Na boa, os problemas da Sigma vão além de problema de AF, a maioria é problema mecânico:
http://www.lensrentals.com/news/2008.09.12/the-sigma-sagaAs lentes Sigma são as que eles menos alugam, e as que mais quebram:
http://www.lensrentals.com/news/2008.11.30/lens-repair-data-20When we looked at our actual numbers things were even more bleak: Sigma lenses failed at a rate of 30% per year, compared to less than 5% for Canon, Tamron, Nikon, Tokina, and Zeiss.Lens Annualized Repair Rate Typical Problems
Sigma 120-300 f2.8 84.6% Zoom mechanism, calibration, autofocus
Sigma 150-500 OS 45.5% OS, Autofocus, zoom
Sigma 120-400 OS 44.4% OS, Autofocus, zoom
Sigma 50-500 28.3% Zoom mechanism, autofocus
Sigma 100-300 f4 22.2% autofocus
Tamron 70-200 f2.8 22.2% tight mount (Canon), autofocus
Canon 17-55 f2.8 IS EF-S 19.6% IS, Err99
Canon 10-22 EF-S 15.8% barrel separation
Nikon 70-200 f2.8 VR 13.1% zoom mechanism, manual focus clutch
Sigma 18-200 12.7% barrel separation
Nikon 17-35 f2.8 12.5% calibration
Canon 50 f1.2 13.3% Calibration
Sigma 30 f1.4 12.3% calibration
Canon 100-400 IS 12.3% Zoom tension ring, Err99, calibration
Nikon 80-400 11.9% Electronic issues
Canon 85 f1.2 11.7% Electronic issues