It's mostly significant when the lens is used wide open, though zooming in does appear to limit its presence.Ĭorner shading isn't a huge factor with the Sigma 17-50mm, producing (at its most notable) extreme corners which are just over a half-stop darker than the center of the frame, when the lens is used at 17mm and ƒ/2.8. The Sigma 17-50mm ƒ/2.8 produces CA of the magenta-green variety, noticeable in areas of high contrast, and predominantly in the extreme corners. The sharpest point for this lens appears to be 50mm at ƒ/8, where performance is essentially tack-sharp across the frame.

These are small numbers, so it's doubtful that you will see a meaningful impact in real-world photography, but it's interesting to note. Zoomed in to 21mm, 28mm or 50mm, corner performance is actually slightly better at these focal lengths, but there is an interesting deviation at 35mm where we see corners similar to those at 17mm. The preceding is actually a good descriptor for performance in the other focal lengths, with a single noteworthy observation. Stopping down to ƒ/16 or ƒ/22 produces light softness across the frame. As the lens is stopped down this corner softness abates gracefully, offering excellent sharpness by ƒ/5.6 (which is actually the best setting: diffraction limiting appears to set in at ƒ/8, noticeable by ƒ/11).

Used wide open at wide angle (17mm, ƒ/2.8) produces the ''worst'' corner softness - but even here it is comparatively light, matched nicely with a generous central area which is very sharp. This corner softness is improved as the lens is stopped down, and there are a few points of tack-sharp image quality. The Sigma 17-50mm ƒ/2.8 OS offers sharp central performance throughout its zoom range, with varying degrees of corner softness.

Nikon lens filters 50mm