Visual acuity measured with the Dyop test varies significantly from ETDRS results under mesopic and blur conditions, ...

Most of the surface is a sheet of light blue, and you can't see through it, even though the water is clear. But right above you, there's a round window of transparency.

The circle of confusion is the blur circle formed when a point source of light, such as a distant star, is out of focus. The phenomenon, which appears as a small circle on the image plane, can be ...