The Sigma 20mm f/1.8 EX DG Aspherical RF is an ultra-wide prime for full-frame cameras, featuring an unusually fast f/1.8 aperture at 20mm. RF indicates Retrofocus formula for SLR mirror clearance; EX = Sigma professional grade; Aspherical = aspherical elements for distortion control.
20mm focal length, f/1.8 maximum aperture (fast for ultra-wide), DG (full-frame), EX/RF Aspherical formula, Nikon F mount.
The f/1.8 maximum aperture at 20mm is uncommon among ultra-wide primes and provides excellent low-light capability for interior, astrophotography, and available-light documentary use. Distortion at 20mm requires post-processing correction. Sharpness is good from f/2.8.
Inspect for fungus, haze, and coating condition. Test at f/1.8 for any field curvature or coma at corners. Confirm Nikon F mount engagement. Check aperture blades. Test at f/5.6 for optimal performance.