I haven't tested many but Sigma does make some good glass, especially when you factor in the price.
The little 19mm f/2.8 and 30mm f/2.8 Sigma primes I have for the Sony NEX-6 are two little fantastic lenses, affordable and light. Their image quality is pretty much just as good as they are cheaply built; and those things are all made of plastic!
Nonetheless, you should check out the video that Sigma put out about their lens manufacturing. It's quite good and poetic, well worth the 5 minutes. I actually want to buy Sigma now!
Labels: Japan, Lenses, Manufacturing, Marketing, Sigma
