Appears to be a misidentified Sigma APS-C zoom — likely the 17-50mm f/2.8 or similar, not f/1.4.
The listing 'Sigma 17-40mm f/1.4 DC' appears to contain a data error — no f/1.4 zoom exists at this focal range from Sigma. This is most likely a misidentified Sigma 17-50mm f/2.8 EX DC OS HSM or possibly the Sigma 17-70mm f/2.8-4 DC Macro OS HSM. The DC designation correctly indicates an APS-C lens. Enrichment is provided based on the most likely identification.
If this is the Sigma 17-50mm f/2.8 EX DC OS HSM, optical performance is excellent for an APS-C standard zoom — widely regarded as one of the best third-party APS-C zooms ever made. Centre sharpness is outstanding from f/2.8, and OS stabilisation provides effective handheld shooting. The constant f/2.8 aperture is a significant advantage over kit zooms.
Available in multiple mounts including Canon EF-S, Nikon F DX, Sony A, and Pentax K. Filter thread is 77mm on the 17-50mm version. Weight is approximately 565 grams. Build quality is solid with the EX professional designation. HSM motor provides fast, quiet autofocus. The OS stabilisation is effective at around 4 stops of correction.
Common and affordable on the used market. Check for AF accuracy and OS motor function. If this is indeed the 17-50mm f/2.8, it is one of the strongest value propositions in APS-C zoom lenses — delivering constant f/2.8 performance with stabilisation at a very competitive price. Verify the actual model before purchase.