A lens ensures a sharp and correct image of the test object in industrial image processing. Important parameters such as sharpness, contrast, luminous intensity and distortion can be optimized.
- C mount: Standard connection 1"-32 UN, 25.4 mm diameter, flange distance 17.5 mm
- CS mount: 1"-32 UN, 25.4 mm diameter, 12.5 mm flange spacing (compatible with C mount via adapter)
- F mount: Bayonet connection with approx. 44 mm diameter, usually for high-resolution or large sensors (can be adapted to C mount)
- S mount: M12 × 0.5 mm thread, common for embedded systems
The choice of lens connector depends on compatibility with the camera connector and sensor format. In open systems, C mount lenses are mostly used in industrial image processing, while integrated lens systems with auto-focus modules often use S mount connectors.
The aperture setting affects both the amount of light on the sensor and the depth of focus.
A small aperture increases depth of focus, but reduces the amount of light. A large aperture lets more light into the sensor, but significantly reduces the depth of field. A smaller aperture in combination with external illumination is the best choice for many applications.
Distortion refers to the geometric distortion of an image:
- Pincushion distortion (1): Edges are curved inwards
- Barrel distortion (2): Edges are bent outwards
Precise measurement applications require low-distortion lenses. In many cases, a software calibration can correct the distortion.
Telecentric lenses focus not only in the image center, but also at the edges of the image vertically on the object. They generate images with a fixed image scale regardless of the object distance and are used for:
- Accurate measurement applications
- Objects with height differences
Entocentric lenses, on the other hand, show lenses in perspective.
A lens is optimally focused by first fine-focusing in the live image until the maximum transitional contrast at the edges is reached. The aperture should then be adjusted so that the minimum depth of field can be reached and the correct level of focus can be determined for the relevant application.
After focusing, the focus is secured with locking screws to ensure a stable setting even when vibrations and shocks occur. For industrial systems, motorized focusing units are also available, which enable precise and reproducible focusing.
To select a suitable lens, the following questions should be clarified:
- Does the thread fit on the camera?
- Is the focal length suitable for the sensor and field of view?
- Is the resolution sufficient in relation to the pixel size?
- Is the image circle large enough for the sensor?
- Is there distortion or edge blur?
- Does the lens need to be particularly robust?
- Is the luminous intensity sufficient for the exposure time?
- Is a protective tube required?
