The Sigma 20mm f/1.8 EX DG lens is a wide-angle prime lens designed for full-frame and APS-C digital SLRs. It was announced on December 1, 2000, and remains popular for its combination of good build quality, a fast aperture of f/1.8, and decent optics.

Here are some key features of the lens:

• Focal Length: 20mm
• Maximum Aperture: f/1.8
• Minimum Aperture: f/22
• Minimum Focus Distance: 20cm (7.87 inches)
• Weight: 520g (1.15 lbs)
• Length: 90mm (3.52 inches)
• Lens Mounts: Canon EF, Nikon F (FX), Pentax KAF, Sigma SA Bayonet, Sony/Minolta Alpha.

The lens incorporates a Dual-Focus (DF) mechanism, which makes it easy to hold the lens since the focusing ring does not rotate during auto-focus. It also provides adequate focusing torque during manual focusing. The use of aspherical lens elements in both the front and rear lens groups effectively compensates for distortion, spherical aberration, and astigmatism.

However, the lens has some drawbacks. It is known for having strong vignetting, especially at f/1.8, and the corners can be quite soft at wider apertures. The lens is sharp in the center at f/4 and even sharper at f/5.6

Overall, the Sigma 20mm f/1.8 EX DG lens is a good option for those looking for a wide-angle prime lens with a fast aperture, but it may not be the best choice for those who require edge-to-edge sharpness at wider apertures.