Shooting with the 61MP Sony A7R IV gives the resolving power of the Sigma 20mm F2 DG DN | Contemporary a good test – more than the 47MP Foveon sensor that Sigma states it uses to assess lens’ performance can. However, the 20mm F2 DG DN | C does a great job and there’s an excellent level of detail visible in my images.
The level of sharpness right across the frame is very impressive from wide-open (f/2) to about f/11. However, at f/16 the sharpness of the corners reduces a little and if you pixel-peep images that were shot at f/22, you’ll find the corners look rather soft. However, thanks to the short focal length there’s plenty of depth of field at wider apertures. For example, an aperture of f/8 and focusing at the hyperlocal distance of 1.69m, delivers an image that is sharp from 0.92m to infinity. Using an aperture of f/11, the closest point of sharp focus is 0.75m and the sharpness extends to infinity. It means that there shouldn’t be many occasions when you need to close down beyond f/11.
Nevertheless, the 20mm F2 DG DN | C has a closest focusing point of 22cm, and if you approach that at the wider apertures, it’s possible to blur the background significantly. And if you do so, you’ll find that the bokeh is very attractive with rounded rather than triangular small highlights.
I noticed very strong vignetting in the viewfinder when shooting at the widest apertures, however, the in-camera optical corrections or the correction profiles in Adobe Camera Raw (or other processing software) remove it. In fact, they are a little to zealous because they can make the corners look slightly too bright and I’m inclined to darken them a little at the processing stage.
The correction profiles, either in camera or in software, also do a great job of correcting the barrel-like bowing of straight lines that are seen without a profile.
I spent a lot of time using the 20mm F2 DG DN | C in bright sunshine and try to capture flare. However, even with the light passing over the front of the lens, it does a brilliant job of resisting it. Clearly, Sigma’s coatings are very effective.
Also, after scouring my images, I’ve concluded that the lens controls chromatic aberration very well.
The Sony A7R IV has an excellent autofocus system and as a result, most subjects I photographed were rendered sharp very quickly. As is often the case, it’s only when the subject is close to the nearest focusing point that you start to see some hunting. That’s when it would be nice to have a focus limiter switch. I was still usually able to get the subject sharp by turning towards something a little further away to get the camera and lens in the right zone, before reframing to focus on my intended subject.
Alternatively, it’s possible to focus manually and with the camera’s MF Assist option activated, the image is magnified as soon as the focus ring rotates. The camera also shows a distance scale in the viewfinder or on the screen to help guide the direction of the adjustment.
When the focus is adjusted from infinity to the closest focusing point, the view in the viewfinder changes as if the lens has zoomed in slightly. This effect is known as focus breathing and it could be problematic in videos with a subject moving towards or away from the camera. It is also likely to cause problems for anyone focusing stacking, but again, the extensive depth of field offered by the lens should make that unnecessary.