Objective lenses (from now on referred to as just lenses) are translucent objects that focus light beams through refraction.
The first theories of optical magnification by using curved lenses came about a thousand years ago. These theories were further developed in the 12th century. At this time, the reading stone was created, which was placed over written text to make it easier for people to read. Already at the end of the 13th century, lenses were being used to correct vision and the first eyeglasses came about.
Modern camera objectives are comprised of individual lenses and groups of lenses. Depending on the shape of the boundary surface of the lens and the refraction index of the lens material, the refractive properties of the lens are different.
There are many different kinds of lenses; the simplest kind is the spherical lens. They are made up of rounded surfaces of a sphere. Spherical lenses can be divided into two categories: convex lenses and concave lenses.
Convex lenses focus or converge light to a single point. The point where the light converges is known as the focal point. The distance between the focal point and the middle of the lens is referred to as the focal length, which is a very important term in photography. Convex lenses are characterised by an outwardly curved outer surface, thus they are thicker in the middle than at the edges.
Concave lenses, on the other hand, are curved inward and are thicker at the edges than in the middle. They diverge the light rays that pass through them.
In addition to these types of lenses, there are also mixed forms. So-called meniscus lenses have both a convex and concave side. Meniscus lenses are used in camera objectives that are made up of several lenses and lens groups, in order to compensate for aberrations.