What is a refractive error?
A refractive error is a mismatch between the focusing power of the eye and its length, so light does not come to a focus on the retina. Short sightedness, long sightedness and astigmatism are the three common forms, and each is described by a prescription.
- Subject
- Vision problems
- Topic
- Blur, night vision & strain
- Kind of question
- What is…
- Last reviewed
- 22 September 2026
In more detail
Refractive error is not a disease and does not damage the eye by itself. The reason clinicians still examine the whole eye before treating it is that the same blur can sit on top of a condition that does need treatment.
What this answer depends on
- The prescription and how stable it has been
- The health of the cornea, lens and retina
- Whether the blur is explained entirely by the prescription
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Where this answer sits
It is one of 483 answers in the vision and eye health knowledge base, filed under Vision problems & prescriptions in Blur, night vision & strain . The subject page carries the answers either side of it, and the hub is searchable.
This information is for educational purposes and does not replace an examination by a qualified eye-care professional.
General answers cannot tell you what is happening in your own eyes. Only an examination by a qualified ophthalmologist can do that.
Sources & further reading
We prefer primary sources. Where a claim comes from a manufacturer, a single study or a regulator, it is labelled as such below.
- EyeSmart: eye health information for the public
Patient-facing material from the largest ophthalmology professional body, covering conditions, tests and treatments outside refractive surgery.
Used for Cross-checking patient-facing explanations of non-refractive eye conditions; Describing what tests such as dilation and OCT involve.
Link checked 23 September 2026