Sigma's older 20mm f/1.8 wide prime for DSLR — pre-Art era with fast aperture for astrophotography.
The Sigma 20mm f/1.8 EX DG is an older wide-angle prime predating Sigma's Art lineup, offering f/1.8 speed at 20mm for full-frame DSLRs. It was one of the few affordable fast ultra-wide primes available in the early 2000s.
Optically adequate — soft in the corners wide open with noticeable coma, but sharp in the centre from f/4. The fast f/1.8 aperture was useful for astrophotography despite the corner issues. EX designation indicates quality construction. Aspherical elements help control distortion.
Canon EF mount with 82mm filter thread. Approximately 520g. No weather sealing. Full-frame coverage. No stabilisation. Minimum focus distance 20cm — excellent for environmental close-ups. The front element accepts standard 82mm screw-on filters.
Available used very cheaply. The Sigma 20mm f/1.4 Art replaced it with dramatically improved optics, especially coma control. Only worth considering at bargain prices. The corner coma makes it less suitable for astrophotography than the Art successor. Check for smooth focus ring.