Fast Sigma 20mm f/1.8 ultra-wide for Nikon F — popular for astrophotography.
The Sigma 20mm f/1.8 EX DG Aspherical RF is a fast ultra-wide prime for Nikon F mount, offering a dramatic 20mm perspective with an exceptionally fast f/1.8 aperture. The combination of extreme width and fast speed made this lens particularly popular for astrophotography and low-light ultra-wide creative work.
Optical quality is decent with the aspherical element controlling aberrations at wider apertures. Wide open at f/1.8, the lens shows some softness and coma in the corners typical of fast ultra-wide designs of this era. Stopped down to f/2.8-4, performance improves significantly across the frame.
Build quality follows Sigma's EX standard with a robust barrel and smooth focus ring. The lens is large for a 20mm prime, reflecting the optical complexity of achieving f/1.8 at this focal length. The 82mm filter thread accommodates large filters, and the lens includes a petal-type hood.
On the used market, the Sigma 20mm f/1.8 offers fast ultra-wide capability at very affordable prices. While superseded by newer designs with better corner performance, its fast aperture at 20mm remains appealing for astrophotographers and low-light enthusiasts working on a budget.