Skip to main content
Eye Atlas
Tests at the clinic

Ultrasound, OCT and other eye exams: what each one shows

Home tests can hint that something is off; the instruments in the exam room give the real answer. Here is why each exam is done, how it goes, whether it hurts and which problems it's ordered for.
Find an examLab: try the instruments
Sudden vision loss, flashes with a curtain, severe pain or an eye injury — see a doctor the same day
Eye surfaceCorneaAngleLensVitreousRetinaMaculaOptic nerveOrbital fatPathway to the brainLight pathPressure inside the eyeEye length

OCT — optical coherence tomography

A micrometer-level cross-section of the retina and optic nerve, with no radiation. Details

Pick an exam — the diagram highlights what it checks

Reference

What each exam does

Pick an exam: what it shows, how it's done, whether it hurts and when it's ordered.

The basic eye exam

What happens at almost every eye doctor visit: vision, pressure and a look at the eye under a microscope.

Scans and measurements

Instruments that take cross-sections, photos and precise measurements of the eye. Results can be saved and compared a year later.

How well the eye works

Tests of how the eye performs rather than how it looks: visual field, retina and nerve signals, tears.

Common question

OCT, ultrasound or a dilated exam — what's the difference

These exams don't replace each other: each has its own depth, precision and blind spots.

OCT — light, micrometers

The best way to see the macula and optic nerve layer by layer. But light needs a clear path: through a dense cataract or blood in the eye there is no image.

Try it on the instrument

Ultrasound — sound, millimeters

Lower resolution, but sound passes through a cloudy lens, blood and opacities and shows the whole eye, even the orbit behind it.

Try it on the instrument

Dilated exam — the periphery

OCT mostly images the center. Retinal tears behind new flashes and floaters are usually in the periphery, which a doctor examines most reliably through a dilated pupil.

Try it on the instrument

Diagnostic lab: work the instruments yourself

Ten live instruments with real physics: from the eye chart and slit lamp to ultrasound, OCT, a fundus camera, perimeter, tonometer and topographer. Pick an instrument and what's in the eye, then work like a doctor: try lenses, move the probe, line up the camera, take the scan. Resolution, noise and alignment errors behave as on real devices.

By situation

What's usually ordered for…

A typical set — the doctor makes the final plan after examining you.

Before the visit

How to prepare for eye tests

  1. Bring your glasses and contact lenses in their case — you may need to take them out.
  2. Make a list of medicines and eye drops, including steroids, blood pressure pills and hydroxychloroquine.
  3. Before biometry, topography and tear tests, ask how many days to go without lenses and drops.
  4. If you've had eye pain with rainbow halos or were told you have a narrow angle, tell the doctor before dilation.
  5. Keep your OCT, visual field and photo results: in glaucoma and AMD the trend matters more than a single result.

If your pupils will be dilated

This is how a dilated fundus exam, angiography and some other tests go.

  1. 0 min

    Drops

    They sting a little.

  2. 20–30 min

    Wide pupil — the exam

    The doctor can see the retinal periphery, which can't be examined through a small pupil.

  3. 4–6 h

    Light is glaring, near vision blurry

    That's normal. Don't drive — bring sunglasses.

  4. After

    It wears off

    Longer in children and people with light eyes.

What you can check at home

Acuity, the Amsler grid, color vision, astigmatism: home tests don't diagnose, but they show what to tell your doctor.

All tests

These descriptions are for reference: the set of tests, methods and preparation vary between clinics. Your eye doctor decides what you actually need.