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Well... I don't have the Sigma 50mm 1.4 ART lens, but I do have the Sigma 35mm 1.4 ART lens. That being said BOTH lens have the same performance, except for focal length. The only problem I have had with my 35mm 1.4 is that the auto focus on it isn't 100% accurate so photographing people at 1.4 can be difficult at times. The image quality is second to none in it's price range.
 
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