The retina contains two forms of photosensitive cells - rods and cones. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells, which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, they do not distinguish between colors, and have low visual acuity (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colors (wavelengths) of light, which allows an organism to see colour.

The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task, which primarily requires looking at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing the individual to observe distant stars.

Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to electromagnetic radiation of a particular wavelength and intensity (i.e. a colour visible light) the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The opsin in both opens ion channels on the cell membrane, which leads to the generation of an action potential (an impulse, which will eventually get to the visual cortex in the brain).

This is the reason why cones and rods enable organisms to see in dark and light conditions.

Colour is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen.

Visual Acuity

Visual acuity (VA) is one of many components of the visual perception sense and is defined as the eye's ability to resolve fine details. VA is a quantitative measure to see an in-focus image at a certain, standarized distance. VA is the most common measurement of visual function that is performed in a clinical setting.

Visual acuity can be measured with several different metrics.

Cycles per degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from angular resolution. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.73 millimeters at a distance of one meter. A horse can resolve about 14 CPD (0.62 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m).

A diopter is the unit of measure of focus.

Many humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees in diameter also meets the definition of legally blind.

Visual acuity is registered documenting if the test was for distant or near vision, the eye evaluated and if corrective lenses (i.e. spectacles or contact lenses) were used:

Distance from the chart

D (distant) for the evaluation done at 20 feet (or 6 meters).

N (near) for the evaluation done at 14 inches (or 35 cm).

Eye evaluated

OD (lat. oculus dexter) for the right eye.

OS (lat. oculus sinister) for the left eye.

OU (lat. oculi uterque) for both eyes.

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