When used to photograph a round subject, a fisheye lens does not so-readily show its barrel distortion properties.
This full frame fisheye lens image also shows the normal gradation found in a blue sky - no circular polarizer filters are available for this lens and therefore are not the cause of the uneven blue sky seen here.
Camera and Lens Settings
15mm f/11.0 1/125s
5616 x 3744px