Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

Lens Glass and its Purpose


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A photographic lens consists of pieces of polished and carefully shaped glass used to refract light rays so that an image of a desired object or scene is formed on the rear wall of a camera. If we look at the main terms of a lens we need to first consider:




Focal plane


The surface (plane) on which an image transmitted by a lens is brought to sharp focus, the surface or area at the back of the camera occupied by the film or sensor.
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The following is an advert for Canon but it is an excellent description of the issues of lens design.

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Optical center


The optical centre of a lens is a point, usually (although not always) within a lens, at which the rays of light from two different sources entering the lens are assumed to cross.

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What is Principal Focus?

The principle focus is the point where light rays originating from a point on the object converge. The principal focus is defined as the point where a beam parallel to the principal axis appears to diverge converges from a point on the principal axis after passing through the lens.

Infinity


This term is not easily described. When light is reflected from the point of an object it produces a cone shaped dispersal, the closer the point is to the lens, the greater is the angle of the spread of light rays from the object.

As the object point gets farther away from the lens, the angle of spread becomes less and less until a distance is reached at which the rays from a single point, for all practical purposes, can be considered parallel. This distance is known by the term infinity. For all practical purposes, light rays from a distant object or an object at 200 or more meters away may be considered to be parallel.

So what is the main purpose of a lens?

A photographic lens is designed to transmits more light than a pinhole could do. It therefore increases the brightness and improves the sharpness of an image. The basic principle of a lens, any lens is relatively simple.



First, consider an image formed with a single pinhole. Next, consider another pinhole above the first. This pinhole forms a second image. If these two images could be made to coincide in some way, the result would be an image with twice the amount of light so would be twice as bright as the original. If we add another four holes and these are made to coincide with the center one, the result would be an image with five times the light and thus be five times as bright as the image made by the one center pinhole.

This is achieved using the principle of refraction, this principle make these four images coincide with the centre one. By placing a prism behind each pinhole, you would be causing the light that forms each of the four images to be refracted and form a single image. In other words, the more pinholes and prisms used, the brighter or more intense the image. A lens simply represents a large number of prisms.

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But to create the prisms to achieve the desires focused light output the optical nature of the glass needs to be considered. The best optical glass is Crown Glass. Crown glass was produced from a melt of the oxides of silicon, aluminium, calcium, lead, potassium and sodium. The more refractive flint glass, as used for crystal glass, was produced by increasing the amount of lead oxide. Dispersion also increased with refractive index, and early lenses could not simultaneously be corrected for colour and astigmatism. Remember back in the 1800s and early 1900s only black and white images were produced so some of the complexity of differing wavelengths could be corrected with filters. This is why black and white photography often needed lens filters to block some wavelengths.

In the middle of the 19th century, after completing his training to become a mechanic, at just 30 years of age Carl Zeiss (1816-1888) started his own business in Jena. Carl began the official operation of his "Werkstätte für Feinmechanik und Optik" on November 17, 1846. A historic date. Zeiss first worked without any employees, constructing, repairing and optimising different instruments by himself. Carl enlisted the help of physicist Ernst Abbe (1840–1905) to help with the design of his lenses.

Pioneering work by Abbe in collaboration with the Schott glassworks in Jena gave a range of new glasses with the necessary properties, and the first anastigmatic lenses were possible. The glass­ making technique used compounds of barium, boron, phosphorus and fluorine to give increased refraction without increased dispersion. Barium oxide was the most important ingredient.

Zeiss and Abbe built up a close friendship, and not only achieved wonderful scientific pioneering achievements but outstanding sociopolitical achievements. After Carl Zeiss' death in 1888, Abbe successfully transferred his shares of the company and the glassworks, along with those of the Zeiss family, into the Carl Zeiss Foundation. At first he wanted to secure the company's existence by making it independent of his personal interests. Abbe drew up the statute in 1896. stating that from then on, the company's profits were to benefit the University of Jena and Jena's population. The legal regulations were also pioneering. He laid down legally enforceable workers' rights at a time when there was no such thing as labor laws and the relationship between the employer and the employee was still patriarchal.

The next significant advance was in the late 1930s, when glasses were made which included oxides of rare earths such as lanthanum, tantalum and thorium but no silica. Even higher refraction was given with relatively low dispersion.

The chart shows the composition of the glass from this time.

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And each single element of the lens often need different glass composition as well as form.


The advantage of high refractive index glasses is that both the radial (sagittal) and tangential focal points move closer to the focal plane, and the Petzval sum is also improved.

Thorium compounds are no longer used in modern lenses because of their radioactive nature, although some of the older lenses do have these radioactive properties. Similarly, the highly toxic oxides of mercury, beryllium and tellurium are no longer used. The oxides of zinc, zirconium, cadmium, lithium, niobium and tung­sten are all used as necessary.
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The wide range of glass types (as many as several hundred may be offered by one manufac­turer) are all necessary to give more flexibility to a lens designer. Computer-aided studies of existing designs or of new ones in development may indi­cate that significant cost savings can be achieved through using a particular type of glass. It may be available or may have to be made. Trial melts with variations in the ingredients are used to produce new glasses, but suitability for practical use also depends on other properties such as glass colour and resistance to weathering or mechanical abrasion.

There is a considerable cost difference between the cheapest and the most expensive glasses, and some are not available in quantity, being made only infrequently, perhaps because the raw mate­ rials, e.g. tantalum, are in short supply. Some lens manufacturers develop glasses solely for their own use, Leitz for instance.

Glasses of lower density are now being pro­duced without loss of refractive properties. This makes lenses lighter in weight and may reduce costs, as glass is sold by weight.

Manufacturing methods

The highly pure raw materials are carefully analysed before melting in pots. The melt may be pressed or cast or allowed to cool into chunks. Reheating and annealing of approved-quality glass allow it to be shaped into lens blanks to assist the surface­ shaping process. Blocks and discs for prisms or filters may be sawn from slab glass.


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Until about 1950, glasses were melted in clay or ceramic crucibles and these materials were often attacked by the melt. It proved almost impossible to remove all minute air bubbles, and for many years such bubbles in lens elements were taken as proof of a quality lens made using highly refractive dense glass. Nowadays, all melts are made in platinum crucibles to avoid deterioration as well as trace contaminations, but one of the quirks of old lenses is these small bubbles. Air bubbles in lens elements were very common in older lenses.

By the late 70's and early eighties, when lenses was made, they were becoming less common as manufacturing techniques improved. If they are, in fact, just air bubbles, you should be fine. Embrace the imperfections.

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.

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