according to anastigmatic optical systems whose design is called, four or more elements are incorporated to control the refraction of light transmitted through the lens and prevent the presence of optical aberrations in the images.
Recall that a piece of glass (or other transparent material), carved with the intention of changing the path of light to transmit, is a lens and these optical instruments, play a fundamental role in the development of photography from its early stages.
To start including lenses in cameras, it was discovered that the most basic way to achieve focus of an image is double concave, but this carved, it has certain imperfections
The light passing through the periphery of the circumference, is focused at a different distance than is transmitted by the center (spherical aberration). Another type of aberration that commonly occurs in the double convex is the point where the rays incident at an angle, “focus” shine as fuzzy comets (Coma).
Looking improve performance lens developed “Meniscus”, characterized in that one side of the lens is concave and the other is convex. These parts greatly improved the conditions of approach and to avoid the presence of aberrations at the end thereof, the opening of the lens is limited to a fixed aperture of f / 11. This design, it still represents to this day the basic principle for many “Point & Shoot” camera focal length and although most focus problems disappear, unfortunately exhibit a phenomenon of distortion known as “Pincushion or number sign” when the diaphragm it is placed on the concave side, and “barrel distortion” when the opening is limited on the convex face.
One of the proposals that appear to solve the problems of distortion in the meniscus, is achieved by placing a couple of them identical and placing the diaphragm in the middle, thus distortions are canceled. This arrangement is known as “fast rectilinear” and represents one of the first optical solutions incorporating more than one element.
Another problem observed in the optical approximations of one lens, is that color frequencies containing a light beam is re-routed to pass through the lens at different angles of refraction depending on the oscillatory speed, so that the influence on the focus point, distances vary. To this aberration is known as Chromatic and visually recognized by the overflowing color it occurs in the forms.
In the search for a solution for chromatic aberrations, he began experimenting with two elements working together (usually one positive and one negative), and this type of combined pieces were called Achromatic and are cemented together to form a single unity.
Achromatic elements considerably solved chromatic aberration, however, they had their own Achilles heel … astigmatism, which occurs when there is a point of convergence between the sagittal and tangential light rays passing through the lens planes.
Today three cemented elements act as one and are called apochromatic, dramatically improving the performance of their predecessors are used.
In 1893, H. Dennis Taylor designed an arrangement using three elements: positive Two flanking the center one negative (convex Double), who managed to eliminate most of the aberrations. The arrangement it is known as “The Cooke triplet”, and this design is the starting point for many renowned optical designs that evolved later as “Argus Cintar”, “Zeiss Novar”, “Yashica Yashikor”, “Schneider Radionar” and “Lomo LC-a’s Minitar”.
Some triplets arrangements were reached also referred to as anastigmatic, although most manufacturers and scholars of optics, consider this category starts from systems with 4 or more items. Triplets can present soft focus at the ends, so its best performance is in the middle openings.
A decade later, Zeiss experiment with models of 4 elements to develop Tessar, replacing the background element with an achromatic and design reverberated in virtually all manufacturers made their own versions as “Leitz Elmar”, “Schneider Xenar”, ” Yashica Yashinon “,” Agfa Soiinar “among many others.
At the start incorporating 6 elements in the designs, the openings could reach higher levels as f / 2 and more.