Photographic lens design

Designing a lens that reproduces colour accurately is also important as is the production of an evenly lit and sharp image over the whole of the film or sensor plane.

For the lens designer, achieving these objectives will also involve ensuring that internal flare, optical aberrations and weight are all reduced to the minimum whilst zoom, focus and aperture functions all operate smoothly and predictably.

Photography is a highly competitive commercial business and both weight and cost constrain the production of lenses.

The use of many lens elements serves to minimise aberrations and to provide a sharp image free from visible distortions.

Other kinds of aberrations such as coma or astigmatism can also be minimized by careful choice of curvature of the lens faces for all the component elements.

This is partly to do with the history of lens making but also because grinding and manufacturing of spherical surface lenses is relatively simple and cheap.

Caesium[1] and lanthanum[2] glass lenses are now in use because of their high refractive index and very low dispersion properties.

It is also likely that a number of other transition element glasses are in use but manufacturers often prefer to keep their material specification secret to retain a commercial or performance edge over competitors.

This arrangement, while simple to design and construct, has some limitations, not least the rotation of the greater part of the lens assembly including the front element.

This could be problematic if devices such as polarising filters are used that require accurate rotational orientation irrespective of focus distance.

Most modern lenses for 35mm format rarely provide a stop smaller than f/22 because of the diffraction effects caused by light passing through a very small aperture.

Very-large-aperture lenses are commonly made for other types of optical instruments such as microscopes but in such cases the diameter of the lens is very small and weight is not an issue.

All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed.

Such a mechanism only works effectively for exposures of several seconds or more and carries a considerable risk of inducing camera shake.

By the end of the 19th century spring tensioned shutter mechanisms were in use operated by a lever or by a cable release.

However, because projected images are almost always viewed at some distance, lack of very fine focus and slight unevenness of illumination is often acceptable.

From then on novel products appeared in rapid succession which brought the Zeiss company to the forefront of optical technology.

In 1890, Rudolph designed an asymmetrical lens with a cemented group at each side of the diaphragm, and appropriately named "Anastigmat".

Marketed in 1894, it was called the Protarlinse Series VII, the most highly corrected single combination lens with maximum apertures between f/11 and f/12.5, depending on its focal length.

Rudolph also investigated the Double-Gauss concept of a symmetrical design with thin positive menisci enclosing negative elements.

One of the most significant designer was the ex-Ernemann man Dr Ludwig Bertele, famed for his Ernostar high-speed lens.

Bertele's Sonnar series of lenses designed for the Contax were the match in every respect for the Leica for at least two decades.

The last important Zeiss innovation before the Second World War was the technique of applying anti-reflective coating to lens surfaces invented by Olexander Smakula in 1935.

At first both firms produced very similar lines of products, and extensively cooperated in product-sharing, but they drifted apart as time progressed.

Jena's new direction was to concentrate on developing lenses for the 35 mm single-lens reflex camera, and many achievements were made, especially in ultra-wide angle designs.

Over the years its licensees included Voigtländer, Bausch & Lomb, Ross, Koristka, Krauss, Kodak.

Another licensee active today is Sony who uses the Zeiss name on lenses on its video and digital still cameras.