June 13 2008

Preliminary Comments:
*The zoom tension was VERY "sticky" early on, but since has loosened up and is ACCEPTABLE.
*The lens LACKS FULL-TIME MANUAL focus even though it has "HSM" autofocus. But the AF is NOT particularly quick, though it is quieter than Sigma's normal gear-driven units. I'd say the AF speed and noise is very similar to the Canon 50 f1.4's micro USM unit, maybe a hair slower. It certainly is not as good as Sigma's ring HSM or Canon's ring USM. I REALLY wish it had FTM.
*The lens has a very strange issue. It seems to engage the OS feature MORE THAN IT SHOULD. You hear a "faint radio white noise" MUCH OF THE TIME, even when you are not using the OS. It will turn off after about 45 seconds of non-use, but once you AF, it goes on again. When I halfpress the shutter, the OS engages (as it should). But when I release the shutter button, instead of the usual 2 second shut-off delay, it just continues for about 45 seconds. Looking through the VF, you can see the OS trying to still stabilize the image, even long after you've released the shutter. It *must* be draining the batteries! See this DPR thread showing others experiencing the same thing (read through all the posts thereafter) http://forums.dpreview.com/forums/read.asp?forum=1029&message=28250678 .
*The OS mechanism is VERY effective. I'm getting 4 stops (1/13th shutter speed) easily and 5 stops (1/6th shutter speed) is certainly attainable for reasonably sharp images.

Focal length to maximum aperture is as follows:
18 - 24mm = f4
25 - 40mm = f4.5
41 - 70mm = f5
71 - 125mm = f5.6

Closing Comments:
The 18-125 OS DC HSM is a VERY NICE walkabout lens. The focal range is EXCELLENT (certainly better than MOST of its competition). Overall the imaging quality is GOOD TO VERY GOOD, though it does exhibit some strong light falloff at the wide end with noticeable barrel distortion as well --- not surprising though for a zoom of this type. Flare control is EXCELLENT (similar to the 18-200 OS), and CA's are not a major problem at all. The lens delivers GOOD CONTRAST reproduction and images can *pop* with even the most basic of post-processing. The micro HSM AF is decent; nothing to write home about. It certainly is NOT as quick as its ring HSM sibling. I do wish it had FTM focusing. The size and handling and build is VERY NICE and ergonomically well-designed. SOLID FEEL to the build, and the focus and zoom rings have a GRIPPY, tactile feel that makes handling and focusing and zooming very efficient. Its build-feel EASILY SURPASSES the EF-S grade of Canon. Weighing in at about 18oz, the lens has some "heft" to it but is certainly NOT HEAVY. The OS unit is HIGHLY EFFECTIVE and stabilizes very smoothly in the VF. The only issue at this point is the strange continuous buzzing (read above comments). It doesn't seem to affect overall performance, but it seems to me that Sigma should address it. Battery life, I would think, is being affected, at the very least. I also wonder if it will deteriorate the OS mechanism at a faster pace.

So how do I rate it against its competition? I think as a general, walkabout, all-purpose lens, especially for travel and wanting to go lightweight and inconspicuous, this lens is definitely NEAR THE TOP of the list. I still, after much usage and thought, give the TOP CHOICE to the Sigma 18-200 OS for its TOTAL CONVENIENCE with similar imaging quality (even their MTF charts are similar). I don't find a meaningful difference in AF speed between them, and while the 18-200 OS has a little more AF noise, it isn't objectionable. The 18-200 OS gives you an extra 75mm at the long end, and is also THE FASTER lens when you look at their focal length to max aperture breakdown. Both are significantly faster than the 1785 IS. Another good combo is the Canon 18-55 IS + 55-250 IS for lightweight going-about. But they DON'T offer the overall convenience of the singular 18-200 OS. After using all these combinations (and still owning the 55-250 IS), I still think the 18-200 OS is THE BEST ALL-IN-ONE lens for APS-C currently. There are times where I'd much rather take the 18-200 OS (which I currently do not own) over the 17-55 f2.8 IS (which I do currently own). But the 18-125 OS finds itself VERY NEAR THE TOP for a nice APS-C walkabout lens. Again, EVERY LENS CHOICE INVOLVES SOME SORT OF COMPROMISE, so it boils down to what YOUR particular needs/priorities are, and choosing based on those things. In conclusion, I have no reservations RECOMMENDING the new 18-125 OS DC HSM if the focal coverage is what you want, with OS, and in a relatively small and light package.

NOTE: Here are my 18-200 OS tests, comparisons, and images:
http://www.pbase.com/lightrules/1785isv18200os
http://www.pbase.com/lightrules/1os2is
http://www.pbase.com/lightrules/zoomzoom
http://www.pbase.com/lightrules/norcalpics

Sigma Link http://sigmaphoto.com/lenses/lenses_all_details.asp?id=3342&navigator=6