Sigma 18-50/3.5-5.6 DC

Sigma introduced this digital-only lens in the late 2003 and it resembles closely the Canon's 18-55 EF-S kit lens, that is sold with the 300D. Sigma 18-50 however is not an EF-S lens, but it will fit all Canon EF (of course there also Sigma, Nikon and Pentax mount versions) compatible bodies. Image circle of this lens is smaller than normal, as it is intended for cameras that have a crop factor of 1.5x or higher. Therefore you cannot use this lens on a full frame camera without significant vignetting.

This lens will definitely be of interest to many DSLR-owners who are looking for an inexpensive wide angle solution. After seeing one sample photo on a message board at dpreview.com I was curious to find out, how it would perform on my 10D.

This lens is cheap. It costs about 120 euros and besides the metal mount, it's almost completely made of plastic. The general feel is rather solid though, if you keep the price and construction material in mind. It's also very small for such a wide-angle zoom. The exit pupil is very small, as you can see from the picture below.

Zoom ring is nice and stiff and the lens extends when zooming. Manual focus ring is a little bit small, but if you use the supplied lens hood, you can focus the lens by turning the hood. Sigma warns that manual focusing should not be tried out when the focusing switch is set to AF, as it may damage the AF motor. Taking the hood off without turning the focusing ring is very difficult, so watch out there! I think that the hood is small enough to be kept on always. The 18-50 also rotates when focusing, so if you want to use a polarizer you have to reset the polarizer position every time after focusing. AF motor is very loud (but surprisingly fast), even louder than Canon EF 50/1.8. At 50mm the AF seemed to focus a little too near, but that can be my fault as well.

Optics

Optical performance of the lens was better than I had expected from a lens as cheap as this.

Let's look at a sample taken at 18mm. The weather was really depressing here when I performed the test, sorry about that. I'll take better pictures when it's nice and sunny here. =) The highlighted areas are the parts that can be seen in the 100% crop chart below. Crops for the 28 mm and 50 mm are taken from different parts. They were taken in RAW format, and no in-camera sharpening (or either sharpening in post processing) was applied.

There was a clear barrel distortion at wide end.

Some chromatic aberration was visible in the picture. If I overexposed the sky, there was a clear CA (or is that color fringing?) visible. These crops are from the corner of the image and were taken wide open at 18mm. First one shows a properly exposed sky and second one overexposed sky. Amount of CA decreased when the lens was stopped down.

Then it's time for some unfair comparison: The Sigma vs. Canon 28-80/2.8-4L. Both lenses were set at 28mm and the image was taken with f/8. It is notable that the Sigma is clearly wider, although the camera reports focal length as 28mm with both lenses. The Canon produces warmer image although both images were shot with the same white balance setting (cloudy). When viewing the full sized image, the Canon is somewhat sharper.

Sigma 18-50/3.5-5.6 DC

Canon 28-80/2.8-4L

Samples

You can find full sized images of these samples from the link at the end of the page.

18mm, f/5.6, ISO 400, 1/125 s

50mm, f/5.6, ISO 400, 1/90 s

Conclusion

I've used the lens quite little but is seems to perform well for the price. It offers very inexpensive (for the price of a single Canon extension tube) wide angle that is moderately wide with 1.6x crop factor cameras as well. Optical quality is not that bad. If I want to go point'n'shoot, this is the lens I take with me - light, usable focal length of 28.8mm - 80mm and produces pictures of acceptable quality.

Some fullsized pictures can be found from here.