Myodisc Lenses and Franklin split lenses
Myodisc Lenses are a type of corrective lenticular lens which have a concave fitment on the front surface of a carrier lens. Myodisc lenses are used in cases of extreme myopia, commonly known as short-sightedness or near-sightedness. This is due to light focusing in front of the retina rather than on, causing objects which are far away to look blurred. Myodiscs are the cheapest option available for minus powers greater than -12 diopters however, they are not always prescribed as they can cause scotoma which is an altered or diminished section of sight. Due to this they are only recommended when someone has extremely bad eyesight to the point of nearly being blind. As a lenticular lens, it magnifies from different angles when viewing through them. It is advised to avoid drill mounts for this type of lens.
Franklin split lenses are a type of bifocal which were originally invented by Benjamin Franklin so that he did not have to swap between two types of lenses. The lens is created from two different lenses; one half of a distance lens attached to the bottom half of a near lens. Benefits of these are the large field of view they provide wearers, the optical centres of both section of the lens are at the line of division meaning there is no vertical prismatic jump. Furthermore there are not as many chromatic aberrations, this type of lens is fully customisable for prisms or centres and is suitable for all indexes. On the other hand, an irritating drawback is the reflections which can be produced by the dividing lines. Much like myodics, these lenses should not be used with drill mounts.