The requirements for a modern lens are very complex. It is designed to provide a wide range of light, to provide the best image quality across the entire image, regardless of whether it is close-up or landscape, and a large focal range. The whole in a small and light form at a reasonable price.
Light break
The desire and the reality are clearly different, because some things can only be combined with difficulty, in particular the exquisite picture quality in small design and at an affordable price. In this article we look at the different technical aspects of the lens construction and show the possibilities and limitations in the production.
Lens shapes
A lens usually consists of an array of different lenses or individual lenses that are joined to form elements. Light is broken at the transition from one medium to the other. On the one hand, this can be the entrance of the light from the air into the glass, the transition between two joined glasses or the exit from the glass into the air. If one imagines the light as a ray through the lens, the refraction is a kinking of the beam at each transition.
Lens elements
The refractive index of the material used describes how strongly the light is refracted. Vacuum has a refraction index of exactly 1, air of about 1.0003, water 1.33 and optical glasses between 1.5 and 2. The refraction is caused by the different propagation speed of the light in different materials Br>
Aspherical lenses
If the light beam hits a transition exactly in the solder, the direction does not change. If the light beam hits obliquely on the transition, it is refracted either towards the solder or from the solder. This is greatly simplified as in the case of a car in which the wheels of one side are braked more strongly than on the other, which results in a change in the direction of travel. Special cases such as diffraction or reflection on mirror elements for imaging in lenses are treated later.
The shape is decisive for the property of a lens. A simple glass pane whose surfaces lie exactly parallel to one another (plane parallel) can not be used as a lens. The refraction at the entrance of the light from the air takes place in the same intensity as it exits the glass from the air. Several parallel light beams are still parallel to each other after the penetration, no image can be formed. The beams were shifted only in parallel.
One differentiates between collecting lenses and scattering lenses. A collective lens bundles incident light at a point behind the lens, while scattering lenses distribute the incident light as if it were from a point in front of the lens.
The focal length describes the distance between the main plane of the lens and the focal plane. The main plane is a simplification of the lens on a thin layer with the property of the entire lens. The focal plane is the plane at which parallel rays pass through the lens.
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In a collective lens the focal range is positive, with a scattering lens negative. Flexible and changeable lenses, similar to the human eye lens, have already been introduced several times, but a real market maturity has not (yet) been achieved.
Thus, current lenses consist of a combination of many lenses which each have fixed properties. For example, while the Canon 2.8 / 28mm is constructed of 5 elements, there are 23 elements in a Sigma 2.8 / 120-300 mm.
A lens always has two surfaces, into which the light enters or exits. The lens surface of one side may be planar, concave, or convex. Plan means that the surface is not curved and is perpendicular to the main axis. A concave surface is curved towards the center of the lens, while convexly convex outward. When a lens shape is specified, the two data are combined in the order of the incident light beam. A plan-convex lens is therefore planar on the entrance side and convex on the exit side.
For a single lens, the most important characteristics can be defined by the choice of material and surface shape on two sides. In order to achieve certain properties in an optical system, individual lenses are combined into lens elements. In this case, two lenses are either joined together by means of an adhesive in such a way that they each split a surface or are held in such a way that they have a very thin, defined distance from each other, which is filled with air
Material
As a result, the lens designer has more possibilities to achieve the desired properties since now three or four surface shapes and two different materials can be combined. In particular, color errors due to different properties of lenses as a function of the wavelength of the incident light can be greatly reduced.
The surface of a lens is in a simple case a spherical cut, and the strength of the refraction is determined by the radius of the spherical portion. Spherical lenses (from the Greek "sphere") are relatively simple to manufacture, but have an imaging error - the spherical aberration. In this case, incident, parallel light beams are not all bundled at one point, but rather more or less distant from the main axis.
Refractive index
More complex lens shapes are aspheric lenses which do not exhibit this error, but are significantly more complex to manufacture. The surface is designed in such a way that parallel light beams all pass through the same focal point.
In order to reduce the imaging errors, the lens manufacturers combine a wide variety of glass grades, but at the same time increase the price. Another relevant cost item is lens making.
