Sigma 17-40mm F1.8 DC Art Review: Constant f/1.8 APS-C Zoom

The Sigma 17-40mm F1.8 DC Art is a high-end standard zoom for APS-C mirrorless cameras, built around a constant f/1.8 maximum aperture across the whole focal range. It belongs to Sigma's Art line, and it is one of very few zooms of any kind that opens wider than f/2.8 at every focal length. With the usual 1.5x crop factor it behaves like a 25.5-60mm lens in full-frame terms, a flexible reportage-friendly range rather than a conventional 24-70mm equivalent.

The lens is designed to cover the APS-C image circle, so it works natively on APS-C bodies and on full-frame cameras switched into crop mode; the image circle is too small for uncropped 24x36mm capture. Sigma offers it in Sony E, Canon RF, Fujifilm X and L-Mount. Owners of Nikon or Micro Four Thirds systems are not catered for. On the Canon RF version the 1.6x crop factor shifts the equivalent range slightly, to roughly 27.2-64mm.

The defining feature is that f/1.8 remains available at 17mm and at 40mm alike, supported by a circular 11-blade diaphragm. Constant-aperture zooms usually stop at f/2.8 or f/4, so this is unusual. In practice it means exposure settings do not have to change as the focal length changes, hand-held shooting in weak light is far more comfortable, and shallow depth of field and pronounced bokeh are much easier to reach than with a conventional standard zoom. The 11-blade diaphragm produces rounded out-of-focus highlights and gentle transitions between sharp and blurred areas.

The optical formula uses 17 elements in 11 groups, including four SLD glass elements and four aspherical elements. Sigma states that particular attention was paid to correcting axial chromatic aberration and sagittal coma, and applies its Super Multi-Layer coating to control flare and incident light. The manufacturer's claim is prime-lens-class performance. Zooming and focusing are both internal, so the barrel does not change length in use, which helps on a gimbal, and the lens is sealed against dust and splashes.

For its specification the lens is remarkably contained: 535 g, about 11.6 cm long and 7.3 cm in diameter. That is the advantage of designing for APS-C. Sigma's own full-frame lens in this class, the 28-45mm F1.8 DG DN Art, is quoted at 960 g, 15 cm long and 8.9 cm across.

Despite the modest size the control set is generous. There is a manual aperture ring with a lockable automatic position, switchable between one-third-stop clicks and a continuous, de-clicked action by way of a second switch. Two customisable buttons and a manual-focus selector complete the layout. There is no optical stabilisation, and consequently no switch for it.

All three rings are pleasant to use. The zoom ring has a short throw that makes it quick to travel from 17mm to 40mm, though it is arguably a little too free-running. The manual focus ring is wide, comfortable and smoothly damped; because it drives the autofocus motor electronically it has no hard end stops. The front element carries a protective surface treatment. On handling, the lens does not put a foot wrong.

The recorded laboratory measurements were made on a 61-megapixel full-frame body used in its APS-C crop mode, which delivers 6240 x 4160 pixel files, roughly 26 megapixels. Files were captured as fine-quality JPEGs with the camera's automatic lens compensation active, so digital corrections for vignetting, chromatic aberration and distortion were in play. Sharpness was assessed with a dot chart and a blur-unit analysis derived from a low-frequency MTF measurement, which tracks the way the eye perceives detail: the closer the figure sits to zero the sharper the result, with blur becoming perceptible around -1.5, visible around -2.5 and strong around -3.5.

The results for this lens are excellent. Sharpness is very high from the widest aperture and at every focal length, and consistency from the centre to the edges is equally strong. The behaviour is coherent across the range rather than peaking in one narrow configuration, which supports the prime-like claim made for it.

Standard zooms are rarely badly affected by distortion, and in-camera digital correction was active during the measurements, although such corrections tend to be less thorough on a third-party lens than on a native one. Measured average distortion is low: 0.3% at 17mm, 0.25% at 28mm and 0.2% at 40mm. Chromatic aberration, also corrected in camera, is negligible at around 0.03%.

Vignetting is the clearest optical weakness. It is present and visible wide open at f/1.8, most obviously at 17mm, and only disappears completely from f/4 at every focal length. It is straightforward to remove in post-processing, and some photographers deliberately keep it to close the frame and draw the eye towards the centre of the composition.

Autofocus uses Sigma's HLA linear motor, which is quiet, fast and precise. Sigma also states that focus breathing has been suppressed, which matters for video work. Minimum focus distance is 28 cm at all focal lengths, giving a maximum magnification of about 0.2x, or roughly 1:4.8. That is enough for occasional close-ups but not for macro work. There is no optical stabilisation, so hand-held shutter speeds depend on in-body stabilisation and on the f/1.8 aperture.

As a standard zoom the 17-40mm range is genuinely flexible, comfortable for landscape, reportage and portraiture. It is shorter at the long end than the 24-70mm and 24-105mm ranges full-frame users are accustomed to, and 60mm equivalent is the one figure portrait photographers may find limiting, although plenty of photographers work happily at 50mm.

Within its own category the lens has no direct rival: no native Canon, Sony or Fujifilm APS-C zoom matches a constant f/1.8. The nearest alternatives trade aperture for reach, among them the Tamron 17-70mm F2.8 Di III-A VC RXD in Sony E, the Sony E 16-55mm F2.8 G and the Fujifilm XF 16-55mm F2.8 R LM WR II.