Next, the rim of the pit begins to constrict. This may be a small mutation, or it could just be that the creature is covered by a slime layer anyway. In either case, the jelly or slime helps to hold the shape of the pit, and helps to protect the light sensitive cells from chemical damage. And, the jelly keeps mud out. The aperture continues to decrease.
Visual acuity increases until the aperture gets so small that it begins to shut out too much light.
There will come a point when the aperture is the perfect size. A bigger aperture gives worse eyesight, and a smaller one gives worse eyesight. This is now much like the eye of a nautilus. It can only be improved by adding a lens. To get a lens, one mutation is needed.