Sigma 17-50mm f/2.8 EX DC OS HS M £330/$370
It packs a punch at a penny-pinching price
Both of the Sigma DX-format lenses on test have ultrasonic motor-based autofocus systems. While not as fast as most ring-type ultrasonic systems, and while lacking in full-time manual override, the relatively small motor does enable a compact build. Even so, this is the chunkier of the two Sigma lenses and, at 565g, is 100g heavier than the 17-70mm.
It beats the other Sigma DX lens in terms of having a constant-aperture f/2.8 rating, but falls shorter in maximum telephoto reach. It’s also an older design with an ‘EX’ designation, denoting ‘professional grade’ build quality, although the standard of construction doesn’t feel any more robust than that of the newer 17-70mm lens, and neither have any weather-seals.
Helped by two FLD (Fluoritegrade Low Dispersion) elements, contrast and centre-sharpness are good throughout the zoom range. At wide apertures however, edge and corner sharpness are disappointing. Autofocus speed is no match for that of the Nikon DX lens.