Conclusion

The Sigma 20mm F2 DG DN is a a super-sharp, very well built, fast wide-angle prime lens that is pretty affordable, but ultimately it suffers from some significant image quality issues when compared to its main rival, the Sony FE 20mm f/1.8 G.

Its main selling points are undoubtedly its superlative build quality, vintage feel and perfect balance between size and speed.

Following in the footsteps of the other I-series lenses, the Sigma 20mm F2 DG DN is an incredibly well-built lens considering its mid-range price-tag,

From the weather-proof all-metal construction to the incredible attention to detail, such as the beautifully damped aperture and focus rings, ribbed lens hood, redesigned focus switch and the clever, and free, magnetic lens cap, it's clear that Sigma have lavished a huge amount of time and love on this lens.

A traditional aperture ring is a feature that we love on any lens, and Sigma's implementation of it on the 20mm F2 DG DN is nigh on perfect.

It has a lovely action, positive enough movements between the 1/3rd aperture stops, and a pronounced gap between f/22 and the Auto setting so that you can't inadvertently select it.

The only thing that we would have liked to see is a way of de-clicking it for video use, but then this is true of all the I-series lenses that Sigma have released so far.

The stepping motor auto-focus system is virtually silent, satisfyingly quick, and almost 100% reliable.

This lens is a decent enough performer optically, being pin-sharp throughout virtually all of the frame, even when shooting wide-open at f/2. Chromatic aberrations are well controlled and the bokeh is quite appealing for such a wide-angle lens thanks to the 9-blade aperture.

Vignetting is very heavy when shooting wide-open at F2, though, and barrel distortion is all too apparent unless you turn on the in-camera lens distortion controls, which successful corrects the JPEG files (but not the RAWs).

Compared to its main rivals, the Sigma 20mm F2 DG DN is much smaller, lighter and cheaper than its faster big brother, the 20mm F1.4 DG HSM.

There's also the Tamron 20mm F/2.8 Di III OSD M1:2 to consider, although that suffers from a sluggish autofocus system and high levels of distortion.

The real fly-in-the-ointment for this lens, though, is the outstanding Sony FE 20mm f/1.8 G, which crucially offers significantly better image quality with less distortion and vignetting than the new Sigma 20mm F2. The Sony lens is admittedly more expensive (another £$200), but in our view it's worth the extra investment.

If you need a fast 20mm prime lens for your Sony Alpha full-frame camera, then you should still consider the Sigma 20mm F2 DG DN, especially if you're on a budget and you can live with the rather strong distortion and heavy vignetting.

Ratings (out of 5)
Design	4.5
Features	4.5
Ease-of-use	4.5
Image quality	4
Value for money	4