Optics - Basic knowledge

During the design and manufacture of a lens, unavoidable aberrations occur. In industrial image processing they are undesirable, in photography and film they are sometimes deliberately used. Typical aberrations include spherical and chromatic aberration, distortion, coma and astigmatism.

Spherical aberration (left) is an error that leads to loss of sharpness in the image. Light rays in the peripheral area of the lens are not imaged to the same point as rays through the centre of the lens. This results in a slightly blurred circular area rather than a single point in the image plane (sensor). Chromatic aberration (right) occurs when different wavelengths of light are refracted to different degrees. This becomes visible in the image, for example, as a coloured edge at the edges.

Distortion is a geometric aberration that distorts the image. Straight lines are partly displayed in a curved way. The error is particularly visible at the edges of the image. Depending on the focal length and type of lens, the distortion is pincushion-shaped (left, especially with telephoto lenses) or barrel-shaped (right, especially with wide-angle lenses). The centre shows an image without distortion. Distortion can be easily corrected by image processing software.

Coma is an asymmetry error in optics and occurs outside the optical axis in the image field when not all rays meet in the same point. In the image, coma often shows up as a tail or a drop-shaped image of a pixel. Astigmatism occurs when horizontal and vertical ray bundles meet in different image planes. Depending on the focus, only horizontal or vertical structures are then sharply imaged as a point and the other as a line. Astigmatism also occurs, for example, in the case of astigmatism of the cornea in the eye and can be corrected there by means of cylindrically ground glasses.

 In physics, optics is the study of light. It deals with light sources and propagation, reflection and refraction of light. Optical images, the function of lenses, magnifying glasses and cameras as well as colours are also part of this field. At Schneider-Kreuznach, optics are manufactured in the fields of industrial image processing, photography and film. 

In industrial image processing, the areas of application are diverse: from quality control in production to flashers in road traffic or as a partner to science: Schneider-Kreuznach has already cooperated with NASA several times, such as in 2021 on a rocket flight to the James Webb Space Telescope. In the photographic sector, Schneider-Kreuznach sells high-quality photo filters under the B+W brand. These can be used to create a specific atmosphere or to avoid unwanted effects such as reflections. With photo filters, a good result can be achieved right from the start without having to use an image editing programme later. This is because image information cannot be added later, because software is mainly suitable for retouching. In film, you want to convey emotion and mood. Diffusion filters such as the "Hollywood Black Magic" or the Radiant Soft give a soft shimmer, conceal skin blemishes and conjure up a beautiful image. The goal in film is a beautiful bokeh, i.e. a successful transition between sharp and blurred image areas. Our filters are used for shows or films: The True-Streak effect filter was used during the filming of The Weeknd's halftime show at the Super Bowl in 2021. The Radiant Soft filter was used in the shooting of the German series "PARA - Wir sind King".
 

The simplest form of camera is a pinhole camera. It is a black box with a hole through which light falls onto the opposite side, creating an upside-down and side-inverted image. A modern camera is nothing different in principle. In place of the hole is the lens and where the image is formed is a light-sensitive sensor or film. If the lens is detachable, usually only the housing with the sensor and lens mount is called a camera.

More information on this can be found in the Basics - Lenses.

Optical lenses have different shapes, functions and applications. The light is refracted at the air-glass transitions and changes its beam direction. In general, the more curved a lens is, the more light is refracted. Lens surfaces are usually convex (outwards, see illustration on the left) or concave (inwards, see illustration on the right). A typical biconvex converging lens has two outwardly curved surfaces and focuses the light. A typical biconcave diverging lens has two inwardly curved surfaces and diffuses the light. 

In lenses, a combination of different lens shapes is used to reduce aberrations as much as possible and thus achieve the sharpest and most contrasting image possible on the sensor.

Light is a form of electromagnetic radiation whose properties can be described by various models. The wave model and the radiation model are often used. Light is visible to the human eye in different colours when the wavelength is in the range of approx. 380-750nm (nanometres). Below 380nm we speak of the ultraviolet (UV), above 750nm of the infrared (IR) range. Blue light is short-wave, while red light is long-wave.

In principle, light propagates in a straight line, which is why we also speak of light rays and represent them graphically as lines. Accordingly, the path of light through a lens is called a ray path. A multitude of light rays form a light bundle that usually spreads out conically from the light source. Light sources are divided into natural light sources, e.g. the sun, and artificial light sources, e.g. luminaires and flash units for illuminating objects in photography, videography and industrial image processing.

