The design is the main characteristic of an ophthalmic lens. Convergent, concave or progressive, there is a solution for everyone:
Single vision lenses have the same optical focal point or correction over the entire area of the lens. Most non-presbyopic wearers will need single vision lenses to answer their visual needs. As a solution for presbyopia, single vision lenses only correct near vision. To see things far away, you need to remove your glasses or look over them.
Progressive lenses provide a smooth transition from distance to near vision, allowing the viewing of all intermediate distances. Progressive lenses are the optimum solution for presbyopia. These are designed to restore clear and accurate vision at all distances, without needing to constantly change glasses. The upper part of the lens makes it possible to see things far away, the central part to see the middle distance and the lower part to see close up.
Bifocal lenses usually have a straight or visible line that defines separation of the top area which is for distance vision and the bottom area reserved for reading. There are some round designs available however the straight top bifocal lens is most popular. The trifocal lens has three focal distances. The third focal distance, placed in the middle of the lens, is utilised for viewing intermediate distances that are not covered by the top or bottom sections.
Minus Lenses - Concave
The thickest part (the base) of a minus lens is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the centre of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.
Plus Lenses - Convex
The thickest part of a plus lens is in the middle and the thinnest part is on the outter edges. The light is bent toward the centre and the focal point moves back. The stronger the lens, the closer the focal point is to the lens.
A lens' power is determined by its material, as well as by the angle of its curve. Power measures the extent to which light is bent as it passes through the lens, and is expressed in dioptres (D). The higher the dioptre, the stronger the lens is. A sign preceding the dioptre power on your prescription indicates the type of lens. A minus sign indicates that the lens is concave, while a plus sign indicates that the lens is convex.