Focus of contact lenses
Monofocal contact lenses are those that have a single focus. They are those that serve to compensate myopia, hypermetropia and stigmatism.
If there is presbyopia, then we need to focus from far to near vision. Then, they are called multifocal contact lenses. The most common are with aspheric geometry (the curvature is changing and this allows different powers) or with concentric rings (a zone for distant vision and another for near vision). They give a simultaneous view of different images, whether they are remote or nearby. It is the brain that has to select the one that interests in each moment (it is the same that we do when in a conversation with several interlocutors we have to select the voice that we want). In the semi-rigid there is another modality, which is rarely used today, in which the vision is alternating (the eyelids raise the lens with the lower part with the near power).
There are other types of contact lenses that are little known, since they are very new. In people whose myopia is increasing, it has been seen that they have a greater peripheral hyperopic blur. Now there are contact lenses of reverse geometry that correct myopia centrally and peripherally, neutralizing the hyperopic peripheral blurr in order to reduce this progression. The results of recent clinical studies show that this is the case.