None of the following visual impairments is a medical condition but simply a quirk of the eye caused by its non-standard structure. If the eye is anatomically healthy, eyesight can be corrected with glasses or contact lenses. (Medical conditions may be accompanied by visual impairments, but in this case this is a consequence and a pathological change).
Short-sightedness or myopia
Short-sightedness is a visual impairment wherein you can see well close-up but your vision is unclear looking further away. The eyes’ light refraction power is too high or the eyeball is too ‘long’, so the light that falls into the eye is focused in front of the retina. Therefore the retina, which obtains the necessary information for vision, receives a foggy image. Short-sightedness is corrected by glasses or contact lenses with a negative prescription.
Farsightedness or hyperopia
Farsightedness is a visual impairment wherein you can see well further away but your vision is unclear when looking at things close-up. The eyes’ light refraction power is too low or the eyeball is too ‘short’, so the light that falls into the eye is focused behind the retina. The retina therefore receives an unclear image. Farsightedness is corrected with glasses or contact lenses with a positive prescription.
Astigmatism can occur alongside myopia or hyperopia and is caused by the uneven shape of the eye (uneven cornea or lens). The image that the person sees may have shadows, be stretched or distorted because the light that falls into the eye is focused in two different places. The visual impairment is corrected with cylindrical lenses or toric contacts.
Presbyopia or the need for reading glasses
Presbyopia usually occurs after turning 40 and manifests itself in difficulty seeing close-up. Throughout our lives, the elasticity of the lens decreases and the eye cannot make full use of its natural ability to adjust in order to see clearly at different distances. As a result, reading glasses are needed. As presbyopia worsens, you might also develop a need for glasses so as to see further away.