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Sigma 18-50mm F2.8 EX DC

Review Date: Dec 18, 2005 Recommend? no | Price paid: $470.00 | Rating: 3 

Pros: Good focal range, lightweight, compact, solid build, sharp center
Two copies couldn't focus, purple fringe/CA excessive wide, extreme edge/corner softness

Oh how I wanted to keep this lens. The perfect walkaround for my XT. Solidly built, not too big or heavy. Fair price. Includes a useful lens hood. A really nice optic.

The flaws compared to the competition (17-40/4L) are softer edges, some objectionable CA/purple fringe at wide angle (which really doesn't improve at all stopped down) and no full time manual focus or USM (although focus is still adequately fast and quiet). These seemed like very fair trades for the lower cost, smaller size, and less weight of this lens.

Unfortunately I was never able to actually enjoy it because it just refused to AF properly. My first copy couldn't really AF on *anything* at any distance. Though there were some acceptable shots at 50mm. If you tried to use AI Servo it would just never settle and hunt wildly. Obviously broken. Fortunately I had purchased from Amazon which has free exchanges.

My second copy snapped decisively to focus for most things and my hopes were raised. But it failed at infinity and focal lengths 28mm and wider. I started to wonder about my camera, but all my other lenses had no problems. I even borrowed a kit lens so I could try identical focal lengths and aperatures. The kit lens nailed infinity focus every time, but the Sigma would randomly end up anywhere in the range from 30ft to well past infinity. The sigma also missed some closer targets the kit lens had no trouble with (all of these shots tripod mounted of course to ensure identical view and focus point).

Indoor tests at short distances (where it could focus reasonable accurately) showed a nice and sharp at center wide open but unsharp corners that really took until past f8 to clear up. I was also suprised at the amount of purple fringes that showed up just on my white and black focus target - this wasn't a way over exposed sky behind a tree or anything, just a piece of paper. Also the second copy that could almost focus showed a slighly less sharp bottom edge of the image compared to the top.

So, if I had ever gotten one of these to AF properly I would have probably given it a 8 at a minimum, probably a 9. If both top and bottom were equally sharp probably a 9 for sure. Never a 10 because of the purple fringing. After two copies couldn't focus though I have to give it 3 for Sigma's horrible QC.