Sigma's early fast ultra-wide — the 20mm f/1.8 EX DG for Nikon F with macro close-focus.
The Sigma 20mm f/1.8 EX DG Aspherical RF for Nikon F is an early fast ultra-wide prime with close-focus capability. At f/1.8 and 20mm, it was one of the fastest ultra-wide primes available. The RF designation indicates rear-focusing. The aspherical element provides wide-angle correction. DG coatings for digital sensors. A pioneering fast ultra-wide specification.
Good for the era — the f/1.8 at 20mm was rare when released. Sharpness respectable. The aspherical element helps at wide apertures. The rear-focus design prevents barrel extension. Close-focus capability enables dramatic wide-angle perspectives. The Sigma 20mm f/1.4 Art is the dramatically improved modern replacement.
Nikon F mount. Filter thread 82mm — large for 20mm. Weight approximately 520 grams. Build quality solid EX professional. HSM motor for quiet AF. No image stabilisation. The 82mm filter adds cost. The Sigma 20mm f/1.4 Art supersedes with dramatically better optics.
Available at very low prices — superseded by the f/1.4 Art. Check AF and optics. The f/1.8 at 20mm was pioneering. The Art version is dramatically better. Only at very low prices. The close-focus capability remains interesting for creative wide-angle work. The 82mm filter is the practical drawback.