Why is my vision blurry only sometimes?

It is easy to think of focusing as the job of the lens and cornea alone. But light meets the tear film first, and a smooth tear film is a precise optical surface. When it thins and breaks into patches, the surface becomes irregular and vision blurs — not because anything inside the eye changed, but because the outermost surface did.

Blink, and the film is restored, and clarity returns. That cycle, repeating through the day, is what people describe as vision that will not settle.

Fluctuating vision from dry eye tends to be worse for sustained near work, worse late in the day, and noticeably better right after a blink or after using drops. People often notice it most when reading, driving at night, or at the end of a long stretch at a computer.

By contrast, blur that is constant, or that does not change with blinking, is more likely to be about prescription, the lens inside the eye, or something else — which is an optometric examination rather than a dry eye assessment.

Plenty of it is outside your control: age, genetics, hormone changes, autoimmune and inflammatory conditions, and previous eye surgery including cataract surgery and LASIK. Medications matter too — antihistamines, oral contraceptives, and some anti-anxiety and antidepressant medications all reduce tear production. Smoking makes symptoms worse, and so does contact lens wear.

None of that changes the treatment much, but it does explain why dry eye often arrives at a particular stage of life rather than out of nowhere.

Fluctuating vision is the symptom that most often pushes people to act, because it interferes with work in a way that grittiness does not. It is also a useful measure of progress: as the tear film stabilises through a course of treatment, the fluctuation usually settles before other symptoms do.

If your vision has changed and you are not sure which of these it is, book the eye examination first. Dry eye treatment is not the right starting point for a change in sight.