system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are lenses round but the camera sensor and resulting image square?

For SLRs the attachable lenses are circular, yet when you look through the camera, as well as the image taken, the shape is rectangular.
gpt
When making lenses for a camera, it's difficult, if not impossible, to achieve a uniform distortion of light throughout a rectangular lens. A circle is very uniform and makes all refraction of light through the lens the same. Rectangular image sensors are also easier and more cost-efficient to make than circular ones, assuming you'd even want a circular sensor.

Basically, it's the cheapest and easiest way to manufacture.