Overall a very convincing performance.
|F16, Canon EOS 5D||F8, Canon EOS 5D|
Background Blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and large aperture. The 50mm F1.4 can be made to blur even relatively close backgrounds into oblivion at wide apertures, a huge advantage for portrait shooting, and despite the presence of an aspherical element, the bokeh is rendered generally very attractively. Focused close and shot wide open, more distant objects can take on an almost impressionistic, painterly look (the ideal lens for budding van Goghs perhaps?).
|F2, Canon EOS 450D||F1.4, Canon EOS 5D|
|50% crop, upper right||50% crop, top centre|
This lens is shows higher levels of lateral chromatic aberration than other 50mm F1.4s in our studio tests, presumably a consequence of Sigma's use of an aspherical element to deliver improved sharpness across a larger region of the image circle at wide angle. However it's only very rarely visible in real-world shots, and therefore essentially inconsequential, the worst example we could find out of hundreds of shots is shown lower left.
Perhaps more problematic, though, is the presence of quite strong bokeh chromatic aberration, which is magenta in front of the focus plane and green behind. This is most visible at wide apertures with their associated extremely narrow depth of field, and can result in a strong fringing artifacts in high contrast regions.
|F2, Canon EOS 5D||F1.4, Canon EOS 5D|
|100% crop||100% crop, upper centre|
Corner softness at wide apertures
The most obvious result from our studio tests is that this lens exhibits fairly extreme corner softness at wide apertures on full frame, and it's possible some potential buyers will be concerned by this issue. In this regard it's important to appreciate that with the extremely small depth of field afforded by 50mm F1.4 lenses, and assuming a reasonably centrally-placed subject, the likelihood of any object in the corners of the frame being remotely in focus is in fact minimal, and corner resolution therefore near-irrelevant.
However for those who still like to fret about such issues, and are possessed by the unnatural urge to shoot planar subjects face on at unusually wide apertures, then not to worry, we've saved you the trouble and the results are shown below. Even at F1.4, central resolution is high (although contrast is rather low), but the corners are extremely soft, and this is exacerbated by darkening due to vignetting. However stop down to F4 and the image quality has picked up substantially; central sharpness is now pretty impressive, and the corners show a far more acceptable result. It's worth noting that this is a significantly better performance than most zoom lenses (relatively few of which even open up to F4 at 50mm), illustrating the optical advantages offered by primes in normal everyday shooting.
|Canon EOS 5D||Canon EOS 5D|
|100% crop, centre||100% crop, centre|
|100% crop, top left corner||100% crop, top left corner|