Sigma 18-200mm f/3.5-6.3 DC lens is one of Sigma's DC lenses specifically designed for APS-C type dSLR cameras and has a reduced imaging circle, which means it will not be usable on full-frame cameras (the lens will actually fit the full-frame camera, but will vignette heavily because of the smaller imaging circle). Released in early 2005, the lens retails at around US$300 (as of March 2007), and is quite affordable for main-stream consumers. The field of view of this lens resembles that of a 29-320mm lens on a full-frame camera.
Sigma 18-200mm f/3.5-6.3 DC is an interesting lens in sense that it is covers one of the longest zoom ranges among modern SLR lenses. To achieve such an extreme focal length, the lens uses two inner-zoom tubes, which extend during zooming. The lens is constructed of 15 elements in 13 groups and includes 2 Special Low Dispersion (SLD) glass elements and two hybrid aspherical lenses designed to correct chromatic aberration. The lens is surprisingly compact and lightweight, weighing only 405g (14.4oz) and measuring 70x78.1mm (2.8x3.1in) at 18mm focal length, but extends significantly towards the longer zoom range. The lens delivers a mind-boggling 11x zoom, has a minimum aperture of f22, maximum magnification of 1:4.4 and accepts 62mm filters (the lens incorporates an inner focusing system, which prevents front elements from rotating so using a circular polarizer is possible).
The build quality of the lens is surprisingly good (at least for a budget level lens) - the overall finish resembles that of a Signa's EX professional grade line of lenses. Inner tubes wobble a bit but otherwise the lens looks and feels very solid. Rubberized zoom and focus rings are smooth and the lens even has a lock switch which will prevent the lens from accidentally extending during transportation. Auto-focus is reasonably fast, but is a bit noisy. The lens also supports full-time manual focusing.
The factory box includes Sigma 18-200mm f/3.5-6.3 DC lens, front and rear caps, lens hood, manual and warranty card. Sigma manufactures the lens for Canon, Nikon, Pentax and Sony mounts.
|Lens Composition||15 elements in 13 groups|
|Angular Field||69-7 degrees|
|f-stop Scale||f/3.5-f/22, camera-controlled|
|Lens Hood||Petal shaped (included)
Sigma 18-200mm f/3.5-6.3 DC was average in the field. Vignetting was mostly under control except at 18mm, where you might want to stop down to achieve best results. Chromatic aberration persisted throughout the zoom range primarily around borders. The lens produced best results in the 18-50mm focal length range. Both center and borders in this zoom range were reasonably sharp – quite a surprise for me. From 50mm to the telephoto side of the zoom things started to fall apart, with borders suffering most. At 200mm the lens was noticeably soft both around borders and in the center and stopping down did not seem to help much, which was quite disappointing.
As with most lenses with wide angle of view, Sigma produced quite noticeable barrel distortion around 18mm. Artifacts started to disappear at 35mm and up. The biggest disappointment however was the auto-focus, which was consistently hunting, especially around 200mm (and not only in low light conditions). Another disappointment was with the extendable barrel – the dual cams would collapse under their own weight whenever I pointed the lens up (any angle over 60 degree would do the trick). Mind this issue whenever shooting under an angle.
Finally, throughout my tests I noticed that the lens would produce images that are somewhat warmer then usual – not that I would complain about it, but some users might find it less acceptable. On a positive side, the extreme zoom coverage of the lens was quite useful in some situations and I did not have to change lenses as often in order to go from a wide, landscape-type range, to telephoto, portrait-type photography. One of these days lens manufacturers will finally figure out how to make lensed that cover everything from ultra wide to super tepephoto and are also sharp throughout the supported focal length. But until that day comes I’d be using lenses with more conservative zoom to achieve better results.
The lens showed mixed results in the lab. Center was very sharp from 18mm to 50mm, bu then started to deteriorate rather quickly and at the telephoto side of the zoom was less then satisfactory. Unfortunately, quality of borders suffered throughout the zoom range but at 200mm were absolutely dismal. You would get best results between 18-35mm, where the lens is sharpest stopped down to f/8 or f/11. Here you could get very good quality 11in and decent 16in prints. Conclusion? Not easy to sum in a single sentence - decent performance at the wide zoom and below average quality at the telephoto side. The lens might be OK as your first lens but it comes short in image quality to be seriously considered by prosumers and professionals.
Interestingly enough, Sigma 18-200mm f/4.5-6.3 had higher then average degree of chromatic aberration at the telephoto range (~2 pixels), whereby 18mm range showed very mild CA (less then 1 pixel). Stopping down did not really help much unfortunately, and chromatic aberration persisted (to some degree) throughout the aperture range.
Lately most lens manufacturers took the habit of trying to develop lenses with extreme coverage. Some may argue that such extreme zoom lenses are targeted at first-time dSLR buyers who might be willing to invest initially only in one lens before they become comfortable with the new system. Others may argue that the whole segment exists because some customers are more price sensitive and are more likely to buy a single lens that can serve all their needs. Whatever the reason might be, pretty much every reputable lens manufacturer now has at least one lens with such an extreme focal length coverage in their portfolio (with Tamron having the widest selection). The problem is that all such lenses are rather average (image quality-wise), so the alternatives mentioned below are not necessarily much better then Sigma 18-200mm f/3.5-6.3 DC reviewed here. But if you are curious what else is available, then I would suggest taking a look at Canon EF 28-200mm f/3.5-5.6 USM (review) lens, which has a slightly more conservative zoom range and is also slightly faster. Since this is a regular EF lens, you will loose the wide aspects of the lens when shooting on APS-C ype cameras. Another lens that might interest you is Tamron AF 18-200mm f/3.5-6.3 XR Di II (review) or Tamron AF 18-250mm f/3.5-6.3 XR Di II LD Aspherical IF (review), both of which are specifically designed for dSLR cameras with 1.6x crop sensor. Another Tamron lens you might want to consider is AF 28-300mm f/3.5-6.3 XR Di LD Aspherical IF (review).
There's nothing special about Sigma 18-200mm f/3.5-6.3 DC lens that would make you love or hate it. It's just an average lens, with an appeal to mainstream consumers looking for a 'walkaround' lens with an extreme focal length coverage. As such it does not really excel in anything, but that might be OK too, especially once we consider its competitive price. So, if you cannot (or don't want to) buy dedicated wide and telephoto zoom lenses (with more conservative zoom range that is), then this lens might be a viable choice. For everyone else, there are better alternatives available on the market.