Stop down if you want even brightness across your image.
Or, on the other hand, open your lens to the wides stop if you want vignetting – often, for instance in portraits, it is an advantage.
Here is my ceiling, shot with my Canon 7D, with the 35mm lens fully open at f/1.4:
And the same shot at f/2.0, just one stop from fully open:
Can you see how the first one is vignetted, meaning it is darker along the edges? Save both images and flip rapidly between them, if it is not obvious to you here. You’ll see the difference, I promise.
(Yes, and this is on a crop camera. Let’s debunk the myth that this only happens on full-frame cameras.)
So: want vignetting? Then open your aperture all the way. Or want even brightness? Stop down by one or two stops.