Design and features
Photographers looking for a versatile walk-around lens often gravitate toward the wide-angle to telephoto models that all the major camera brands make. For something that offers a little more length in a cheaper package, third-party vendors like Sigma are here. This lens is the far-reaching 18-250mm f/3.5-6.3 DC Macro OS HSM, which is quite the mouthful of a name. It's the first Sigma lens to use a brand new material called Thermally Stable Composite (TSC), which helps reduce any expansion or contraction when the temperature changes.
The lens offers an approximate reach of 13x optical zoom. It remains very compact despite its focal length, and is actually smaller than the Canon 18-135mm, which covers a shorter range. At just 470 grams it won't overwhelm any of the crop-sensor SLRs it's designed for. We conducted our testing using the Canon EOS 600D, though it also comes in Sony, Nikon or Pentax mounts. Image stabilisation is available in the lens for the Canon and Nikon mounts, offering four stops of compensation.
The Sigma (right) alongside the Canon 18-135mm lens (left), which is actually a fair bit longer in physical length.
As hinted by the name of the lens, the 18-250mm is also a competent macro performer. It offers approximately a 1:2 magnification ratio, with a minimum focusing distance of 35cm. The lens uses a 62mm filter thread, and has three switches on the body: one to turn on image stabilisation, one for switching between auto and manual focus and one for locking to prevent lens creep. A petal-style lens hood is provided in the box.
There are two rings on the lens, one to zoom and one to focus. When in automatic focus (AF) mode on the lens switch, you cannot rotate the focus ring in order to override with manual focus.
The Sigma is a very sharp third-party lens, and exhibits this throughout its focal length. It doesn't compare to the sharpness delivered from a prime lens, naturally, but it is impressive, given that these sorts of zooms tend to deliver results that fall on the soft side. This sharpness is maintained across the frame, and is the most evident when shooting between the aperture range of f/4-f/11.
There's a small degree of chromatic aberration, or fringing, evident on high-contrast areas, but this is only noticeable when looking at shots at 100 per cent magnification and can be reduced in post-processing.
Here's an example of the two extreme focal lengths on the Sigma 18-250mm.
Bokeh, or the out-of-focus areas of light in the image, is particularly pleasing. The 7 blade design produces smooth-looking and mostly circular bokeh. Colour rendition is very good, with accurate reproduction of natural scenes.
The bokeh in this shot (particularly the red circles from the car's brake lights) is nice and smooth, with a rounded characteristic at f/5.6.
Autofocus is swift and accurate, though this performance will also depend on the camera body used with the lens. Some barrel distortion is evident at the wide end, but can be corrected in post-processing.
Despite its effective image stabilisation, the 18-250mm performs best when it is given lots of light. On rare occasions, we found that we couldn't get a steady shot, even with shutter speeds of around 1/125 in less than bright situations.
Exposure: 1/320, f/11, ISO 400
Exposure: 1/500, f/9, ISO 400
Photographers often have to compromise when it comes to third-party lenses, particularly ones that cover such extreme focal lengths as the 18-250mm. This Sigma model is excellent value for money, offering good sharpness and performance, with a lengthy zoom.