First, what do you actually mean?
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Two quite different observations share this description. The first is that one eye looks darker to you or to other people, something you notice in the mirror or in photographs. The second is that vision through one eye seems darker, dimmer, or less vivid than through the other. The appearance of the eye and the performance of the eye are separate questions, so it helps to know which one you are describing.
When the eye looks darker (in the mirror)
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When one eye looks darker, you are almost always noticing one of two things.
The first is the pupil. The pupil is the dark opening at the centre of the coloured iris, and a larger pupil looks like a bigger, darker circle. About one person in five has a naturally occurring difference in pupil size, called physiological anisocoria, which is harmless and lifelong. What matters is whether it is old or new. A longstanding difference, especially one that others in your family share, is very unlikely to be significant. A new difference in pupil size, particularly with a drooping lid, double vision, or pain, can point to a problem such as Horner syndrome or a third nerve palsy, and should be assessed promptly.
The second is the colour of the iris itself. Some people simply have two differently coloured eyes, known as heterochromia, which is usually present from early life and entirely benign. A change in the colour of one iris that comes on later in life is different, and worth having looked at.
- A new difference in pupil size appears with a drooping eyelid, double vision, or pain
- One pupil becomes large and the eye is painful
When the vision seems darker: the optic nerve comes first
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If what you mean is that vision through one eye is darker or dimmer, rather than that the eye looks different, the priority is the same as for an eye that seems too bright: the optic nerve must be checked first. A darker, dimmer image, with washed-out colour and reduced contrast in an eye that looks entirely normal, is the classic picture of an optic nerve problem, whether from glaucoma, inflammation of the nerve, or a reduced blood supply to it.
The sign an examiner looks for is a relative afferent pupillary defect, or RAPD, a reduced pupil reaction on the affected side that takes only moments to test with a torch. Because these conditions range from the slow and silent, such as glaucoma, to the sudden, a persistent sense that one eye sees more dimly than the other should be examined properly. This is covered in full on the companion page, Why is one eye brighter than the other?, since a dimmer eye and a brighter eye are two sides of the same observation.
When to act
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A gradual change warrants a routine but definite eye examination. A sudden darkening of vision in one eye, over hours or a day, should be treated as urgent. If a new difference in pupil size appears alongside a drooping lid, double vision, or pain, seek same-day assessment.
Common questions
Is it normal for one eye to look darker than the other?
Often, yes. Around one in five people have a naturally larger pupil on one side, which makes that eye look a little darker, and it is harmless if it is longstanding and you have no other symptoms.
Why does vision seem darker or dimmer in one eye?
When the eye looks normal but sees more dimly, the optic nerve is the first thing to check. Glaucoma, inflammation of the nerve, and a reduced blood supply to it all dim the vision and wash out colour on one side, and an examiner can test for this with a simple torch. A gradual change should be examined, and a sudden one treated as urgent.
When should I be seen urgently?
Seek prompt assessment if a new difference in pupil size comes with a drooping lid, double vision, or pain, or if the vision in one eye darkens suddenly. These need same-day or urgent review.
