Eye Exam Tests Explained: Vision, Prescription, Pressure and Retina
Reading an eye chart is only one part of an eye examination. A prescription test asks how lenses improve focus; an eye-health examination looks for disease. Understanding the purpose of each test helps you give useful responses and understand why another test may be needed even when you can read small letters.
Tests of clarity and focusing
- Visual acuity: measures detail seen at a specified distance, with or without correction. Results may use Snellen fractions, decimal notation or logMAR. A 20/20 result is not proof that the retina, optic nerve or peripheral vision is healthy.
- Refraction: estimates focusing error and refines lens power using objective measurements and your responses. The result contributes to a glasses prescription; contact lenses also require a fitting assessment.
- Astigmatism assessment: refraction measures cylindrical correction, while keratometry or corneal topography assesses corneal curvature. A corneal map does not directly measure every contribution from the internal lens.
During “one or two?” comparisons, say if choices look equal, fluctuate after blinking or are both blurred. Do not guess what answer is expected. See how a glasses prescription is measured.
Tests of how the eyes work
- Color vision: tests such as Ishihara plates screen particular color deficiencies, especially red–green. One plate test does not characterize every acquired or inherited color problem.
- Stereopsis: assesses binocular depth perception. An abnormal result is a clue, not a diagnosis of amblyopia or strabismus by itself.
- Alignment and movement: cover tests and movement observations examine coordination, restriction or involuntary movements.
- Pupil responses: light responses can provide information about visual pathways and neurological function.
- Visual fields: assess sensitivity across the field of vision. Automated tests require steady fixation; tell the examiner if you lose position, become tired or do not understand the task. Not every missed light means disease.
Tests of eye health
Slit-lamp examination uses magnification and a light beam to inspect structures such as the lids, cornea and lens. With additional lenses, the clinician can also examine structures at the back of the eye.
Tonometry measures pressure using methods such as an air puff or contact applanation. Pressure alone neither confirms nor excludes glaucoma. The optic nerve, visual field, corneal factors and other findings may need consideration.
Retinal and optic-nerve examination assesses the back of the eye, often with pupil dilation. Photography and OCT may add information when indicated. No single scan automatically replaces the clinical examination.
Prepare for the appointment
Bring current glasses, lens packaging if relevant, medications, prior reports and your family eye history. Describe when a symptom began, which eye is affected and the task that makes it noticeable. Ask whether contact lenses need to be removed beforehand and whether dilation is planned.
Dilating drops can temporarily blur vision and increase light sensitivity. Bring sunglasses and arrange transport when needed; do not drive while vision is affected or against the clinic's advice.
How often should tests be repeated?
The interval depends on age, symptoms, diabetes, family history, previous findings and current treatment. A universal age-based calendar cannot cover every patient. Screening, a routine prescription review and monitoring an established disease have different purposes. Ask for a written return interval and the symptoms that should bring you back sooner.
Do examinations prevent eye disease?
They can help detect disease and guide treatment, but do not guarantee prevention. Sudden vision loss, a new curtain or flashes with many floaters, severe eye pain or new double vision should not wait for a routine appointment. See recent vision changes and urgent warning signs and limits of home vision tests.
Sources and review
Reviewed October 29, 2026. General education, not an individual diagnosis or treatment plan.