When objects are illuminated, part of the light is reflected and can be perceived by a camera or even the eye of a viewer. Depending on how much light and which colours are reflected, the object appears brighter or darker and also coloured accordingly. Light that is not reflected is either absorbed or transmitted. Black objects absorb the incoming light almost completely, while polished glass transmits the incoming light almost completely. Lenses, for example, are blackened on the inside as best as possible to minimise unwanted reflections from the mechanics. The lenses, on the other hand, are polished as best as possible and usually anti-reflective coated so that the light rays are disturbed as little as possible on their way through the lens to the sensor/film at the lens surfaces.

In order to manufacture optical lenses, glass is first needed. This is produced by companies such as SCHOTT from various materials through a process of melting and cooling. Schneider-Kreuznach buys a wide range of glass types, as glass with different refractive properties is needed to manufacture a lens.

Individual lens blanks are cut out of the delivered glass blocks. These are then ground, polished, centred and tempered by precision opticians (m/f/d). During processing and after completion, the precision opticians use high-precision measuring instruments to measure whether the lens has the desired properties. It is a matter of thousandths of a millimetre. Once the lens is finished, trained industrial mechanics (m/f/d) assemble it together with mechanical components into a finished lens. Dust-free clean room conditions are important here. 

The mechanical components are produced in advance in the assembly department by cutting machine operators (m/f/d), among others. Different trades have to work together before a finished lens can be made from a glass block and a metal tube.

The focal length affects the angle of view a lens will produce for a given sensor size. The shorter the focal length, the larger the angle of view of the lens (wide-angle lens). The longer the focal length, the smaller the angle of view of the lens (telephoto lens). We speak of a standard focal length when the focal length roughly corresponds to the sensor diagonal - below this is a wide-angle lens, above it a telephoto lens.

Standard lenses have a focal length that roughly corresponds to the sensor diagonal. In the 35 mm format, for example, the diagonal is 43.2 mm; accordingly, lenses with focal lengths of approx. 40-60mm are called standard lenses. The image impression created by standard lenses corresponds more or less to human visual habits. Lenses with a focal length significantly shorter than the sensor diagonal have a larger angle of view than standard lenses and are therefore called wide-angle lenses. In the 35mm range, these are usually focal lengths of 35mm and shorter. Wide-angle lenses are often used in landscape and architectural photography, where the effects of an accentuated foreground combined with a disproportionately reduced background can create a very interesting pictorial effect. Wide-angle lenses are also often a solution when a subject needs to be shot to fill the format from a relatively short distance. For the lens manufacturer, it is a great challenge with wide-angle lenses to reduce distortion and vignetting to such an extent that they are no longer perceived as disturbing in the images. In contrast to wide-angle lenses, the focal length of telephoto lenses is always greater than the sensor diagonal. This results in an angle of view that is significantly smaller than that of wide-angle and standard lenses. Telephoto lenses are often used where details of a larger scene need to be shown in full detail or where a greater shooting distance needs to be bridged. Typical examples are animal and sports photography, but also paparazzi often use telephoto lenses in order to be able to photograph unnoticed from a greater distance. Due to physical conditions, telephoto lenses usually have a relatively large front lens and are also usually significantly longer and heavier than standard and wide-angle lenses.

Many lenses have a filter thread at the front. A filter, but also other accessories such as a lens hood or a special filter holder, can be screwed in there. You can find more information on this in the explanations on filters.

In order to focus at a certain distance, individual or all lenses in the lens are moved. In terms of the human eye, the focus is the point that we see sharply. If we are too close or too far away from an object, we see it out of focus. Then the focus must be corrected.

The so-called camera mount is a thread or bayonet that connects the lens to the camera. This allows different lenses to be used on one camera.

Lenses can have different requirements: In film, we want to tell a story and depict moods; in industry, what counts is an image that is as sharp and detailed as possible. The same applies to our filters. Depending on the application of the lens, you have to pay attention to the following:

•    At what distance does the lens need to focus?
•    Do you want to focus on something in motion or at a standstill?
•    Are certain wavelengths of light (for example long-wave IR radiation) of interest that are invisible to the human eye?
•    How big does the lens have to be to be able to image my object?
•    Do I want an image that is as sharp and detailed as possible or a beautiful bokeh (balanced transition between sharp and blurred areas of the image)?

For this reason, we explain separately the areas of application of industrial and film lenses.

Machine vision can be used for many things: Quality inspections in production, traffic monitoring, medical technology or in space travel. In many areas of life, optical systems are integrated into our everyday lives without us being aware of it. Due to the diverse applications, there are lenses with different properties in all kinds of sizes and shapes.

