Completely black corners usually mean mechanical vignetting: something is physically blocking part of the lens’s view.

Common causes to check:

• misaligned lens hood, especially a petal hood rotated incorrectly
• wrong hood for the lens
• thick filter rims, especially on wide-angle lenses
• stacked filters, such as a polarizer on top of another filter
• using a lens that doesn’t cover full frame
• something loose inside the lens

A small aperture (high f-number) can make the obstruction appear more sharply defined, so the corners look black instead of just dark.

Since you’ve seen this with multiple lenses, first inspect any hood and filters. Remove all filters and test again, then reattach one at a time. Also confirm the lens hood is the correct one and properly oriented. If the issue remains with no hood or filters attached, check whether the lens is intended for full-frame coverage or may need service.