The iris is a colored annulus (def: a ring-shaped object, structure, or region) behind the cornea but before the lens. The iris contains radial muscles that allow it to change the size of its inner hole, the pupil. Only light passing through the pupil proceeds further into the eye.

Light passes through the pupil then strikes the transparent crystalline lens. The lens is surrounded by muscles called the ciliary body, which can pull at the sides of the lens. When the ciliary muscles are relaxed, the lens is stretched radially, flattening it and reducing its optical power; the light entering the eye is now brought to a focus as far from the lens as possible. The ciliary muscles tense to exert a compressive force on the lens: its diameter shrinks, the lens becomes thicker, the optical power increases, and the focal point moves closer to the lens. Thus when the muscles are relaxed, the lens has its longest focal length and when the muscles tense, the lens is focused on nearer objects.

Light focused by the lens falls on the retina, a thin layering of cells covering about 200° on the back of the eye. The retina contains two types of photosensitive cells: rods and cones. Cones are primarily responsible for color perception; rods are limited to intensity, though they are ten times more sensitive to light than cones. Rods are physically smaller than cones, so they can be packed denser, giving greater spatial resolution. 

There is a small region at the center of the visual axis known as the fovea, which subtends only 1 or 2° of visual angle. The structure of the retina is roughly radially symmetric around the fovea. The fovea contains only cones, and it is here that we find the densest collection of cones on the surface of the retina. Moving outward from the fovea, rods begin to appear among the cones, and at the edge of the fovea there are more rods than cones (see diagram below). Traveling further on a radial path from the fovea, the rods begin to form rings around each increasingly infrequent cone. The highest density of rods appears at about 20° from the fovea. In total, the human eye

contains about 120 million rods and 6 million cones. Since the optic nerve contains only about 1 million fibers, the eye must perform a lot of processing before the visual signal ever reaches the brain. 

There are three types of cones in the human eye, typically called S, M, and L (named respectively for their peak response to relatively short, medium, and long wavelengths), with peaks located at roughly 420, 530, and 560 nm. The response curves for these cones (as well as the rods) are asymmetrical; the drop-off at the high-frequency side is sharper than at the low-frequency side. Thus the shorter wavelengths are more readily absorbed than the longer wavelengths for all three ranges. Both rods and cones may be considered the ultimate in visual sensitivity: a single photon carries enough energy to produce the chemical reactions that change the electrical potential at the cell's membrane, signaling the arrival of light at that cell .

The signal carried by the change in membrane potential makes up the entire message sent by a photoreceptor to the rest of the visual system. Thus the only message sent by a rod or cone is that light has arrived and stimulated the photopigment; there is no information transmitted describing the wavelength of the photon. This effect is called the principle of univariance (The principle of univariance states that an individual receptor cell can be excited by different combinations of wavelength and intensity, and therefore cannot differentiate between a change in wavelength and a change in intensity). The likelihood of absorption of a photon by a particular cell is a function of the spectral sensitivity of the receptor and the intensity of the incoming light (e.g., if the receptor is 30% sensitive at some wavelength, any particular photon may not be absorbed, but about 30 of every 100 will). The time-averaged output of a photoreceptor is related to the number of photons received over some recent interval, but there is no way to determine the frequency distribution of these absorbed photons. It is only by combining the results of many photoreceptors with different spectral sensitivities that the visual system is able to reconstruct intensity and color descriptions of the incoming signal; this reconstruction is believed to happen at a very early stage in visual processing. The principle of univariance may at first seem puzzling. Suppose that the eye contained many distinct color sensors with different, narrowly defined bands of absorption. Although they might be as dose-packed as cones, the number of sensors for any particular frequency band in a fixed region would necessarily be fewer than if only three types of cones occupied the space, thereby sacrificing spatial color resolution. The human eye has evolved with a compromise of three sensors, which gives good color sensor density in the retina and a sufficient amount of color information to recompute the spectral information of the incident signal. Either the number of sensors or their density could be theoretically increased at the expense of the other.

The chemical processes that occur inside a photoreceptor last several milliseconds, and additional photons that strike the receptor during that time add to the overall response. Thus the output of a receptor is really a time-averaged response, an effect called temporal smoothing. In effect, the sensors impose a low-pass filter over their time response, though the cutoff frequency of that filter changes with respect to the background light level: when there is little light arriving, there is little smoothing. The effect of temporal smoothing leads to the way we perceive light that blinks, or flickers. When the blinking is slow, we perceive the individual flashes of light. Above a certain rate, called the critical flicker frequency (or CFF), the flashes fuse together into a single continuous image. Far below that rate we see simply a series of still images, without an objectionable sense of near-continuity. Under the best conditions, the CFF for a human is around 60 Hz [389]. In contrast, a bee has a CFF of about 300 Hz. We note that as with most other visual phenomena, the flicker rate is dependent on many factors.

The electric potential generated by a photoreceptor travels through a series of relay cells, and eventually results in a series of voltage pulses being transmitted along a nerve fibre to the brain. The rates at which these pulses are produced provide the signal modulation- a higher rate indicating a stronger signal, and a lower rate a weaker signal. Zero signal may be indicated by a resting rate, and rates lower than this can then indicate an opposite signal. The pulses themselves are all of the same amplitude, and it is only their frequency that carries information to the brain. The frequencies involved are typically from a few per second to around 400 per second. It might be thought that, as there are four different types of receptor, the rods and the three different types of cone, there would be four different types of signal, transmitted along four different types of nerve fibre, each indicating the strength of the response from one of the four receptor types. However, there is overwhelming evidence that this is not what happens. While much still remains unknown about the way in which the signals are encoded for transmission, the simple scheme shown below can be regarded as a plausible model. The strengths of the signals from the cones are represented by the symbols, ρ , γ ,and β (L, M, S).