Why is one eye brighter than the other?

It helps to separate two things that are described in the same way. The first is discomfort: light feels too strong, harsh, or even painful in one eye, which is what doctors call photophobia. The second is a difference in appearance between the eyes: the image through one eye simply looks brighter, sharper, or more colourful than through the other, even in ordinary light. The causes of these two are quite different, so knowing which one you are describing is the sensible place to begin.

If the brightness is really discomfort, with light feeling too strong in one eye, the explanation usually falls into one of a few groups.

The first is a larger pupil on one side. The pupil controls how much light enters the eye, so if one pupil is bigger than the other, that eye admits more light and can feel dazzled or uncomfortable in bright conditions. A difference in pupil size is worth understanding in its own right, and is covered on its own page.

The second is a problem on the surface of the eye. The cornea is the clear front window of the eye, and if its surface is disturbed by a scratch, an infection, or inflammation, that eye becomes sensitive to light and is usually red and uncomfortable as well.

The third is inflammation inside the eye, known as uveitis. Light sensitivity is one of its hallmarks, typically alongside a red, aching eye and sometimes blurred vision. Uveitis needs prompt ophthalmic assessment.

If instead you mean that the vision through one eye looks brighter or more vivid than the other, the way to think about it is usually the reverse of how it feels. It is rarely that one eye has become too bright. It is that the other eye has quietly lost a little of its brightness, colour, or contrast, so the normal eye now looks brighter by comparison.

When an eye loses brightness and colour in this way while looking completely normal from the outside, the first thing an ophthalmologist wants to rule out is a problem with the optic nerve, the cable that carries the signal from that eye to the brain. Optic nerve conditions of any kind, including glaucoma, optic neuritis (inflammation of the nerve), and a reduced blood supply to the nerve, all weaken that signal. The affected eye sees a little dimmer, colours look washed out or less saturated, red in particular, and contrast falls away.

The telling sign is one an examiner can check in seconds with a torch, called a relative afferent pupillary defect, or RAPD. Because the affected nerve carries the message about light less efficiently, the pupil of that eye reacts less briskly when the light is moved onto it. It is painless, it takes only moments, and it is exactly what is looked for when someone reports that one eye no longer matches the other for brightness or colour. Because glaucoma can be silent until it is advanced, and the other causes need timely treatment, a genuine and persistent difference in brightness or colour between your eyes deserves an eye examination that checks the optic nerve, the colour vision, and the pupil reactions.

A brightness difference that has crept in over months should still be checked, but it is not usually an emergency. A difference that appears suddenly, over hours or a day, should be treated as urgent, because a sudden loss of brightness or colour in one eye can reflect optic neuritis, a reduced blood supply to the nerve, or a retinal problem, all of which need prompt assessment. If you are unsure whether the change has been gradual or sudden, treat it as sudden and seek same-day review.