I have to admit that I was genuinely excited when I heard the announcement for this lens. I had previously reviewed the Sigma 24mm f/1.4 and was really impressed by its image quality and design, and the thought of a 20mm version was very appealing indeed. Not only is this an excellent focal length for landscapes, but the f/1.4 maximum aperture means that on paper at least, it should also be an ideal choice for those photographers keen on shooting the night sky.
Its nearest rival would probably be the Zeiss 21mm f/2.8. This is a lens with an excellent reputation for optical quality and it is popular with both landscape and astrophotographers. The original Distagon was not weather-sealed, but the updated Milvus is. Both versions are manual focus only, whereas the Sigma 20mm features autofocus. I shot regularly with the Zeiss 21mm for a few years, so I’m familiar with its capabilities and was interested to see if the Sigma, at roughly half the price, could match its performance.
In terms of styling, the new Sigma closely resembles the other lenses in the Art range, with a sleek, minimalist look. It has a quality feel with a smooth manual focus action. It’s bulky and heavy, but balanced well when mounted on my Canon 5Ds. One slight disappointment is the depth of field scale, which maintains the minimalist theme of the lens design: it only has markings for f/8 and f/16. One of the benefits of prime lenses is the ease with which you can set the hyperfocal distance, provided the depth of field scale is adequate – which it isn’t with this lens. More information on the focusing distance scale would also be appreciated. The lens isn’t weather-sealed, but at its price point, and given the rest of the lens’ specifications, there is little cause for complaint, especially when you compare it to the Zeiss 21mm Distagon.
The most obvious feature of its design, though, is the large front element and fixed, petal-shaped hood. This is a necessary byproduct of the ultra-fast aperture but it does mean that using filters can be problematic. Fortunately, however, LEE Filters have just released an adaptor for the Sigma, which means that you can use it with their SW150 system.
The most important question is how does this lens perform in the field? Well, there is good news and bad news. First, the good news: at mid-apertures, sharpness across the frame is stunning and overall image quality is very impressive, with good contrast and color. There is some mild distortion, but for an ultra wide lens it performs very, very well. It renders scenes slightly on the cool side – rather reminiscent of the Zeiss 21mm – but if you don’t like the look, this is easily corrected. At mid-apertures, this is a lens that easily matches the Zeiss for quality, and it has the added benefit of fast, quiet and accurate autofocus, should you need it.
As this is a lens which could potentially appeal to astrophotographers, I took it to Iceland with me to see how it performed when photographing the Northern Lights. Lenses that I have previously used on these trips include the Zeiss 21mm f/2.8 Distagon, the Canon 16-35mm f/2.8L II and the Canon 14mm f/2.8L. The first thing I noticed was what a difference the extra two stops makes when looking through the viewfinder. Even in the near-pitch black of the Icelandic night, I was able to see enough to compose accurately through the viewfinder – with other lenses that I have used there has always been an element of guesswork involved. The second benefit was that, thanks to the extra two aperture stops, even with a fairly faint display of the aurora, I was able to keep ISO and shutter speeds down to workable levels.
Unfortunately, we now come to the not-so-good news, which is the image quality at maximum aperture and infinity focus – the settings you need for the night sky. While centre sharpness was still very good, the right hand edge of the image – not just the corners, but also the entire edge – was soft. This was not limited to the extreme edge, but extended nearly halfway to the centre before becoming sharp. Stopping down obviously improves things, but you need to stop down to at least f/4 before the right edge sharpens to acceptable levels and, to be honest, my Canon 16-35 f/4L delivered better results wide open at f/4. While it’s possible that this right-edge softness is unique to the copy of the lens that I tested, I can only comment on what I see with the lens that I’m testing.
Coma – an optical aberration which distorts point light sources such as stars – is another issue which is important to night sky photographers. Coma is certainly present in the lens, but not at a level that would put me off it.
This was a lens that I really wanted to love. If I had been completely happy with its performance wide open, I was planning to buy one to use as my ‘aurora’ lens on future Iceland trips. As it is, I’m still trying to decide what to get to fill this particular hole in my camera bag.
So, the bottom line is, should you buy this lens? Well, for general landscape photography, it’s an excellent choice – in the aperture range you likely want to shoot at, it’s bitingly sharp and with excellent contrast and color. With a street price of around £600, it seems like exceptionally good value. However, if you are considering it for astrophotography, I’d recommend testing a couple of copies if possible and taking some sample shots at maximum aperture to see how the edge performance is.