Im not the author of this but it is for sharing
EYE ANATOMY: all birds have a small separation between the anterior (cornea and lens) and posterior (eye ball) sections of the eye. within this separation there is a ring of 12-15 tiny bones. there are 2 striated (long) muscles that attach these bones to each other and they connect the posterior and anterior sections of the eye. these muscles and bones are used to focus on objects. in birds, the cornea and lens change their curvature in focusing by using these muscles and bones...only the lens does this in mammals. this anatomy gives birds a much more accute focusing capability (refractive power) than humans.
the retina and fovea control sharpness and color contrasts. within the retina, cone cells (color absorption) and rod cells (light absorption) exist. nocturnal animals such as deer, walleyes, and whiporwills have a greater number on rod cells for more light absorption for their nocturnal existence. most birds; however, have a greater number of cone cells because they rely on color so much. cone cells allow for sharpening of images no matter where light strikes the retina. sparrows have counts as high as 400,000 cone cells per square millimeter, ducks have approxiamtely 650,000 per sq./mm and eagles are in the neighborhood of 1,000,000 per sq./mm....consequently, humans have about 200,000 per sq./mm.
the fovea is known as a deep depression within the retina. like a ditch. this depression actually allows for interpretation of depth perception and movement. a birb's fovea (ditch) is deeper within the retina than a humans. this ditch acts as an elongated piece of glass for extra power. it acts as a pair of binoculars basically. some birds that are fast flying such as falcons and doves have two foveas for detecting movement and depths at high speeds. (this is where dave's explanation of eye position comes into play). eye position and fovea placement are directly related. the faster flying the bird is, the more forward the eyes are placed on the head for a more binocular vision. the slower the flying the bird is, the further back the eyes are positioned on the head for better peripheral vision. think about an eagle compared to a goose.
COLOR VISION: the presence of cone receptor cells, which contains the visual pigments, suggest that most birds have well-developed color vision. color vision is based on visual pigments, which actually converts the electromagnetic energy of light into neural energy. translation...cone cells turn light absorption into what our brain sees as color contrasts. within these cone cells of birds, color oil droplets are present. these oils act as filters especially to lighter colors. behind this theory lies the answer to the exceptional visibility of whites on darker colors to geese.
the richness of colors and accutness for birds is beyond us as humans, but this is my interpretation of avian vision as explained to me. there is much more to it than this, but this is as simple as i could get.