If the aperture angle is very large, a strong distortion of the optics must usually be expected. Thus it is advisable for many applications to use a lens with a larger focal length and to increase the working distance instead in order to minimise the perspective (measuring) error.

The size of the image y´ is defined by the camera sensor, i.e. it must not be calculated most of the time. The measures are mostly given in inches, however, they are not real inch values but equivalents of historic tube cameras with an outer diameter of the glass tube of 1 inch.

The appropriate optics can be calculated using simple formulae. Yet the results calculated only represent approximate values, since a simple lens system is taken as a basis. In practice, however, 4-7 lenses (groups) interact in order to make the image as error-free as possible. Nevertheless the simple calculations are sufficiently precise most of the time in order to calculate the working distance correct to several millimetres or centimetres depending on the image field.

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Entocentric lenses have a fixed aperture angle. Therefore the object field to be viewed can be increased or reduced by increasing or reducing the working distance.

The object distance refers to the distance between the object (inspected part) and the centre of the optic lens groups. Unfortunately, the free working distance between the object and the front edge of the lens cannot be calculated without detailed knowledge of the lens design, but most of the time it is 2 - 3 centimetres lower.

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Usually no focal length is indicated for telecentric measuring lenses or macro-lenses, which would serve to calculate and select the optics. These lens types are characterised by the image scale β (beta). It can be calculated very easily.

The object size G is usually the range to be detected which must be viewed by means of the camera. This value, too, is normally predefined and known, after all this is our test object with a little bit of surroundings.

The most important parameters for the calculation are the image field, sensor size, working distance and focal length of the optics. The first two values, however, are mostly defined by the application so that only the working distance for a particular focal length, or vice versa, must be calculated.

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The focal length is given in millimetres and is the distance between the optic centre of a lens and the focal point. All rays of light inciding in parallel intersect in this point. The focal length of the optics depends on the optical power of the lens.

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A telecentric lens has the image scale β = 0.1. As a fraction this corresponds to 1/10. A camera with a 1/2" sensor and a size of 6.4 x 4.8 mm therefore serves to capture an object of 64 x 48 mm. If the same lens is used on a 1/3" camera, only 48 x 36 mm can be inspected.

1/1" 9,6 mm × 12,8 mm 16,0 mm (diagonal) 2/3" 6,6 mm × 8,8 mm 11,0 mm 1/1,8" 5,1 mm × 6,8 mm 8,5 mm 1/2" 4,8 mm × 6,4 mm 8,0 mm 1/3" 3,6 mm × 4,8 mm 6,0 mm

On the other hand, extreme wide-angle optics (fisheye lenses as well as endoscopes) serve to solve special inspection tasks. In case of small working distances, the side of a part, a countersink or a bore can be inspected in this way.

The focal length f´ virtually serves to calculate the required lens and is thus the most important specification to characterise an entocentric normal lens. The larger the value of the focal length, the larger its telephoto properties, small focal length figures represent wide-angle and fisheye lenses. In general, lenses with a short focal length tend to have stronger distortion than optics with a longer focal length, however they are mostly more light-intensive and more compact. It can generally be recommended to work rather with a longer focal length in case of large working distances.

Which focal length is required in order to capture an image field of 150 mm when using a 1/2" sensor and 300 mm working distance? y´ = 6.4 mm y = 150 mm a = 300 mm f ' = 300 / ( 150 / 6,4 +1 ) = 12.3 mm The focal length f' is 12.3 mm. In practice, lenses with a focal length of 12 mm or 12.5 mm are produced and marketed. In addition it is important to make sure that the lens meets the quality requirements of the sensor (standard optics, megapixel lens, colour-corrected optics, etc.) and whether the lens is capable of exposing the full size of the sensor. C-mount lenses, for example, are capable to expose at the maximum 1/2", 2/3" or 1" sensors, depending on the design. If the maximum image circle diameter of the optics is smaller than that of the sensor, strong image shading (vignetting) appears on the margin.