Review of the Sigma 18-50mm DC f:2.8 EX LensThe Sigma 18-50mm f/2.8 lens is an autofocus lens optimized for APS-sized digital sensors that provides an extremely useful range that is 35mm equivalent of 28.8mm to 80mm. With a constant f2.8 maximum aperture, this lens fills a glaring hole in Canon's lens choices for APS sized sensors, such as the Canon 300D, 10D, or 20D. Canon does have an 18-85mm IS lens, however, that only works on EF-S mount cameras however, the Sigma works on all Canon APS-sized digital cameras which support the EF mount. The Sigma lens is particularly appealing not only for its large constant aperture, but also for its very compact, convenient size (3.3 inches long, 15.6 ounces). The lens feels like it's just the right size and weight on my Canon 10D.
The PackageThe Sigma 18-50 comes packaged with a petal-shaped lens hood and a padded soft lens case. The 18-50 lens is made of a fairly attractive plastic which has a velvety matte appearance with tiny sparkles. It looks classy, and feels well-constructed. The lens also has a lock switch that prevents the lens from extending, which is convenient when you are carrying the lens around on your camera. It has a manual focus ring and zoom ring that each rotate smoothly. It also takes a 67mm filter size.
The focusing speed is good. Although not blindingly fast, it is quite reasonable in low light as well as bright light. The lens motor is quieter than my Canon 50mm f/1.8 II, but noisier than my 28-135mm USM IS.
Image QualityThe overall image quality seems to be good to excellent. On the wide angle 18mm end of the zoom range, there is very noticeable barrel distortion, but zoomed to 50mm there is very little.
Performance wide open at f/2.8 is good at the lower range of the zoom, with some softness in the center, slightly more softness in the corners. At the 50mm end, however, it is noticeably softer at f/2.8. At f/4.0 and f/5.6, sharpness is very good. At apertures f/8 through f/16, sharpness is excellent. Below, I show some test images, shooting tripod-mounted, with a cable release (but didn't use mirror lockup- it shouldn't make much of a difference, but, do the test yourself if you're that picky) at ISO 400 on a Canon 10D. No sharpening was done on the images.
100% Crop, at 50mm, f/2.8
Wide open, at 18mm, there is some vignetting (slightly darker in the corners), however, this is probably rarely a problem in most real-world images, and also, it is easily corrected with software. I saw very little purple fringing. Although there is some amount of purple fringing seen in extreme lighting conditions, I found it to be quite well-controlled and not enough of an issue to worry about in most real-world scenes, except when there are extreme bright and dark edge boundaries. The lens seems to give pictures a slightly warmer tone than my Canon 28-135mm IS.
Correcting Lens Deficiencies in Software
Sigma's lack of depth of field markings can also be somewhat fixed by software. There are many depth of field and hyperfocal distance calculators on the web. One example is DOFMaster which lets you print out your own customized depth of field chart for your particular lens and camera (for the Canon 10D, use a circle of confusion size of 0.019, for the Nikon D70, use 0.20).
In the Real WorldIn the real world, the Sigma 18-50 is a great lens. Images were generally excellent. The reasonably light weight of the lens made carrying the 18-50 around a pleasure. Pictures were sharp and contrasty. In low light situations, when you need to shoot wide open at f/2.8, it is noticeably softer.
Because the 18-50 makes a reasonable choice for landscapes, I was extremely disappointed that the lens lacked markings for the depth of field. This was particularly annoying because when I was out in the field and wanted to set the lens to the hyperfocal distance, I could not. This is a glaring oversight, given that this is supposed to be a Sigma "EX" series, top-of-the-line lens.
ConclusionThe Sigma 18-50mm F2.8 EX DC lens is a nice, light weight, bright lens. It serves well as a general-purpose lens, and its bright f/2.8 aperture is wonderful, especially given its compact size and light weight. The lens is sharp at f/8.0 to f/16 throughout its range, very good at f/5.6 and f/22, and usable at f/2.8 though rather soft. Remember, there may be sample variance, so since my tests were done only on one copy of the lens, it may not represent the performance of another unit of the lens. It has a good finish, a nice feel, and its price is reasonable for its performance level. Its most glaring omission is the lack of a depth of field scale.
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