Schneider-Kreuznach also manufactures customised lenses if a standard lens cannot meet all requirements. The following standard lenses can be found in the Industry business unit: 

•    Lenses with fixed focal length, e.g. our OPAL industrial lens.
•    Telecentric lenses, e.g. our SYLVINE lenses
•    SWIR lenses, e.g. our industrial lenses CHAROITE, CUPRITE and CRYOLITE
•    Lenses with extremely high light intensity, e.g. our industrial lens ONYX
•    Lenses for long distances, e.g. our TOPAZ lenses
•    For capturing the smallest details, e.g. our DIAMOND lenses
•    Lenses for line sensor applications, e.g. our SAPPHIRE lenses
•    Lenses with additional beam splitter, e.g. our ZIRCONIA lenses

With film, the shots are not viewed individually, but 25 frames/second. Our eye cannot capture every single frame, but perceives the movement with a certain amount of blurring. The requirements for cine lenses are therefore different from those for photo and industrial lenses. Besides a sharp motif, camera people are also interested in bokeh and flares. The out-of-focus areas and the sharpness gradient as well as the reflection of the highlights are crucial when choosing the right lens. 

Typical of cine lenses is that they are usually offered in sets, consisting of several focal lengths of one series, they have the same colouring and the same image effects, this makes post-processing easier. Film lenses are easily recognisable by their long focus travel and gear rings for follow focus. It is also typical that the aperture and focus ring are in the same place for all focal lengths. This saves time and therefore money when shooting.

In contrast to other lenses, the aperture on film is given in T-stops, i.e. the amount of light that passes through the lens, and not in F-stops, the size of the aperture. In addition to the classic spherical (also called prime) lenses, there are also special lenses such as anamorphic lenses for widescreen photography. Schneider-Kreuznach ISC04all combine all of these. The set consists of 3 mini prime lenses the ISCOSpherical in the focal lengths 43, 58 and 85 mm for classical shooting or with the ISCORAMA attachment for anamorphic shooting.

Anamorphic lenses allow widescreen images to be captured without the need for a widescreen sensor. They are often used in the film industry to create a "cinema effect" with a wide image. During shooting, the image is squeezed so that it can be completely imaged on the sensor. In post-production, the image is desqueezed to the original aspect ratio. Macro and micro lenses are special lenses optimised for capturing very small objects. In photography, they are often used for insect and flower photography. In industry, they are used where very small defects need to be found, such as in the automatic inspection of mobile phone and computer displays. And of course, microscope lenses are known from the medical field, where they are an important component of microscopes.

With shift lenses, the optical axis can be shifted in relation to the sensor. If you have ever photographed tall buildings with the camera tilted upwards when travelling, you know the plunging lines that make the building appear distorted. If you use a shift lens, on the other hand, you can photograph the tower or skyscraper with the camera pointed horizontally so that the lines are shown vertically on the image, which appears much more natural to the human eye. Mirror lenses are a special type of telephoto lens. In addition to lenses, at least 2 mirrors are built into them. This makes the lens much shorter, lighter and usually cheaper. However, the imaging performance is worse compared to conventionally constructed lenses. In addition, the image in the out-of-focus areas shows the typical curls of mirror lenses, which are perceived as unnatural by most viewers. For this reason, mirror lenses have never become established, at least in photography. In contrast, mirrors are often used in telescopes for optical imaging. 

Telecentric lenses have a very special imaging property that distinguishes them from all other types of lenses and is mainly used in industry. Telecentric lenses show objects as if you were looking at them from a very great distance. For example, if you look into a pipe with a normal lens, you will always see the inner wall. With a telecentric lens, on the other hand, you will not see the inner wall and the tube will only appear as a ring. This makes it very easy to determine the diameter, e.g. using image processing software. Accordingly, telecentric lenses are mainly used in systems for measuring components. Another interesting fact is that the front lens of a telecentric lens always has to be larger than the object to be measured do not show the "classical" focal length. Due to the changed angle, partial areas are not imaged. For example, if you look at a cooking pot from above, the telecentric lens would capture the rim and bottom of the cooking pot, but not the inner walls. Telecentric lenses are mainly used in industrial image processing to perform exact and reproducible measurements. With tilt lenses, the optical axis can be changed in relation to the sensor and tilted or swivelled. This changes the position of the focal plane in the image focus area. This can be used, for example, in landscape or architectural photography when foreground and background are to be equally sharp. But they are also used in product photography, where the plane of focus can be placed along a tabletop, for example, so that all the products presented are in focus even at different distances. This is also an advantage in architectural photography: if you stand close in front of a large building, you will not get it completely in focus. By tilting the tilt lens, the angle changes and you get a sharp photo. Tilt-shift lenses combine the possibilities of tilt and shift lenses.

In digital cameras, the image of a photograph is captured by a sensor: When light hits the sensor, it is converted into electrical charges. These charges contain information about the colour and brightness of each individual pixel. The data is digitised and stored in an image processor. The more pixels a sensor has, the higher the resolution of the image - which is accordingly expressed in pixels.