This setup has an unusual peculiarity in that our eyes view things upside down mechanically. That's because the process of refraction via a convex lens flips the picture, resulting in a totally inverted image when it reaches your retina. Your brain then reassembles the image into its proper orientation. The fact that the retina is attached to the back of the eye means that it must be located behind the lens. Thus, when the lens bends light from objects behind it, only certain parts of the image reach the retina. All other parts are blocked. The result is that only certain parts of the image get focused on the retina, while the rest is ignored. This is why you see images flipped upside down on the retina. The human retina contains two types of photoreceptors: rods and cones. Rods are most numerous, making up 95% or more of all retinal cells. They are very sensitive to light but have a narrow focus; thus, they need to be stimulated many times per second for clear vision. Cones account for about 5% of retinal cells. Because they are sensitive to color, they are responsible for our ability to see detail such as shadows and contrasts in colors. Rods are highly sensitive to light but have a narrow focus. Thus, they need to be stimulated many times per second for clear vision.