Sigma 30mm F1.4 DC HSM "Art"
Vignetting, also known as light falloff, can be observed when the corners of the image are noticeably darker than the remainder of the frame.
After testing the Sigma 30mm F1.4 Art in our studio, we found vignetting to only be prominent wide-open. Once the lens is stopped-down, the vignetting quickly fades, and it becomes negligible by F2.8. The table below shows sample images taken at various aperture settings; the rightmost column shows the difference in luminance between the center and upper-left corner of each frame.
The 30mm exceeded our expectations when it came to vignetting, as stopping down to F2 or smaller renders corrections virtually unnecessary in practice. We do wish that in-camera corrections were available when shooting wide-open, however, as at F1.4 the vignetting can be overwhelming when left untouched. As with distortion corrections, Pentax cameras only support in-camera falloff corrections for genuine Pentax lenses, so the vignetting cannot be corrected in-camera when shooting with this lens.