Why are my two eyes different?

Almost nobody has two identical eyes. A difference between them is normal, has a name when it is large — anisometropia — and matters less for its size than for whether it is changing.

Small differences are the norm

A quarter or half a diopter between eyes is unremarkable and usually unnoticed: the brain fuses the two images without effort, and the correction handles the rest. Your two eyes also differ in dozens of other small ways you never think about.

When it gets a name

Beyond about 1.00 D of difference, the term anisometropia applies. Two things start to matter:

• Image size. A stronger lens magnifies (plus) or minifies (minus) slightly more than a weaker one, so the two eyes receive images of marginally different sizes — aniseikonia. Above roughly 2 D of difference this can be uncomfortable in spectacles.
• Prismatic effect off-centre. Looking away from the optical centre of two unequal lenses introduces different prism in each eye — most noticeable looking down through reading glasses.

Contact lenses avoid both, because they sit on the eye and move with it — which is why a large difference is often a reason to prefer them.

The trend is what to watch

A stable 1.50 D difference over a decade is a characteristic of your eyes. A difference that grows from 0.25 to 1.50 D over three years is a change in one eye, and the reason for it matters. Neither is visible from a single prescription — the gap on today's paper looks identical in both cases. Only the history distinguishes them, which is precisely the sort of question a lifelong record answers in seconds.