Average Reviews:
(More customer reviews)(See my update at the bottom of this review.) After buying my Canon xSi I quickly concluded that I was not going to be satisfied unless I had a zoom of some kind to go with it. For a walk around lens, the modest 18-125mm range that this zoom offered, which is moderate wide angle to moderate telephoto, sounded very appealing. I am not all that keen on sports or wildlife photos, so I eschewed the available super zoom lenses, not just for cost but for weight, size, and image quality issues.
I've done pretty extensive tests on this lens, taking many tripod shots out my window in Chicago, trying different apertures and focal lengths, and comparing them to my two Canon lenses, and found it compared favorably. I found the Sigma's photos to be good looking and always in focus.
The OS works pretty well, and I was able to take two perfectly clear hand held shots at full 125 mm telephoto at only 1/15 of a second, though I had to use my best technique, as I suspect at full tele it's not hard to overwhelm the OS. Two more identical shots taken without OS were both blurry.
I've taken a few hundred shots with this lens so far and have no focus problems. Focus is fast and quiet. I took a number of shots of seagulls in flight at 125mm, a pretty challenging focus situation, and every one was in focus.
At 18mm this lens is a tiny bit less sharp than Canon's well-regarded 18-55mm IS kit lens that came with the camera, but as you zoom in the Sigma gets better, particularly at 50mm. (see update below about this)
The telescoping part of the lens is very tightly constructed, with no looseness or feeling of imprecision at all, unlike the Canon kit lens. It has a zoom lock, but this doesn't seem to be necessary as there is no lens creep. I found this lens to be very well built and solid.
This is not a light lens. At about a pound it is twice as heavy as the Canon 18-55mm IS kit lens. Still, some of the superzoom digital lenses by Sigma and Tamron such as the 18-250mm weigh quite a bit more.
Overall I would say this is a very nice lens and I would buy it again. If you want the option to have one lens you can leave on the camera while traveling around, and if you're hooked on wide angle as I am, and if you also like telephoto but don't need a 200 or 250 mm superzoom and all the image quality compromises that can entail, this is the best and only choice for Canon APS-C digital cameras that I know of at this time, and I would recommend it.
UPDATE: After owning this lens for a year I finally figured out that the reason it tended to be soft at the wide (18mm) end was because it was mis-focusing. I am embarrassed that it took me that long to figure it out, but there it was: Point at a distant object and it was focusing at 2m.(The problem quickly disappears above 18mm) I went to the Sigma site, registered my lens's serial number and it said that I was eligible for the additional two years of warranty coverage (as was originally promised in the Amazon product listing), and that I had 22 months left. So far so good. When the came back the paperwork said "Lens not under warranty" but it also said "Adjusted A.F. data courtesy N/C", which meant they fixed it, no charge. I was probably one of the first people to buy this lens, in June 2008, and it sounds like they may have since updated the firmware that controls the autofocus function. In fact the lens now works better, perhaps much better, at 18mm. I can now take shots at F/4.0 and they are often quite sharp, at least in the center. I can now say that the optics in this lens are in fact capable of taking very sharp photos even wide open, but if yours has the same problem mine had you will have to focus manually to achieve this unless you get this "AF data" update. Hopefully the lenses being sold now don't have this problem. More recent reviews than mine seem to indicate that this issue has been resolved.
I would also suggest that Sigma put more resources into their customer service. Being told on the phone and in writing that the lens wasn't under warranty when their website and their printed warranty clearly said that it should be was a bit troubling.
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Sigma 18-125mm f/3.8-5.6 AF DC OS HSM Zoom Lens for Canon Digital SLR CamerasThe lens is equipped with Sigma's own unique OS (Optical Stabilizer) function. This function uses two sensors inside the lens to detect vertical and horizontal movement of the camera and works by moving an optical image stabilizing lens group to effectively compensate for camera shake. This system offers the use of shutter speeds approximately 4 stops slower. It also automatically detects movement of the camera and compensate for camera shake when shooting moving subjects such as motor sports. Its compact construction has an overall length of 88.5mm (3.5"), maximum diameter of 74mm (2.9") and weight of just 450g/15.9oz (Sigma mount). This lens has a minimum focusing distance of 35cm (13.8") at all focal lengths, and has maximum magnification of 1:3.8, making it ideal for close-up photography. SLD (Special Low Dispersion) glass, a molded glass aspherical lens and two hybrid aspherical lenses provide excellent correction for all types of aberrations. The super multi-layer coating reduces flare and ghosting. High image quality is assured throughout the entire zoom range.
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