The nearest point from which one can still see clearly without difficulty is called the proximity point. In a normal (emmetropic) human eye this point is about 30 cm in front of the eye; even closer at a young age. The most distant point from which one can still see sharply is called the vertepoint, which is normal for an ordinary eye.
For many people, however, these points are at different distances.
- If both points are too close, this is called myopia (short-sightedness). The point of proximity is then much closer than 30 cm, and the vertepoint is far before infinity. One can therefore see sharply too close, and in the distance one sees out of focus. This is corrected with a negative lens, which brings the vertepoint back to infinity and the proximity point at approx. 30 cm.
- If both points are too far away, this is called hyperopia (farsightedness). The vertepoint is then, as it were, ‘beyond’ infinite, or far behind the person (so it has become a virtual vertepoint). Without glasses, the eye of a farsighted person will constantly accommodate to correct the eye error, causing symptoms such as headaches, eye strain and difficulty concentrating. The hyperopia is corrected with a positive lens, which places the proximity point at about 30 cm and the vertepoint at infinity. This allows the wearer to see sharply in the distance, without having to make an effort to constantly accommodate the eye lens; the complaints will disappear.
In addition to the eye itself, the required strength of a spectacle lens also depends on the distance between the eye and the lens, the so-called cornea distance. This distance is usually 12 to 14 millimeters and is determined by the frame. The greater the distance to the eye, the stronger a negative lens should be and the weaker a positive lens should be.