The eye works by passing light through a lens which is then picked up by receptors on the back of the eye. A shutter called the iris controls the amount of light we pass through to the receptors. Resolution of what we can physically focus on is influenced by two factors directly related to the operation of the iris. First, up to a point, the more light that we allow in to the retina, the better we will be able to stimulate our receptors and discern what we are seeing. The down side to opening up the shutter is that we allow light to enter the eye from a wider area of the lens. The lens is spherical in nature and light entering from the edges is subject to distortions both to the wavelength of the light, and how it is actually bent to be focused on the back of the eye. If we have enough light, that is not so intense that it overloads our receptors, then a smaller opening will generally allow for the best resolution of images. With smaller eye apertures, things in front of and behind the main object we are looking are presented in more detail. This is known as an increased ‘depth of field’.
Not all of the receptors on the back of the eye have the same properties. Those associated with visual acuity are largely concentrated in a small area called the fovea. Even at a very small angle from this area, visual acuity and colour discrimination drop off dramatically. Only about a 10 degree cone is suitable for discerning the details of an object. The areas off to the side of this are more sensitive to motion and are the basis for what we call peripheral vision. As well, the optic nerve is attached to the retina at the back of the eye and to one side. At the point where this nerve is attached, the retina does not have vision receptors resulting in a blind spot.