The optical properties of a lens are determined by the optical properties of the individual lenses and elements. In order to influence this, on the one hand, the lens shape and, on the other hand, the material used vary. Depending on the desired effect and technical feasibility, the lens is then made of different materials.
Dispersion
Future
Lens manufacturing
In most lenses, glass is used as a lens material. Flint glass and crown glass form two main groups. Both consist largely of quartz, each modified by their admixtures in their optical or mechanical properties. Flint glass has a higher refractive index than crown glass, but it also has a stronger dispersion.
Since it is much easier to process and therefore cheaper and allows complex shapes, lens elements or complete lenses are also made of plastic. The optics in mobile phone cameras, for example, mostly consist of plastic.
Complex crystals (flourite crystals) are expensive to process and thus expensive. The material used has two main characteristics for optical characterization: the refractive index and the dispersion.
The larger the refractive index, the stronger a light beam is refracted on transition into the material. It is said that the glass is optically denser than air. When a light beam passes from one medium to another, the light beam changes in its direction.
In the transition from air to glass, the light beam is refracted towards the solder; on exiting from the optically denser medium glass into the optically thinner medium air, the light beam is refracted away from the solder. Defined by Snell's law of refraction, the index of refraction is defined by the angle of entry and exit at the transition from the vacuum into the medium to be described. Or alternatively expressed as the ratio of the speed of light in the vacuum to the velocity in the medium.
However, the refractive index of an optical material is not only dependent on the material itself but also on the wavelength of the light. The different color components of the white light, such as red and blue, are therefore broken differently. If now the refractive index varies greatly with the wavelength of the light, the opticians speak of a high dispersion.
A high dispersion results in chromatic aberration, ie the effect that the colored components of the light are not imaged in the same way, thus resulting in color fringing, in particular at edges in the image. In order to minimize the negative influence on the image, individual elements are used in modern lenses which are made of glass varieties with a very low dispersion.
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Nikon identifies lenses with this element, for example with "ED" for "extra-low dispersion", Canon calls these elements "UD" for "Ultra-Low Dispersion" and also the other manufacturers block elements with low dispersion. The dispersion is described by the Abbe number. This is calculated by means of the refractive indices for three different, defined wavelengths of the light.
Lens manufacturing
Now, not only the desired optical properties play a role in the production of optics but also the possible processing of the material. Not every material can be processed at the same time and requires a different amount of effort. The less compromises in optical properties are made, the more specific the glass grades are used, and the more difficult the production.
Simple lenses made of plastic can be manufactured in injection molding in large numbers at a favorable price. Thus, one can explain why complete camera modules for mobile phones are offered for a few dollars.
More complex lenses made of glass or plastic are ground and polished. In doing so, the rotating glass blank is first formed with coarse, then with ever finer abrasive, and subsequently polished. Finally, the lens is cut to ensure that the optical center of the lens is also in the physical center.
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Depending on the glass or plastic, the shaping can also be carried out by pressing. Not all materials allow this, but it is interesting for the manufacturers because it allows a favorable production of aspherical lenses (parabolic curved surface).
After polishing, the lens or the element composed of two lenses is tempered. This coating reduces reflections and optimizes the transmission.
An exciting development are lenses made of metamaterials. These are made up of the smallest elements, so that they represent a new element for the light. This allows hitherto considered properties that are impossible to maintain, such as, for example, negative refractive indices and thus new fundamental lens constructions. To date, these elements have not yet been used outside of laboratories. The future will show whether and how these materials can be used on a large scale.
Also interesting are variable lenses. These lenses behave like our eye lens, so they can be changed in their property by the action of force. The concept promises significantly smaller designs, and the first mini-lenses are now available on the market. Whether these can convince and the technique then also in photo lenses creates, must show.
The combination of several lenses and elements makes the lens clear. The distances and orientations to each other must be strictly adhered to, making a lens a fine-mechanical device.
Metal mountings can usually be manufactured with higher precision, but are more expensive and heavier than plastic fixtures. Since light is not always transmitted through the lens, blackened brackets must be tried to keep the reflection on the various components as low as possible.
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