Which eye scan do you need: OCT, retinal photo, visual fields, topography or axial length

Five scans, five different questions

You have been told you need a scan, or you have seen them listed on a fee page and wondered which one applies to you. The names run together: OCT, retinal photo, fields, topography, axial length. They are not five versions of the same picture. Each looks at a different part of the eye and answers a different question.

The simplest way to sort them is by where they look. Two look at the back of the eye: the widefield retinal photo and OCT. One tests how the eye and brain work together: the visual field. One maps the front of the eye: corneal topography. One measures the whole eye's length: axial length. Knowing which question you have tells you which scan matters.

None of them replaces the exam. They are added to a comprehensive eye exam when the history or the findings call for them, and our guide to what a comprehensive eye exam checks covers the basics that come first.

OCT: the retina and optic nerve in cross-section

Optical coherence tomography uses light waves to take cross-section pictures of the retina, a little like an ultrasound that uses light instead of sound. The American Academy of Ophthalmology describes it as showing the retina's layers and the optic nerve fibre layer, and measuring their thickness so changes can be tracked over time. Nothing touches the eye; you rest your chin on a support and look at a target, and the AAO says scanning takes about five to ten minutes.

Its two big uses are the macula and the optic nerve. Macular Degeneration New Zealand calls OCT a standard investigation for diagnosing and managing wet macular degeneration, where fluid under the retina shows up as a bump in the layers. Glaucoma NZ notes OCT measures the nerve fibre layer around the optic nerve and can detect early damage when the visual field test still looks normal.

The AAO also lists its limits: because it relies on light, it cannot see through a dense cataract, a scarred cornea or a lot of bleeding inside the eye. OCT may be done with or without dilating drops.

Widefield retinal photo: the record you compare against

A retinal photo is a colour picture of the back of the eye: the retina, the optic nerve head, the macula and the blood vessels. Where OCT shows depth, the photo shows breadth, and a widefield camera captures far more of the retina in one picture than a standard one. The picture is stored, so next year's photo can be laid beside it and small changes spotted.

It is the baseline for almost everyone. Diabetic changes, a suspicious freckle, a thinning optic nerve or the early drusen of macular degeneration can all be seen and documented. The National Eye Institute lists diabetic retinopathy, glaucoma and age-related macular degeneration among the conditions a look at the back of the eye is for. At NVISION the widefield retinal photo is included with every eye exam; our technology page describes the camera, and our post on retinal imaging explains how it works.

A photo is a surface view. If it raises a question about the macula or the nerve, OCT is the scan that answers it in depth.

Visual fields, corneal topography and axial length

The other three scans are more specific, and you are unlikely to need all of them.

• Visual field test. You look at a central target inside a bowl and press a button each time a small light appears off to the side. It maps side vision and blind spots. The AAO lists glaucoma, optic nerve problems, strokes and tumours pressing on the visual pathway among the reasons for it. Glaucoma NZ notes it takes about three to six minutes per eye and is repeated to tell whether glaucoma is stable or getting worse.
• Corneal topography. A map of the curve of the cornea, taken in seconds with nothing touching the eye. The AAO says it finds astigmatism and early keratoconus, tracks their progression, and helps identify a contact lens that can improve vision. It is the test that settles whether changing astigmatism is keratoconus, and it is used before and after cross-linking. See our keratoconus page for how it is used here.
• Axial length. A measurement of the eye's length from front to back, taken in seconds without anything touching the eye. Short-sightedness comes from the eye growing too long, so axial length tracks whether a child's myopia is progressing and whether myopia control is working. Optometry Australia's 2025 myopia management guide calls optical biometry the most sensitive and reliable metric for tracking myopia progression. Our post on axial length measurement for myopia control goes further.

What is routine and what is added

At NVISION the widefield retinal photo is part of every eye exam. OCT, visual fields, corneal topography and axial length are added items, and each is listed on our consultation fees page; we do not repeat the figures here because they can change.

Added does not mean upsold. RANZCO's Choosing Wisely guidance is that routine automated visual fields and OCT are not indicated when there is no relevant history, symptom or sign, and that a normal comprehensive exam can rule out further testing for simple refractive symptoms. So a healthy 30-year-old with mild short-sightedness may have the exam and the photo and nothing else. A 55-year-old with a parent who has glaucoma, or a child whose prescription jumped, has a reason for more.

If you are offered a scan, it is fair to ask three things: what question is it answering, what would change if the answer is abnormal, and how often will it need repeating. Our optometrists should be able to answer all three in a sentence each.

When this practice is the wrong place

No scan is the right response to an emergency. Call 111 or go to the nearest hospital emergency department for sudden loss of vision, a shadow or curtain across your vision, flashes with a sudden shower of floaters, a chemical splash, a red and painful eye with light sensitivity, or an eye injury. These need a hospital eye clinic the same day, and booking an OCT for next week is the wrong move.

For a gradual change, such as central vision that has become wavy over weeks or side vision you keep bumping into, an optometrist is the right first call and the scans above are exactly what the visit is for. If you are unsure which you are dealing with, Healthline on 0800 611 116 is free and open 24 hours.

How to decide

If you have no symptoms and no risk factors, book the exam and let the findings decide. If you have a family history of glaucoma, ask whether a visual field test and OCT are warranted at your age. If straight lines look bent or there is a smudge in the centre of your vision, OCT is the question to ask about. If your astigmatism keeps changing, ask about corneal topography. If your child's short-sightedness is rising, ask about axial length.

Then read what happens at your first appointment so the visit holds no surprises.