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Optics

Error-free depiction of the field of view

Entocentric lenses

Entocentric lenses are the most commonly used lens types. Fixed focal lengths and zoom lenses are based on this optical principle and correspond to the human perspective. An entocentric normal lens has a fixed aperture angle of the optics to both the object and the sensor. In the case of entocentric zoom lenses, the angle of view is variable.

Lenses with an aperture angle similar to the human eye are referred to as normal lenses. They have an aperture angle of approx. 50 degrees, with full-frame sensors of DSLRs the sensor diagonal corresponds approximately to the focal length. Be careful when comparing "focal lengths": This information is not comparable if you only look at the absolute numerical value. It is only meaningful if you know the image size (camera sensor).

Characteristic features of entocentric lenses

Objects of equal size which are closer to the optic centre (and thus closer to the camera) are depicted larger. If they are further away, they are depicted smaller.

Even when using fixed focal lengths, the object range to be detected can be adjusted quite well in this way: with an increased working distance, the image detail becomes larger, when reducing the distance, it becomes smaller.

Entocentric lens: Change of image size using different working distances

The working distance can be reduced until the minimum object distance (MOD) specified by the manufacturer is reached. This can be reduced even further with the aid of extension rings, but the user has to accept an increase in optical image errors.

This effect can be very disturbing with wide-angle lenses, as the object is viewed vertically in the centre of the image and increasingly obliquely towards the edge of the image.

A useful measurement can no longer be guaranteed on such images, especially if the parts cannot be positioned precisely. However, fisheye lenses can also be used in this way to inspect the inside of a hole or the side walls of a part. However, this requires extreme focal lengths and very short working distances.

Lens with 25mm focal length

In the middle vertical view of the screw, at the image corners a slight oblique perspective.

Lens with 6mm focal length

At the edges, the perspective is extremely oblique in the direction of the screw thread, in the centre still perpendicular.

As the aperture angle increases (or the focal length decreases), geometric distortion typically increases as well. This is caused by imaging a hemispherical optical space in the flat two-dimensional plane of the camera sensor.

Entocentric lens: image field curvature at sensor level

Expensive lens designs with aspherical lens elements can reduce this effect of image field curvature.
Microlenses can also cause problems on modern sensors if the chief ray angle of the optics becomes too large on the sensor side.

This can cause shading at the edges of the image because the light rays hit the sensor (microlenses) at an larger angle at the edge of the sensor.

Typical applications for entocentric lenses

Entocentric standard lenses are used for most applications such as presence and position checking, pick and place applications, print image inspection, colour applications, OCR and barcode reading.

Entocentric lenses are rather not suitable except for measuring tasks and the inspection of very small image areas (macro and microscope optics).  For these applications, either telecentric or macro lenses are used. However, if simple measurement tasks are to be carried out with non-telecentric lenses, it is generally recommended to use long focal length lenses and to increase the working distance. Only telecentric lenses provide an absolutely parallel perspective without the effects described above.

Adjustment possibilities on the lens

Except for extreme wide-angled and fisheye lenses, entocentric lenses usually provide two adjustment options.

Further information can be found in the "Optical Basics" chapter; helpful online wizards for calculating the required focal length, working distances, aperture angles, etc. can be found in the "Service - Optical Calculations" chapter.

Important for Machine Vision

  • Make sure your lenses are of high optical quality. They should be able to reproduce fine structures on the sensor. Above all, the pixel size of the camera sensor is crucial. The smaller the pixels, the higher the resolution of the lens! 

  • Apochromatically corrected lenses are particularly suitable for colour applications and/or white light illumination (avoidance of chromatic aberration). Specially corrected lenses are also available for IR or UV applications, where the lens design and anti-reflection coating (spectral transmission) are adapted to specific wavelengths.

  • Lenses with locking screws to prevent accidental adjustment of the optics are a must for industrial applications.

  • There should always be a filter thread on the front of the lens. This makes it easy to mount polarising filters, colour filters, etc. to suppress interference. The exceptions are often extreme super wide angle and fisheye lenses with highly curved front elements, where it is usually not possible to mount a filter.

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Optiken Industrielle Bildverarbeitung