Studio Tests (APS-C)

The Sigma 50mm F1.4 DG HSM performs exceptionally well on APS-C in the studio, surpassing most other lenses in its class and essentially matching the vastly more expensive Zeiss Otus 1.4/55. Sharpness is impressive even at maximum aperture, and just gets better on stopping down. Distortion, CA and vignetting are all negligible. This is huge improvement over older lenses such as the Canon EF 50mm F1.4 USM, Nikon AF-S Nikkor 50mm f/1.4G, or Sigma's previous 50mm F1.4 EX DG HSM. (Note though that we don't yet have test results for the recent Sony Carl Zeiss Planar T* 50mm F1.4 ZA SSM.)

Sharpness Sharpness is impressively high even at F1.4, with only the slightest softening towards the corners of the frame. It gets even better on on stopping down, with truly excellent cross-frame sharpness from F2.0 to F8 (indeed in this range the MTF measurements are limited by the camera, suggesting the Sigma has plenty in reserve for higher-resolution sensors). Diffraction starts to degrade the image at smaller apertures, but F16 is still perfectly usable if you need the extra depth of field.
Chromatic Aberration Lateral chromatic aberration is exceptionally low - you're unlikely to see it at all in normal use.
Vignetting Vignetting is negligible, as usual for a full frame lens used on APS-C.
Distortion The 50mm F1.4 shows no distortion whatsoever on APS-C.

Macro Focus

Macro (APS-C) - 118 x 79 mm coverage
Measured magnification: 0.19x
Distortion: Slight barrel

Minimum focus distance*: 37.8cm
Working distance**: 23.4cm
Focal length: 50mm (80mm equiv)
* Minimum focus is defined as the distance from the camera's sensor to the subject
** Working distance is measured from the front of the lens to the subject

In general 50mm F1.4 lenses aren't especially optimized for close-up work, but even so the Sigma puts in a very decent showing. Its measured maximum magnification using manual focus is 0.19x, fractionally higher than specified, and noticeably better than most other 50mm F1.4 primes. Note though that SLRs won't generally autofocus quite this close.

Image quality at minimum focus is rather impressive. Unsurprisingly sharpness isn't great at F1.4, but chromatic aberration is essentially invisible and there's only a tiny bit of barrel distortion. Sharpness picks up quickly on stopping down, and by F4 our flat test chart is impressively sharp right across the frame. Sharpness inevitably starts to degrade on stopping down to F11, as diffraction kicks in, but the trade-off of increased depth of field will often negate this.

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. Here we're looking at issues specific to APS-C users; for a fuller picture, be sure to read the next two pages too.

Wide-open sharpness / corner softness

In previous reviews, we've seen that 50mm F1.4 primes based on film-era optical designs have traditionally not fared too well on APS-C cameras. In particular they've tended to give very hazy-looking images when shot wide open, as a result of the spherical aberration that tends to be associated with this type of lens. The 50mm F1.4 'Art' changes all that, as can be seen in the rollover below.

Sigma 50mm F1.4 DG HSM | Art on Canon EOS 100D

Even at F1.4 the centre of the image is pretty sharp (although there's a slight red tinge around edges from longitudinal chromatic aberration), and the corner isn't bad at all either. Sharpness is excellent from F2 through to F8 right across the frame; stopping down beyond this inevitably introduces a degree of softening due to diffraction. But overall this is a very impressive showing.

Autofocus accuracy and consistency

One persistent problem when using SLRs with fast lenses is autofocus accuracy and consistency. Modern high resolution image sensors place AF systems under extreme scrutiny, and we've often found that APS-C SLRs in particular can come up short. This tends to be very dependent upon the camera body used, and the specific copy of the lens too. It can also vary with the focus point selected, with the outmost AF areas generally more prone to errors than the central ones.

We've shot the Sigma 50mm on both the enthusiast-oriented EOS 70D and the lower-end EOS 100D, and not surprisingly the cheaper camera's AF system struggled more. In particular when shooting at apertures larger than F2.8, it struggled to focus sufficiently accurately to make the most of the lens's sharpness. Below is a fairly typical example of what we saw, using the far right AF point to focus in a distant subject, with the aperture set to F1.4. None of these three replicate shots is perfectly focused, although the first isn't too far off.

Canon EOS 100D, F1.4 Shot 1, 100% crop
Shot 2, 100% crop Shot 3, 100% crop

To be fair though, the Sigma costs rather more than most entry-level APS-C SLRs, so we wouldn't expect many owners of these cameras to buy one. Then again there's a chance that full frame users may well have a small APS-C model as a backup, and for them it's a pity that these cameras can't consistently take advantage of the lens's full potential.