I am sorry to Answer the Sigma Lens Question, Which I have the Fist Sigma Lens in my Life ( And Seem to get the Best one from The Batch/ From MFG. too)---Yes, Some time, My Friends who have Sigma Lens Complaint about Sigma Lens for Canon have a slow or Miss AF---But May be the Quality control of Their MFG. And more than 5 years ago, One of my Dear Friend from Japan, The Optical Engineer, Who work for Canon, he told me that, Sigma company try to save their money, not to buy the Patent of AF/ EOS from Canon, But They Use Reverse Engineer to Make their Lens work with Canon EOS, Yes Tamron Company Buy the AF / EOS patent from Canon, and have a great AF to Match Canon too----Sorry, That is the Truth or not, Or Just Canon Company try to make me Feel Good
, and Want Me to buy only Canon Lenses.
Sorry again that I start to Make two fan Clubs= Ford and Chevy , try to tell another side that their truck a lot better than another brand name Truck.
Yes, If Your AF. ( for Canon EOS Body) of Sigma Lens are not Perfect, Please try to Adjust the Back Focus/ AF. Microadjustment = C. FN III-8.
Yes, Sigma 50 mm. F/ 1.4 ( $ 460 US Dollars), are not Perfected Lens as Canon EF 50. MM. F/ 1.2 L ( $ 1600 US Dollars), But I need a Good Lens for 50 mm, and Save some money to buy Canon Lens EF 1200 mm ( $ 120,000 US Dollars)---That is my Dream before I die ( my Wife might Kill me---Ha, Ha, Ha ).---JUST KIDDING.
I am very glad to be your friend in CR. Yes, I read most of your Answers and Learn a lot of Things = New infor/ New Ideas from you.
Apex, NC., USA.
Here are the comparision Between Canon 50 mm, F/ 1.4 and Sigma 50 mm f/ 1.4---Just move your mouse to left and right , to see the difference.http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=115&Camera=453&FLI=0&API=0&LensComp=473&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=0