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. The Sigma 50mm F1.4 DG HSM fully lived up to the promise shown in the studio tests, performing consistently well at all subject distances and in a wide range of lighting conditions.

Centre and corner sharpness

We've already seen that the studio test results indicate exceptional sharpness from the Sigma, but what does this look like in real images? The answer is in the rollover below - we've deliberately used an uninteresting composition of an interesting subject to show how central and corner sharpness changes through the lens's aperture range.

Canon EOS 6D, Raw + ACR
F1.4
F2
F2.8
F4
F5.6
F8
F11
F16

What you should be able to see here is that central sharpness is already excellent wide open, and only slightly improves on stopping down. The corner crop isn't quite so good, but is still hugely impressive for a 50mm F1.4 prime. However it essentially catches up with the centre on stopping down to F2.8. There's barely any visible change from F2.8 through to F8, then just the slightest hint of diffraction softening at F11. This becomes more marked at F16, but this aperture should still be eminently usable when extended depth of field is required.

Chromatic aberration

Our studio tests reveal that the 50mm F1.4 exhibits exceptionally low lateral chromatic aberration, with essentially no colour fringing towards the corners of the frame. Our real-world shooting confirms this, and shows that Sigma has also done an impressive job of minimizing longitudinal CA too (i.e. colour fringing around out-of-focus elements at large apertures). The overall result is very impressive - there's scarcely anything to worry about here at all.

The examples below are effectively torture-tests for the two types of CA. The first is looking at lateral CA, and reveals just the tiniest amount of blue/yellow fringing along high-contrast edges in the extreme corners; however you have to look really closely to see it. This is a very impressive result, even by the standards of 50mm F1.4 lenses (which are generally well-corrected for lateral CA).

Lateral Chromatic Aberration
F8, RAW + ACR, Canon EOS 6D 100% crop, top left

The second example looks at longitudinal CA, in a situation close to the worst-case scenario (lots of bright chrome against dark backgrounds, with the lens wide open). We've also used an APS-C camera - the EOS 70D - which accentuates the visibility of this type of CA. The image reveals some colour fringing around high-contrast out-of-focus elements - predominantly magenta in front of the plane of focus, and green behind - but for such a fast lens it's actually relatively low.

Longitudinal Chromatic Aberration
F1.4, Canon EOS 70D 100% crop - longitudinal CA
100% crop - magenta fringing 100% crop - green fringing

Vignetting

The tests show that the 50mm F1.4 exhibits about 1.5 stops vignetting wide open on full frame, which is impressively low for its class ('traditional' 50mm F1.4 designs typically show about 2 stops). The fairly gentle falloff pattern means it's not especially intrusive - in many real-world images it will disappear into the natural variations of brightness across the frame. In fact, in many cases we prefer an image with a touch of vignetting to one without - it can help frame the subject. It's also trivial to correct in post-processing when necessary (but note that any in-camera corrections won't work with this lens).

Canon EOS 6D, F1.4, uncorrected
Canon EOS 6D, F1.4, corrected

In practice you'll probably only really notice vignetting with images that have even-toned areas covering much of the frame - typically this means blue skies. The rollover above compares an image shot at F1.4 on the Canon EOS 6D with a version that's had the worst of the vignetting corrected using Photoshop's generic lens correction tools. Which you prefer is very much a personal choice; there's no inherently right or wrong answer here.

Distortion

The studio tests indicate that the Sigma shows very low distortion, and this makes it extremely well suited to shooting geometric compositions such as architecture. These days distortion is extremely easy to correct in post-processing, of course, but it's still nice to see a lens that makes this utterly unnecessary. And while this degree of optical correction often comes with some penalty in corner sharpness, this isn't really the case here - the shot below is pretty much critically sharp right across the frame at F4.

Canon EOS 6D, 1/1600 sec F4 ISO 200

The example above illustrates the 50mm's near-perfect correction - the roof line has been rendered as close to perfectly straight as you're likely to see. We'd probably apply a little perspective correction to this particular shot to correct the converging verticals, and the lens's impressive cross-frame sharpness means that the image file holds up to this manipulation without any trouble at all (click here to see a geometrically-corrected version of this image).

Flare

It's not unusual for fast primes to struggle with flare when shooting into the light - all that glass means there's plenty of surface area for internal reflections. However in our experience the Sigma 50mm F1.4 is impressively resistant to flare, and quite obviously better than older-design 50mm F1.4s. As usual you'll get best results using the hood by default (and it'll also help protect the front element).

In the first example (below left) we've deliberately placed the sun within the frame and stopped right down to F11, which makes any flare patterns resolved and distinct. There's a little radial streaking from the light source, with a few coloured spots across the diagonal from the sun. But there's impressively little effect on shadow detail and contrast.

Canon EOS 6D, F11 Canon EOS 6D, F2, sun just outside frame

The second shot illustrates a tricky situation shooting directly into the light, using a relatively large aperture (F2). Here the sun is directly impinging on the front element meaning that the hood can't help at all; traditionally-designed 50mm F1.4s would probably show extreme veiling flare across the image. However the Sigma has handled this very well, without any obvious impact on overall contrast.

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. The Sigma's fast F1.4 maximum aperture means it can give substantially blurred backgrounds under the right conditions.

For the most part, the lens gives perfectly attractive bokeh too, although it's probably not quite as super-smooth as lenses like the Canon EF 50mm F1.2L USM or Nikon's AF-S Nikkor 58mm f/1.4G (inevitably there's some trade-off for the Sigma's exceptional wide-open sharpness). The examples below give a flavour of how the lens behaves in this regard.

EOS 6D, F1.4 Background detail
Canon EOS 100D, F2 Background detail
EOS 6D, F1.4 Background detail

Overall the Sigma acquits itself well here, with lovely smooth background blur in the close-up and portrait shots. The more-distant background in the third example looks a touch more fussy, although it's still far from unattractive. As always, for best results you should still take some care to choose a suitable background whenever possible, rather than rely on the lens simply blurring everything away for you.

Coma

Conventional fast 50mm primes tend to show quite obvious coma and astigmatism - aberrations which cause point light sources towards the edge of the frame to flare out. This can be problematic when shooting such things as star fields of city scapes at night. In the table below we're looking at how the Sigma fares in this respect, by shooting a bright white LED light placed at the extreme right hand corner of the frame using the Canon EOS 6D. We've deliberately over-exposed the light to show the maximum extent of the effect.

Canon EOS 6D, 100% crops from extreme lower right corner of frame
F1.4 F2
F2.8 F4

Looking at the F4 crop lower right, you can see the actual size (and circular shape) that the LED is supposed to be. Any flaring out from this at larger apertures is an indication of coma aberration. In the case of the Sigma 50mm, it's really pretty small, and unlikely to be a serious problem in real-world shots.