We see things because light bounces off them and passes into our eyes.
An eye works in much the same way as a camera.
The light pases through the transparent part at the front of the eye called the CORNEA. Behind the cornea is a circular shield called the IRIS. The iris is different colours in different people. (When we talk about the colour of someone's eyes, we are talking about the colour of their irises.)
There is a hole in the centre of the iris called the PUPIL. The light passes through this hole to get into the eye.
The size of the pupil changes to control how much light gets into the eye. Sometimes the light is too bright, so the pupil gets smaller and cuts down the light getting into the eye. If you go into a dark place, the pupil gets larger to let more light in. The size of the pupil also depends on how interested you are in what you are looking at. If you are looking at someone or something very interesting, the pupil gets larger. This helps the eye FOCUS or concentrate on the intersting object.
An object IN FOCUS will have a sharp, clear image, that is you see it in detail without blurring. An object OUT OF FOCUS is blurred or fuzzy and can't be seen in detail.
The LENS is a transparent disk behind the pupil that can change shape slightly because it is attached to the eye by small muscles. This means the lens can alter while we are looking at things. When you change between looking at close objects and looking at distant objects, it is the lens in the eye that adjusts to keep the image sharp and detailed.
After passing through the cornea, the pupil and the lens, the light beams end up on the lining at the back of the eye. This is called the RETINA. The retina is like the film in a camera. The retina turns the light beams falling on it into electrical nerve signals. These nerve signals travel down the OPTIC NERVE to the brain.
(I'm not going to describe the detail of how the retina reacts to light, how we see colours or how the nerve signals are produced - all that is too complicated for this page!).
The retina does not work in the same way across the whole of the eyeball. Light coming from different places falls on different parts of the retina. If you look straight at something, the light will fall on the most central part of the retina. If you are looking at something 'out of the corner of your eye' then the light falls on the sides or edges of the retina.
The centre of the retina is the most sensitive to detail and to colour. This means that you get the most detail when you look directly at something. You can still see things even when you are not looking directly at them, but not in so much detail and you can't judge the colour so well.
When the eyesight is affected by Leber's Optic Neuropathy, it is the retina and optic nerve that stop working properly. Some parts of the nerve may even die and so can't send the right signals to the brain any more.
The cornea, pupil and lens still work, but the light can't be converted into the right signals for the brain. The damage happens in the most sensitive part of the retina, where the detailed vision and best colour vision normally sits. That is why people affected by Leber's lose the vision in the centre and can still see 'around the edges'.
Ordinary spectacles correct problems in the cornea or in the lens. They help the eye to focus the light onto the retina properly. When someone is affected by Leber's, the light is already being focused on the retina properly, but it isn't being converted into signals for the brain.