How Vision Works

Light entering your eye is focused onto the retina. Rods and cones begin converting it into signals, retinal circuits process them, and the optic nerve carries information towards the brain.

Light must reach the retina in focus Vision begins when light reflected from a scene enters the eye. The cornea provides most of the eye's fixed focusing power. Light then travels through the pupil, the opening in the centre of the iris. It passes through the lens and vitreous humour before reaching the retina. The lens changes shape during accommodation, especially when focusing on a nearby object. A later lesson examines this focusing process in more detail. The eye does not send a tiny photograph to the brain. It converts patterns of light into nerve signals that undergo processing in the retina and brain. The iris changes pupil size The iris contains muscles that alter the pupil's size. Bright light usually makes the pupil smaller, while dim light usually makes it larger. Pupil size also changes during near focusing and can be affected by medicines, emotions and nerve activity. It is not a direct measure of eyesight quality. Changing pupil size adjusts the amount of light entering the eye. It cannot protect the retina from every damaging light exposure. The retina is living nerve tissue The retina is a layered sheet of nerve tissue at the back of the eye. Its light sensitive cells are called photoreceptors. Rods are highly sensitive in dim light and contribute strongly to peripheral and motion vision. They do not provide colour vision and give less fine detail. Cones work best in brighter conditions and support colour vision and fine detail. The fovea, at the centre of the macula, has the highest cone density. The macula supports detailed central vision. The wider retina also contributes important peripheral vision and motion information. Retinal cells process the signal before it leaves the eye Phototransduction is the process that converts light energy into a cellular signal inside rods and cones. The signal then passes through retinal circuits. Bipolar cells and other retinal cells help organise contrast and changing light patterns. Ganglion cells produce the nerve impulses that leave the eye. Ganglion cell axons gather at the optic disc and form the optic nerve. The optic disc contains no photoreceptors, creating a normal blind spot. The optic nerve therefore carries processed retinal information, not a direct electrical copy from each rod or cone. The visual pathway continues through the brain The two optic nerves meet at the optic chiasm, where some nerve fibres cross. This arrangement sorts information from the right and left visual fields. The main pathway relays visual information through deeper brain structures before reaching the visual cortex. The visual cortex lies mainly at the back of the brain. Brain networks analyse shape, contrast, colour and movement. Conscious visual perception depends on this continuing processing rather than one single visual centre. Binocular vision uses overlapping information from both eyes. Small differences between the two views support depth perception, alongside several other visual cues. Stable vision requires coordinated eye and brain activity Your eyes make rapid movements called saccades to place important details on the fovea. Other eye movements keep targets aligned or stable during head movement. Visual sensitivity reduces around some rapid eye movements, and blinks briefly interrupt incoming light. The brain combines information across time, so these interruptions are often not noticed. This process is more complex than the brain simply inventing every missing detail. Attention, eye movements and several sensory signals all contribute to visual stability. Eye examinations answer different questions Visual acuity measures the ability to resolve fine detail at a set distance. Each eye is tested separately, usually with the person's normal correction. Acuity does not measure the whole visual field, contrast, colour or every retinal and optic nerve function. A normal chart result cannot exclude all eye disease. The pupillary light reflex checks light input through the retina and optic nerve and output through nerves controlling the iris. Both pupils are observed because one light normally constricts both. An unusual pupil response can have several causes. It does not locate the problem or provide a complete diagnosis by itself. A confrontation visual field test is a bedside screening check. Formal perimetry maps central and peripheral vision more carefully when needed. A visual field defect can arise in the retina, optic nerve, optic chiasm or brain. Its pattern provides a clue but does not establish the cause alone. Ophthalmoscopy, also called fundoscopy, allows looking at the retina, retinal blood vessels, macula and optic disc. It does not show the entire optic nerve or directly measure retinal function. A dilated eye examination usually gives a wider retinal view. Retinal photographs, scans or other tests may answer additional questions. Visual symptoms need proportionate assessment New floaters and flashes are often caused by age related change in the vitreous humour. They can also occur with a retinal tear or retinal detachment. Get urgent help from NHS 111 if floaters suddenly appear or increase, you see flashes, or a dark curtain or shadow crosses your vision. Sudden blurring also needs urgent advice. Go to A&E or call 999 if you suddenly cannot see from one or both eyes or develop sudden severe eye pain. Do not drive yourself. Gradual blurring or distortion still deserves an eye examination. It can have several causes and should not automatically be dismissed as normal ageing.

The eye does not send a finished photograph to the brain. Optical structures focus light, retinal circuits process it, and brain networks create conscious visual perception.