How we see
How does the eye work?
Light bounces off what you’re looking at and enters your eye’s cornea, which is a clear covering over your eye. The light passes through your pupil, the black circle in the centre of the iris, to the lens. The lens focuses the light onto your retina – a thin but vital lining on the back of your eye. This acts like camera film, capturing a picture of what you’re looking at. This image is sent to your brain via the optic nerve, which instantly tells you what you’re seeing.
Your ability to see detail is known as ‘visual acuity’. It’s measured both at distance, i.e. what size letters you can read on a chart in the distance, and near, i.e. what size print you can read close up. You may get these measured at the optician and/or eye hospital.
In a healthy eye, when young, the clear lens adjusts to focus near or far to provide detailed images. As we age the clear lens loses its ability to focus on near objects and 'reading glasses' are required for close work.
Your visual field is the entire area across which you can see. It’s divided into two areas – central vision and peripheral vision. Your central vision is in the middle, and your peripheral vision is all round the outer edge. Your visual field is measured at the optician or eye hospital using what’s called a field test. That involves closing one eye, putting your chin on a rest and looking straight ahead. Small lights will flash in your peripheral vision and you’ll be asked to say when you see a flash. This allows the machine to map out your visual field and identify any reduction in or loss of visual field. Your vision in areas of reduced visual field may become misty, blurry or be completely diminished, often known as a blind spot (or scotoma). The visual field test is a useful way to measure the extent of retinitis pigmentosa, detect any conditions related to the optic nerve and to screen for glaucoma. The visual field test can also measure visual field loss due to damage in the visual pathways in the brain such as sight loss due to stroke or brain tumour.
What is central vision?
This is the vision in the middle (or straight ahead). The retina is the thin area at the back of the eye. It’s made up of photoreceptor cells, which consist of rods and cones. Cone cells are mainly at the centre of the retina and the rod cells are distributed across the peripheral retina. Behind the retina is the optic nerve.
The macula is near the centre of the retina and is made up mostly of cone cells, which allow us to see colour effectively and give us sharp, detailed vision (acuity). If you were to have fully functioning central vision, you would be able to see faces clearly and read print (both close up and in the distance).
What is peripheral vision?
The retina is made up of photoreceptor cells called rods and cones and your peripheral vision relies on the rods, which deal with movement, light and night vision. If you have reduced peripheral vision (sometimes called tunnel vision), you may be sensitive to light and may struggle in dull or dark settings or seeing at night. You may also have issues seeing and avoiding obstacles, people and other moving things that are not directly in front of you.