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Normally, I just move the switch to MF to lock on the subject and that's after focusing with the AF. I'm no sucker for the techy terms but my most qualified review of the photos is -- I am very satisfied with the quality.Downside - the AF motor is very slow. Overall , the image quality is very good -- the sharpness is undeniably good. I could not move the zoom range using MF (i must be doing something wrong. And MF is a bit mystery to me. I could not control the focusing during MF. or the instruction in the manual is not clear to me). The focusing motor moves back and forth most of the time before locking on to the subject -- that's what's making it slow -- so I always end up loosing the opportunity shot and have to wait for the subject to return for the shot.
Shipped on time.Great Product.Excellent price.Will buy from them again and again.
I just open the lens and fix it myself, works fine.However, the story did not end there. I bought this lens in December 2007 and just start to use it from this March. The picture quality is good but needs a lot of light and contrast for the AF to work.In June, during an outing, one of the element (glass) fell out of it's holding socket and the lens would not retract to below 1:3.Since I bought this lens in gray market, so that there's no warranty. Just yesterday, during some test shots at home, the lens would not focus and the motor is trying but could not move. I switch to Manual but the focusing ring would not stay at M mode and I can turn the ring freely and it won't adjust the focus (in AF mode, the ring supposed to have resistence from the motor gear and you should not try to adjust manually).If you are thinking of buying this lens, make sure to buy it with warranty or better yet extended warranty.
It constantly searches no matter what light condition I'm in and it never comes out clear (when using AF). This stays on my camera 90% of the time and with f/2.8 it's still useful indoors. This is a very fun lens with great color and clarity. The auto focus also sticks halfway so I've got to give it the extra push, but to save the hassle it's just better to use manual focus from now on. Although I think the lens I received may have been faulty the only problem is the auto focus. I've had this lens for only about 6 months and the AF is pretty much shot. Like I said, I think I've received a faulty lens, but I have read on quite a few Sigma reviews that their auto focus is not very dependable. I love this lens I just wish I could have the luxury of good auto focus like Canon's USM.
Pros:Very good image qualityVery solid buildVery sharp when used with tripodGood priceCons:Extremely slow autofocus, AI servo mode is uselessExtends front element during focusingSlight CA detected in some imagesNot very useful in handheld mode unless lighting is very good (full sunlight)
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