Wondering If anybody thought of using Sigma 18-35mm F1.8 on Metabones Speed Booster for E-M1? If my numbers are right the Sigma setup translates to 26-52mm F1.3. Seems like valuable proposition to me. I am asking because my return policy for Oly 12-40mm is up in 4 days. Any advice from Oly and Sigma users would be awesome.