The dark corners are called vignetting. It’s common, especially with wide-angle lenses used wide open. At large apertures, light reaches the sensor from steeper angles, and parts of the lens barrel, filter stack, or even a lens hood can partially block that light near the edges of the frame. That makes the corners darker.

Stopping the lens down reduces those extreme light angles, so less light gets blocked on its way to the sensor. That’s why the corners usually brighten up at smaller apertures. Many lenses also look sharper when stopped down a bit, which is why your smaller-aperture shots may appear both more even and sharper.

The amount of vignetting varies by lens and focal length, and is often strongest at the widest focal lengths and widest apertures. It can also be made worse by attached filters or stacked filters.

If you want to reduce it, you can:

• stop down the aperture
• avoid filter stacking
• check that the lens hood is correct
• apply lens/vignetting correction in editing software