Vignetting
When imaging a point source with a large amount of defocus, its image has the shape of the exit pupil as seen at that location. If some element of the lens, other than the aperture stop, limits the entering rays from off-axis points, then the pupil has the shape of two intersecting circles. Here, the defocused small lights of a Christmas tree reveal pupil vignetting for ray bundles from those off-axis. With no pupil vignetting, the off-axis images should be circular, or at least slightly elliptical. (Image shot at 1/250 s, ISO 400, with an EF 50mm f/1.4 USM mounted in an EOS M50 camera. The Christmas tree was at roughly 2 m.)
—José A. Díaz Universidad de Granada