The Corners of the Frame
A lens does its worst work at the four places nobody looks, which is exactly where a wallpaper gets seen longest.
Light entering a lens travels farther to reach the corner of the sensor than the centre, and it arrives at an angle. That extra distance costs brightness, roughly following the fourth power of the cosine of that angle, which is why almost every image is a little darker at the edges before anyone touches it. Stopping down from wide open recovers most of it, and the rest is usually corrected in software without being mentioned.
Sharpness falls off the same way, though less politely. A corner can hold detail that the centre would call soft and still look fine, because the eye spends its time in the middle and only glances outward. Photographers argue about corner performance the way drivers argue about top speed, and then crop the corners off anyway.
A wallpaper reverses the arrangement. The middle of the picture spends its life under a window, and the parts left visible are the strips along the edges and the four corners nobody photographed for. What is out there decides how the desktop feels, which makes vignetting less a defect than a frame drawn in light.
