Y. Yin
Update: I sent this lens for Sigma to repair it. after about a week, they sent me an estimate $240 to repair the malfunctioning focus. the warranty is 4 years, mine was a few months out of the 4 years warranty. I checked on youtube, fredmiranda, B&H, adorama of course Amazon and other professional photographers discussion forum about this issue. it seems that it is not an isolated issue, quite some other photographer running into this problem. Sigma service department was just deflecting the real issue of quality to something else. but just couldnt explain the fact that out of the 10 other Nikon Lens I had none of them having any issue what so ever, but 2 out of the 3 Sigma lens I owned gave me trouble. Lessons learned: if you shoot photo professionally, and dont want that embarrassing moment that lens fail you in front of client, and ruin your reputation, then dont try to save a few hundred dollars to buy Sigma, because in the end, they are going to bite you at later time. not only an expensive repair bill, but more detrimentally they ruin your business reputation. I am not saying that this lens never do a good job, they could, the real problem is Sigmas notorious quality control issue, you could end up with a good one lasting for a long time, or you could end up a bad one like I did, it is like rolling a dice gambling your luck, if you think you can afford to put your business reputation in line with Sigmas quality, go for it, mine had proven to be a very bad one. today I give Sigma 35mm F1.4 ART Lens a 1 star, No Sigma in my professional use any more in the future. === After extensive test and use, I found this Sigma Art series 35mm lens having significant issue on Focusing, if I set up camera focus to "FOCUS" not "RELEASE + FOCUS", it gave me quite some very awkward moment in front of client. it simply does not focus at all. and I googled it, found many other users reporting similar issue, I would urge Sigma recall the lens and correct it before release product to market.