Some animals, including humans, have a special area on their retina called the fovea where there is an enormous concentration of these vision cells. In a human, the fovea has 200,000 cones per millimeter, giving us wonderful vision. That's about the same number of visual cells as the finest computer monitor has on its entire screen when set at its highest resolution. The resolution for a person would be similar to setting a computer's screen at a much lower resolution. This ruler isn't bent.
Light hitting it bends as it passes from air to water.
Notice how light passing from air to water makes this ruler seem bent. This refraction can make it hard for eagles to know exactly where the fish are in the water. The first fish young eagles successfully catch are often dead ones floating right on the surface of the water. They miss live prey a lot when they're first learning to fish. Fortunately, with experience they slowly learn how to correct for refraction.