Image Quality

In the following sections we will use DxO data and real life samples to determine just how the Sigma 85mm F1.4 Art compares to the competition.

Sharpness

   
Sharpness  The Sigma is extremely sharp wide-open, so much so that as you stop the lens down, you actually start to see the affects of diffraction much earlier than we would normally expect. Best sharpness is achieved centrally when the lens is stopped down to around F4 and at F5.6 in the corners. The overall performance in terms of sharpness is impressive to say the least.
APS-C Again, on an APS-C sensor the lens is sharpest when shot at an aperture of F4. Meanwhile, vignetting and distortion are virtually non existent.
Chromatic Aberration (Lateral) chromatic aberration is handled exceptionally well across all apertures, with only a slight decrease in already low levels of CA by stopping the lens down to F4
Vignetting  When shot wide open at F1.4, the lens does exhibit a 1.3 stop decrease in light in the corners due to vignetting. This is nearly eliminated by stopping the lens down to F1.8, and completely eliminated by stopping down to F2
Distortion  Distortion is handled exceptionally well, with essentially no measurable distortion.
Transmission (T-Stop) The lens' F-number is a theoretical value, and the actual light transmission value, known as the T-stop, is always fractionally lower due to light losses within the lens. Lenses with more elements, like a complex zoom, tend to be slightly more affected. The measured T-stop for this lens around 1.8, which suggests the lens is letting through a bit less light than the F1.4 rating appears to imply.

How does the Sigma 85mm DG HSM ART compare to the Sony 85mm GM?

The Sigma is incredibly sharp and ticks all of the boxes in terms of overall lens performance, but how does it stack up against the Sony 85mm GM? When both lenses are shot wide open, the Sigma is not only slightly sharper centrally but also more consistent out to the corners as well. Both lenses are sharpest centrally when stopped down to an aperture of F4, with the Sigma maintaining its edge in both sharpness and consistency. The Sony's corners never get better than at F4, whereas the Sigma's sharpen up a little further at F5.6. Achieving the best sharpness in the corners for the Sigma does come at a slight cost to central sharpness but it remains sharper than the Sony ever gets.

In terms of lateral chromatic aberration (CA) both lenses perform very well across the aperture range with Sigma performing better when shot wide-open. By F4 both lenses have eliminated the already low levels of CA present. Vignetting at F1.4 sees a marked increase in the Sony when compared to the Sigma. The Sony experiences a falloff of 1.9 stops in the corners, whereas the Sigma sees a 1.3 stop falloff. Vignetting is completely eliminated in the Sony by F4, as opposed to F2 in the Sigma.

The Sony has a T-stop rating of 1.5, which is nearly a perfect score as the reported aperture is rated at F1.4. Distortion is handled a bit better by the Sigma as the Sony suffers from a small amount of pincushion distortion, while the Sigma is a near-perfect rectilinear projection.