Review of the Sigma 18-50mm f/2.8 EX DC Macro lens
If the f/3.5-5.6 version of this zoom lens seems just too limiting then the constant-aperture f/2.8 will be more appealing. Both lenses share the same matt-black livery with white markings but the f/2.8 version also has a subtle gold ring to indicate its ‘EX’ professional breeding. The mass is almost twice that of the entry-level lens and the filter-ring is now a very substantial 72mm. As a result, the 18-50mm f/2.8 looks and feels much more substantial. It also has a focused-distance scale and a ‘macro’ scale that indicates the maximum reproduction ratio at different focal lengths. Sadly, the maximum ratio achievable is 1:3, which falls some way short of the 1:1 that is required for true macro photography.
The zoom action is a continuous extension as the focal length varies from minimum to maximum but the ring requires slightly more twist than can be provided without changing grip. This is only a minor problem in the small extent to which it slows-down varying the focal length from one extreme to the other.
It is a shame that manual focusing is not possible when the lens is set to AF mode but in other respects the EX lens definitely lives up to its professional specification. Optimum performance comes at f/5.6 and peaks at 0.35 cycles-perpixel when the lens is set to its 35mm focal length position. The resolution figures were above the critical 0.25 cycles-per-pixel value for almost every measurement taken and it is only the 18mm setting at maximum and minimum aperture that falls on the wrong side of the division.
Although chromatic aberrations are always expected to a slight degree it is surprising to see a small but noticeable amount of pincushion distortion. That is not a fatal flaw but it does very slightly take the edge of what would otherwise be a very impressive set of technical results.
Sigma's almost identical 17-70mm f/2.8-4.5 and 18-50mm f/2.8 siblings both look and feel like serious lenses. Neither one is truly a macro lens, despite that specification being in both names, but in other respects the prices are similar enough to allow a buying choice to be made simply by assessing the benefit of a longer zoom range versus a constant f/2.8 maximum aperture.