When you are looking at something that is far away, your eye – if you are perfect sighted – is shaped so that the object is clearly focused on your retina. This means that the image is clear. When you look at something close up, for example to read a book, the muscles inside your eye that surround the lens contract to make the lens change shape. This focuses the light from the book onto your retina.
The lens inside a child’s eyes is elastic, and so can change shape easily to enable them to change focus from looking at something far away to looking at something close up. As we get older, however, the lens naturally stiffens and so it changes shape less easily. This means that the distance up to which we are able to focus gets further away and we are no longer able to focus on things that are close to us, having to hold them further away to see them clearly. This is more noticeable when we want to look at something very close to us, such as when threading a needle. It may also mean that it may take longer for us to focus from looking at something close up to looking at something far away (or vice versa).
This change in focusing tends to become more noticeable when we reach our late thirties or forties as we then find it difficult to focus on things that are at the normal reading distance. It is quite common to see people who are presbyopic holding things away from them in an attempt to see them clearly. As this affects things that are close to you first, your vision of things that are further away – such as the computer – is not affected until later, when your lens has lost almost all of its elasticity.
Because the lens has lost its elasticity, when you are presbyopic you will need glasses to focus on the different distances you need to see. This may mean having separate pairs for distance and reading and maybe for middle distance such as looking at the computer or reading sheet music.