Although monitors and paper photos are rectangular, lenses are (usually) round.
So why are the images generated by round lenses not round? Well .. images of lenses ARE round.
The diameter of these rounds images is called image or image circle. Outside the image circle the picture (hopefully) is dark. If the image circle is smaller than the diagonal of the sensor, the image has dark corners.
When the Image Circle is smaller than the height of the sensor, the image is round and outside of the image circle it’s dark.
Images of Fisheye lenses are typically like that and are then called “circular fisheye image”. Such image are especially helpful for checking whether the axis of sensor and lens are aligned:
The image circle must be in the middle of the (rectangular) screen image.
Maximum (diameter of the) circle that receives (good quality) image information.
The image circle limits the maximum sensor size for which a lens can be used.
An image circle of 6mm limits the use to maximum sensor to 1/3″.
An image circle of 8mm limits the use to maximum sensor to 1/2″.
An image circle of 16mm limits the use to maximum sensor to 1″.