Extending the maximum focal length from its more common standard-zoom upper limit of 50mm to 70mm gives this lens a genuinely useful increase in grabbing power. Despite that increase, its overall size and mass are virtually the same as those of Sigma’s 18-50mm f/2.8 lens. That said, the 50mm lens maintains its f/2.8 aperture at all focal lengths whereas the 70mm lens drops towards f/4.5 as its focal length is increased.

As on Sigma’s other standard-zoom lenses, the rings have a well-defined grip but the focusing ring rotates in AF mode so needs to be kept unobstructed for smooth operation. Once again the ‘macro’ label is not fully justified although the 1:2.3 maximum ratio is heading into the right territory.

In most respects the 17-70mm zoom is very similar to the 18-50mm f/2.8 and if placed side-by-side it might be hard to tell them apart. Only the lack of an EX gold rim separates the 70mm zoom.

The technical charts, however, reveal a clear difference. It is unsurprising that the minimum focal length setting produces the worst resolution figures but the difference compared with the other focal lengths examined is more significant than for either of Sigma’s 18-50mm zooms. Peak resolution is also slightly worse (but still very respectable) at around 0.32 cycles per pixel. Overall it is the lack of consistency in the curves that looks slightly alarming but picture-taking quality is good and that is more important than the beauty of the technical test results.

Bearing in mind that it is cheaper than the 18-50mm f/2.8 and offers a wider zoom range with equally good build quality and ergonomics the 17-70mm f/2.8-4.5 looks very attractive but the performance of the 18-50mm f/2.8 just puts it ahead. That said, cost-conscious potential buyers who favour the extra zoom range and can live without the constant f/2.8 may see things differently.