The Sigma 24mm f1.4 DG HSM Art is a wide angle prime lens, available in Canon, Nikon and Sigma mounts. Announced in February 2015, it’s the second widest prime lens in Sigma’s highly acclaimed Art series which includes the 50mm, 35mm, and 20mm f1.4 models. Expectations are high since the 50mm and 35mm lenses earned our Highly Recommended award; see my Sigma 35mm f1.4 Art review or my Sigma 50mm f1.4 Art review. The 24mm Art is also compatible with Sigma’s USB dock for fine-tuning or firmware updates, and if you change bodies, you can pay to have the mount swapped for your new system.
Large aperture 24mm lenses are quite attractive additions to interchangeable lens bodies of APS-C / DX or full-frame / FX format: On the cropped bodies they become equivalent to 35mm in coverage and f2 in depth of field which works well for reportage / street photography as it’s fast and gives some background isolation. On a full-frame body the 24mm focal length delivers noticeably broader coverage than 35mm or 28mm lenses without being prone to some of the distortions that 20mm can produce. You capture more of architecture or landscapes plus the bright aperture of an f1.4 lens lets you shoot longer under diminishing light, while also allowing you to isolate closer subjects at least a little from the background.
As I also tested Nikon’s own AF-S Nikkor 24mm f1.4G and the manual focus Samyang 24mm f1.4 I can give you a good impression on how the Sigma compares to the competition. All lenses were tested on a 36MP Nikon D810 body.
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Facts from the catalog
As usual I’ll have a look at the technical data first. I’ve rated the features with a [+] (or [++]), when it’s better than average or even state of the art, a  if it’s standard or just average, and [-] if there’s a disadvantage.
Size (diameter x length): 85 x 90 mm. This is pretty decent although it’s funny to see a wide-angle lens being so long. 
Weight: 665g. Pretty heavy, but that seems to be the standard for 24/1.4 lenses as the respective offers from Nikon, Samyang and Canon are around the same weight (and size). 
Optics: 15 elements in 11 groups. This is pretty complex design with 22 glass/air-surfaces and a lot of special elements including two aspherical elements. “Super-Multi-Layer” coating should reduce flare and ghosting. [+]
Closest focus distance is 0.25m which gives you a working distance of around 10 cm when you don’t use the lens-hood. Maximum magnification is 1:5.3. That’s not much but is standard in this lens-class. 
Filter-thread: All 24/1.4 lenses use 77 mm filters. That’s the size that most pro-lenses use. [+]
Image stabilization: No 24 mm prime lens offers optical stabilization except for Canon’s EF 24mm f2.8 IS USM. And with the small magnification at a focal length of 24mm it is no big loss. Still: shooting dim interiors or landscapes in fading light hand-held can prove quite a challenge without stabilization. 
Auto focus: The lens offers silent auto focus with manual-focus override by turning the focus ring. So it works on bodies without AF-motor like the Nikon D3x00/5×00-bodies. That may sound like a standard feature but you can still find lenses with a less optimal manual override mechanism or even with manual focus only (like from Samyang). [+]
Coverage: covers full frame/FX or smaller = very good. [+]
Price: The Sigma costs around 750 EUR (incl. 19% VAT) which is not cheap but a fair price for the ambition Sigma has for this lens. The AF-S 24/1.4G ED Nikkor for example costs more than double that at around 1700 EUR. But the manual focus Samyang is even cheaper than the Sigma at around 550 EUR. [+]
Comes with a nice semi-soft lens-case that is well padded but has no strap, the lens-hood is included, reversible for transport, and the lens-caps are similar to Nikon’s. 
Distance information is relayed to the camera, so the Nikon body can do all the advanced exposure-related stuff with this lens. [+]
Aperture ring: No, which is standard. Only the Samyang offers one. 
Sealing: The lens has a rubber grommets at the lens-mount. [+]
The score in the “features-department” is 0[-]/6/7[+]. So nothing to rave about. But the Sigma has one big plus: it is by far the cheapest way to get a 24/1.4 lens that does AF.
– If you’re a Canon shooter there’s the EF 24mm f1.4 L USM II priced at around 1500 EUR (incl. 19% VAT).
– For owners of Nikon DSLRs the gold-ringed AF-S Nikkor 24mm f1.4G from 2010 is the most expensive of Nikon’s current f1.4 lenses: 1700 EUR. See my Nikon 24/1.4G review.
– If you don’t need AF there’s a manual focus 24mm f1.4 ED AS UMC from Samyang which is the cheapest way to get a 24mm f1.4 lens: It is priced around 550 EUR. See my Samyang 24/1.4 review.
– If you can live with a maximum aperture of f/1.8 or even f/2.0 there are more alternatives: From Sigma for example the 18-35mm f1.8 DC Art (680 EUR), which is a very interesting zoom but covers only APS-C / DX sensors or the 24-35mm f2.0 DG Art (970 EUR). From Nikon there’s the new AF-S 24/1.8G lens (850 EUR) which earned a Recommended in my Nikon 24mm f1.8G ED review.
Focus and build quality
Focus accuracy and repeatability is critical to consistently produce sharp shots. Repeatability (the accuracy of focus on the same subject after repeated focus-acquisition) of this lens is very good (measured 98.7% in FoCal) with no real outliers over a series of 40 shots. To achieve this I had to adjust the lens’s AF using the Sigma lens-dock. Without this the lens produced awful auto-focus results even with AF tuning in camera set to the maximum value. The lens focuses fast: around 0.6 sec from infinity to 30 cm, which is a very good value.
The focus ring is 23mm wide and has no slack/play between its movement and the focus-action and a throw of around 95 degrees, which is OK for accurate focus wide open. The focus ring is easy to grip but movement is pretty stiff. AF-operation is barely audible from the outside and if you record video with the built-in microphone the Sigma proves to have one of the quietest AF-drives around.
The lens is made in Japan and the general impression of build quality is that of a pro-level lens: A high quality metal/plastic construction with nine rounded aperture blades and a tight fitting lens hood. But I miss weather sealing on the lens mount.