Your eye is like a camera
Your eye is like a camera. Your eye has:
- A variable opening called the pupil;
- A lens system, which includes a transparent covering (the cornea, which does most of the focusing) and a spherical lens inside the eye behind the iris (the coloured part);
- A reusable ‘film’ called the retina – a complex layer of cells at the back of the eye;
- Various sets of muscles, which control the size of the pupil, the shape of the lens system (to control the ‘zoom’ function of the eye) and the movements of the eye.
When you look at something, light passes through the cornea and the pupil at the front of the eye. The cornea and the lens refract the light.
The lens inside the eye, between the pupil and the retina, refracts (bends) this light, focusing it onto the retina. The retina is full of sensory cells called ‘rods’ and ‘cones’, which change the photons of light into electrical signals.
Our nerves transmit these signals to the brain, and the brain interprets this as an image.
When you look at something, four things must happen:
- The image must ‘reduce’ to fit onto the retina;
- The scattered light must focus at the surface of the retina;
- The image must curve, to match the curve of the retina;
- The brain must interpret the image as vision.
For the above to occur, muscles attached to the lens contract and relax to change the shape of the lens system and keep the object focused on the retina, even when your eyes move.
Your nervous system controls this complex set of muscle movements.
Most vision problems occur when the eye cannot focus images onto the retina.
The most common problems are to do with the shape of the cornea, the length of the eye, and/or the stretchiness (‘elasticity’) of the lens.
These common issues are known as short-sightedness (myopia), long-sightedness (hyperopia), ageing eyes (presbyopia) and astigmatism. In the vast majority of cases, these can be corrected with laser eye surgery.