The human eye functions by focusing light onto a layer of photoreceptor cells called the retina, which forms the inner layer of the back of the eye. A series of transparent medium is required to complete the focusing. Light entering the eye first passes through the cornea, which provides most of the eye's optical power. The light then continues through the fluid just behind the cornea - the anterior chamber then passes through the pupil. The light then passes through the lens, which further focuses the light and allows adjustment of focus. The light then passes through the main body of fluid in the eye – the vitreous humor and reaches the retina. Cells in the retina line the back of the eye, except where the optic nerve emerges; which causes blind spots.
There are two types of photoreceptor cells, rods, and cones, which are sensitive to different aspects of light. Rod cells are sensitive to light intensity over a wide frequency range, thus responsible for black-and-white vision. Rod cells are not present on the fovea, the area of the retina responsible for central vision, and are not as responsive to spatial and temporal changes in light as cone cells. However, the retina contains twenty times more rod cells than cone cells because rod cells are present over a wider area. Due to their wide distribution, rods are responsible for peripheral vision. Many custom optical prism manufacturer uses precision optics manufacturing process to manufacture custom lenses.
Conversely, cone cells are less sensitive to the overall intensity of light but come in three varieties that are sensitive to different frequency ranges and are thus used in the perception of color and photopic vision. Cone cells are highly concentrated in the fovea and have high visual acuity which means they are better at spatial resolution than rod cells. Since cone cells are not as sensitive to dim light as rod cells, most night vision is confined to rod cells. Similarly, since cone cells are in the fovea, central vision (including the vision required for maximum reading, fine detail tasks such as sewing, or careful examination of objects) is carried out by cone cells. The ciliary muscles around the lens allow the focus of the eye to be adjusted. Near point and far point define the nearest and farthest distance from the eye at which an object can be brought into sharp focus. For a person with normal vision, the far point is at infinity.