Lenses and lens design

Lens design is a complex business. All lenses aim to filter and focus light so that it hits the specified size of the sensor or film strip in the correct place. However, there are a variety of other factors that determine how a camera lens affects the look and quality of the final photo.

Each lens also has its own character as can be seen from the images on this site.

History of Photography Lenses

Photographic lenses have common origins with other types of lenses. The works of the Arabian scholar Alhazen show that the elementary prop­erties of biconvex lenses were known in the 13th century, and convex spectacle lenses were in­ vented before the end of that century.

Newton in 1666 gave the cause of chromatic aberration in such lenses and, believing the prob­lem insoluble. Limits on lens design together with a lack of film sensitivity led to exposure shutter speeds measured in minutes not seconds, necessitating headclamps and white powdered faces.

In 1840 Josef Petzval introduced his design for a lens constructed by Voigtlander. This was an incredible aperture for its day, being some 20 times faster than contemporary designs. Petzval used two dissimilar air-spaced achromatic doub­lets, one cemented, the other air-spaced, with some resemblance to a Lister microscope objec­tive.

In the late 1890s, H. D. Taylor departed from Petzval lenses to tackle field curvature and aberrations in an analytical and scientific way. He showed that by splitting an achromatic doublet into two parts and further splitting the positive element into two parts and creating an air-space either side of the central negative element, the variables could be made just suffi­cient to correct the von Seidel aberrations. This, when manufactured by Cooke & Sons, was called the Cooke Triplet and was a further major advance, as well as being of low cost and simple to make. It was the first lens system that allowed elimination of most of the optical distortion or aberration at the outer edge of lenses. The initial aperture of f6.3 for a 50° field has increased to //2.8 with modern glasses.

The Cooke Triplet

The residual zonal SA was reduced by various derivatives produced by the process of compound­ ing and splitting one, two or even all three single elements.

Many now famous, classic designs started as a triplet. Compounding the rear element with the cemented surface curved towards the stop is a particularly effective design and is the basis of the Zeiss Tessar lens, designed by Rudolph in 1902 with f6.3 aperture as a develop­ment of his Protar lens. The Tessar design of aperture 7/3.5 or //2.8 in its modern form became the standard lens for cameras in many formats, cover­ ing a 60° field.

Zeiss Tessar

Converting both front and rear elements into doublets gives the classic Voigtlander Heliar de­sign of 1902 and later Pentac of f2.9 aperture. Compounding of all three elements was tried, for example by Leitz in 1930 to give an //1.9 Hektor. Splitting the elements, such as in the symmetric­al Aviar of 1920, gave an excellent lens, related to the Celor type. But splitting and com­ pounding gave more scope and allowed very large aperture lenses in the 1920s and 1930s, important in early 35 mm photography.

An example of this is The Zeiss Sonnar design which still widely used. Its particular features are very thick elements, making it a heavy lens, but it is a compact design, not a telephoto, its rear nodal point being well forward in the lens. Correc­tion capabilities are excellent. Its early f/1.5 version was preferred to other designs as its six surfaces gave low flare in the days before lens coatings.

Sonnar

The Gauss type of achromatic doublet lens is a solution giving a less preferred, air-spaced configuration the double Gauss designs covering a 60° field; this was due to Rodenstock and Busch. Early designs included the Ross Homocentric and Meyer Aristostigmat. The outstanding feature was (and is) the flat field at moderate apertures and good correction even with old glass types. The design was dormant until the 1920s when Hollywood demanded a large-aperture, high- quality lens to use.

A further boost to large-aperture anastigmat designs was the advent of the Leica camera in 1925. Derivatives were produced by processes of compounding and splitting to use air gaps. All retaining the highly curved surfaces concave to the stop. Notable early designs were the Zeiss Biotar f1.5 and similar apertures by Leitz and Voigtlander.

Biotar

The design is still very viable, forming the basis for most large-aperture standard lenses, where six and seven elements are still required for f/2 and //1.4 respectively. High-index modern glasses are of great assistance.

The photographic lens is still developing to produce better imagery, based upon the traditional configurations. Evolution of the photographic lens builds upon this easy development but also use computerised ray tracing and design optimisation programs; optical glasses of various properties such as low-weight, high-index and low- dispersion; and other optical materials such as fluorite, aspheric surfaces, floating elements and multi-coating techniques.

Modern Canon RF Lens

See also "Why does the aperture not cause vignetting when it gets smaller?" in the Aperture section HERE

For more information read sections on Optical Design, Glass, Chromatic Errors, Perspective, Lens Flare and Vignetting. Click on item to go to page.