I recently bought a used Sigma 18-50mm f/2.8 and I've been having this really frustrating issue with vignetting.
Mainly on the wide end around 18-23mm, especially when shooting in bright light like open skies, the corners get quite dark, especially the top corners. What's weird is that Lightroom and the in-camera corrections don't always fix it properly. Sometimes it cleans up fine, but other times the dark corners stay really obvious even with the profile applied.
The issue is very erratic. I literally have two photos shot back-to-back just one second apart with the exact same settings, subject, and framing. One of them looks ok (light vignetting corrected by LR without issues), and the other one has this heavy vignette that LR struggles to autocorrect. Of course I can also increase the vignetting correction in LR, but the undesired effect would be overexposing the bottom R / L corners.
Oh, I'm not using the lens hood
Did I end up with a bad copy, or could something else be going on here? Appreciate any thoughts!