This shader shows the extent of your fovea on your visual field:
https://www.shadertoy.com/view/4dsXzM
"Look at any specific point and you should clearly see the extent of your fovea (the stars only seem to be rotating in a small circle at the center of your vision). Move your head back and forth to see it grow/shrink. Works better in full screen mode."
(You need to click the link, not look at the static image.)

