I’ll skip to the end and get to the heart of it: this is, optically speaking, a tremendous lens, and a wonder of modern optical design and lens manufacturing. It’s truly incredible what Sigma has accomplished with this lens: a 20mm ultra-wide lens with an f/1.4 max aperture that is not only usable but critically sharp (in the center) even on a 36 megapixel camera like the Nikon D800E.
But it’s not perfect. It’s big, heavy, has a bulbous front lens that won’t easily accept filters, and my copy (rented) on Nikon F mount failed to autofocus accurately almost 50% of the time, an extremely worrisome failure rate that had me defaulting back to manual focus.
Sigma has been on quite a tear lately with their ART series lenses, which began with the 35mm f/1.4, continued with the 50mm f/1.4 and 24mm f/1.4, and now brings the 20mm f/1.4. For those who don’t know, creating a lens this wide – 20mm – that is this fast – f/1.4 – and that delivers this level of optical brilliance even wide open is astounding, the kind of performance you expect from Leica or a Zeiss Otus, but delivered at a MUCH lower price than those manufacturers would ever dare. And it has autofocus (imperfect as it is…).
It’s not a subtle lens. It’s big. It’s heavy, though it balanced find on my Nikon D800E. It has a bulbous front element a la the Nikon 14-24mm f/2.8, so it you are into filters you’ll have to invest in a Lee holder or something similar to allow you to use filters with this lens.
The remarkable thing about this lens is just how sharp it is – in the center and middle-center portions of the frame – even at f/1.4. I’m talking “able to count my daughter’s eyelashes” sharp. This means you can use f/1.4 realistically for critical shots, and that knowledge really opens up your ability to shoot creative imagery. As this lens, like most ultrawides, focuses very close, you can get super close to a subject and blur out the background by shooting f/1.4 – giving you environmental context but highlighting your subject clearly. Conversely, you can use this lens at f/1.4 to shoot nighttime landscapes or cityscapes by hand – though be warned that the corners and edges are not critically sharp at f/1.4.
I’d be saving up to buy this lens, or at least seriously considering it, if not for the really atrocious performance my copy (rented) had with regard to autofocus. My hit rate was as low as 50% with AF with this lens, even using the central focus point. Sigma (like Tamron and Tokina) have to “reverse engineering” the autofocus algorithms used by Nikon and Canon (which is why some of their lenses fail to AF at all with new Nikon or Canon bodies, at least until a firmware update comes along). It seems like, at least from what this one copy showed, that Sigma still needs to work out some kinks with regard to the Nikon AF algorithm on this lens.
That said, Your Mileage May Vary. I might have gotten a bad lens for a rental – it’s hard for me to say. All I can tell you is how this one example behaved: and, in that regard, the autofocus was terrible.
So: spectacular optics. Truly remarkable. But…wow. I would need that autofocus to work if I was going to buy the lens, because otherwise I’d just stay with my Zeiss 25mm f/2. Not as fast, not as wide, and the extreme corners never fully sharpen up, but just as optically brilliant otherwise and with a nicer manual focus ring and the ability to use filters (and much, much smaller and lighter to boot).
My recommendation is to rent this lens before you buy, and see if you run into the AF issues, and see if the weight and bulbous front element bother you.
Note: All the images used as examples in this article were shot at f/1.4