There are many similarities between the Tamron 17-50mm f/2.8 XR zoom and Sigma’s 18-50mm f/2.8 EX, not least because both use professional-grade optics. Deeper under the skin, they share a seven-blade iris and have almost the same dimensions and mass. But there are also important differences, such as the fact that the manual-focusing ring on the Tamron follows Nikon’s direction convention whereas the Sigma turns the opposite way. The Tamron is also significantly less expensive than the Sigma.

Optical performance is very good. The lens peaks at over 0.35 cycles per pixel in the very useful f/5.6 to f/8 range and, right out to f/22, only drops below the crucial 0.25 figure at the extreme aperture settings of its minimum focal length. Unusually for a fast lens, the Tamron 17-50mm f/2.8 can be stopped right down to f/32 but at this stage the resolution is around 0.2 cycles per pixel, indicating that diffraction has kicked-in to a very significant degree.

The zoom action is just a little stiff and the minimum focusing distance is only 0.27m so, quite properly, there is no suggestion that this might be regarded as a ‘macro’ lens. Interestingly, the front element and filter-mounting ring are significantly smaller than those of Sigma’s equivalent lens, making the Tamron less bulky and, potentially, cheaper to buy filters for. There is slight pincushion distortion at the longest focal length but, despite its smaller diameter, the lens does not seem to have any less light-gathering power.

Despite being proclaimed as an IF lens, the forward-positioned focusing ring rotates while the lens focus is being adjusted and the instructions specifically warn that interfering with the focusing ring may cause serious damage. All of this is a shame because it really can be very difficult to avoid having your fingers resting on the focusing ring. Nevertheless, this is a very well specified and keenly priced lens that deserves serious consideration.