Sigma 18-50mm Lens

By Ken Parks, September 12, 2014

This little gem was picked up so as to handle a focal length range and speed that was not in my toolbox, the Sigma 18-50mm f/2.8 lens. I had a rectangular zoom 8-18mm, a 18-270mm and yet a bit slow, and a 70-200mm f/2.8 zoom lens.

This lens is strictly for digital APS-C type sensor on DSLR cameras, such as Nikon and Canon. The cost is relatively low for this quality glass, about $395-500 depending on the store. There also exist lens mounts for Minolta, Pentax, and the Sigma DSLR cameras.

Despite the fast lens aperture size of f/2.8, the design is compact in size and weight, which is smaller than the heavier full-frame size lenses. It then becomes a good match for the new technology of small mirror-less digital cameras.

The design is composed of a set of aspherical SLD (Special Low Dispersion) groups of elements. And the 7 aperture blades allows for a finer focus and depth of field (DOF). The lens offers a focus distance of 0.28m (about 12 inches) and a Auto-Focus (AF) micro-motor at an acceptable noise level.

The lens has the same EX (excellent) appearance, the EX crinkle finish and rubberized control rings. I found that the focus is relatively smooth and the zooming a bit on the stiff side, which is typical of EF lenses. The zoom mechanism is tight, not allowing for any inner tube wobble often found on cheap lenses. And zoom creeping has not been an issue in any of my shoots.

At the 18mm mark, there seems to be a slight barrel distortion, and at 35mm and 50mm there is very little of distortion. Vignetting seems to be normal and not much of a concern. However, the weakness of this lens at the f/2.8 setting is the noticeable purple chromatic aberration (fringing) when enlarged to 200% in Photoshop.

The quality that exceeds my expectations is the bokeh (shallow depth of field) which I find quite pleasing especially shooting portraits. When it comes to color, Sigma appears to have improved their color correction coating process.