Studio Tests - APS-C format
The Sigma 50mm F1.4 performs very well on APS-C, benefiting as usual from the 'sweet spot' advantages of low distortion and minimal vignetting which are common to shooting full-frame lenses on APS-C. At wide apertures (F1.4-F2), it clearly outperforms either the Canon EF 50mm F1.4 USM or Nikon AF-Nikkor 50mm F1.4 D, providing far more consistent sharpness across the frame and especially towards the corners, coupled with lower axial chromatic aberration; however at smaller apertures the older designs draw ahead.
|Sharpness||The Sigma 50/1.4 gives impressively even coverage across the frame on APS-C, and although it's slightly soft wide open, it's very much towards the top of its class. Sharpness increases progressively on stopping down, with optimum results at F4-5.6; at apertures of F8 and smaller, diffraction takes its toll.|
|Chromatic Aberration||Lateral chromatic aberration is kept reasonably under control, and while measurably higher than other 50mm lenses, is unlikely to be a problem in use. However, more striking is a complete lack of spherochromatism (colour-specific spherical aberration) at F1.4, which is normally visible as non-zero CA values at the centre of the frame, and arguably the major image-degrading issue with fast primes. However the appearance of magenta central CA at F2 indicates that the plane of best focus has shifted slightly to the rear at this aperture compared to F1.4, a symptom of residual uncorrected spherical aberration.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. As usual for a full-frame lens used on APS-C, there's really nothing to worry about here.|
|Distortion||Distortion is very low at just 0.6% barrel, and unlikely to be visible in real-world shooting.|
Extrapolation of test results to Four Thirds Format
The Sigma 50mm F1.4 EX DG HSM will be available in Four Thirds mount, and it's possible to predict its expected performance on this system based upon the MTF50 data for the APS-C format. The Four Thirds frame (17.3mm x 13mm) has approximately 87% of the height of the APS-C frame (22.2mm x 14.8 mm), and the diagonal is approximately 81% of its length. Therefore, the expected data for Four Thirds can be attained by multiplying the MTF50 data (y-axis) by 0.87, whilst considering only the data out to 81% of the frame diagonal (i.e. the x-axis of the graph).
Overall this suggests that the 50mm F1.4 will be a reasonably competent performer on Four Thirds, although somewhat soft wide open and overall unable to match the Olympus Zuiko Digital ED 50mm F2.0 Macro for resolution (not surprising for a lens designed to cover four times the area).
Third party lenses have something of a reputation for less-than-neutral colour balance, with Sigmas in particular subject to widespread anecdotal reports of a distinct yellow tint, so in this test we measure any colour cast introduced by the lens in comparison to the camera manufacturer's 50mm lens (generally considered a good standard for neutrality).
In this test, the camera is pointed towards an evenly illuminated white wall, and light entering the lens completely diffused using an 'Expodisc' white balance filter. A custom white balance is taken using the camera manufacturer's 50mm lens (in this case the Canon EF 50mm F1.4 USM), then exposures made using the lenses under test. The RGB values from the centre of the frame are reported (measured as an 11x11 average). In this comparison we've also included data from Canon's inexpensive EF 50mm F1.8 II.
|Canon EF 50mm F1.4 USM
(220, 220, 220)
|Canon EF 50mm F1.8 II
(210, 211, 214)
|Sigma 50mm F1.4 EX DG HSM
(217, 216, 221)
Here we can see that the Sigma 50mm F1.4 actually shows a very slight colour shift when compared to the Canon EF 50mm F1.4 USM, but mainly towards blue rather than yellow, with a small magenta component. However the Canon EF 50mm F1.8 II also shows a shift of similar magnitude, but this time blue-green. And it must be stressed that while these colour casts are indeed measurable, they are scarcely visible, and if necessary could easily be corrected using a custom white balance.
Specific image quality issuesAs 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.
Softness wide open
As we've come to expect for a full-frame optic used on APS-C, one chink in this lens's armour is a slight lack of sharpness when shot at wide apertures. In this regard it's worth noting that depth of field is so shallow at F1.4 that real-world results are mainly dependant upon focus accuracy, and this lens will tax the abilities of any focusing system, either auto or manual (not to mention the fact that the slightest relative movement between photographer and subject will result in a misfocused image).
For the brick wall connoisseurs, the shots below show what you can expect from a correctly focused image at F1.4 compared to F5.6 (close to the 'sweet spot' for this lens). At F1.4 a reasonable amount of detail is being resolved, just at low contrast; stopping down improves the centre markedly, but has less effect at the corners.
|Canon EOS 450D||Canon EOS 450D|
|100% crop, centre of frame||100% crop, centre of frame|
|100% crop, top left corner||100% crop, top left corner|