Summary
In vision inspection, obtaining a high-quality, clear image requires not only a capable industrial camera, matched illumination, and efficient inspection software, but also a lens appropriate for the camera. Selecting a suitable camera lens involves considering focal length, aperture, field of view, and other parameters. Different settings affect image sharpness and brightness.
Choosing an Industrial Camera Lens
Different lenses will affect the resulting image depending on their adjusted parameters. In practical vision inspection, engineers primarily adjust the following parameters.
Focal length
Longer focal length increases magnification and is suitable for applications that require shooting from a greater distance.
Aperture
Aperture size determines the amount of light entering the lens. A larger aperture admits more light, making it useful for imaging in low-light conditions.
Field of view
Field of view determines the range of objects in the frame. A smaller field of view narrows the visible area, making objects appear larger in the image.
Astigmatism
Astigmatism affects image sharpness and chromatic aberration. Lower astigmatism produces better imaging performance.
Distortion
Distortion impacts image accuracy and stability. Less distortion yields more accurate imaging.
Camera interface
Common camera interfaces include C, CS, S, and F. Different cameras require matching interfaces.
Operating environment
Decide on dustproof, waterproof, and shockproof requirements based on the installation environment of the vision system.
Lens cost
Select a lens with the best cost-performance ratio according to the actual inspection requirements.
Lens Mount and Sensor Format
Before considering specific factors for selecting a lens, you should first consider the available types of lenses. After choosing a lens, the next step is proper mounting to the camera. Sensor size is a decisive factor in selecting the correct lens. High-resolution area-scan and line-scan camera sensors are larger than low-resolution sensors. Sensor size is given in inches and corresponds to the diagonal length of the sensor; the article states 1 inch equals 16 mm.
The technical term for lens interface size, the "target surface," is similar to chip size. The target surface diameter is measured in inches. Ideally, a 1/3" C-mount lens should be mounted on a camera with a 1/3" sensor to fully utilize the available target surface. If the same lens is mounted on a camera with a 1/2" sensor, vignetting will occur. Conversely, using a larger lens with the same focal length on a smaller sensor will avoid vignetting, although the field of view will change. In principle, using a larger lens can be advantageous because it forms a larger usable target surface, allowing image sharpness from center to edge to remain consistent. However, a large portion of the target surface may go unused, which can be wasteful. In that case, lens size becomes less relevant and image size is determined by sensor size. Larger lenses generally cost more.
Lens Resolution
Higher resolution produces sharper scanned images. However, higher pixel counts are not always better if the lens cannot resolve those pixels. Lens resolution is measured in line pairs per millimeter (lp/mm), indicating how many lines per millimeter can be resolved. The greater the number of resolvable line pairs, the higher the lens resolution.
▲ Different resolution effects
The MTF curve (modulation transfer function) describes lens resolution performance from the image center to the edge. The reference for calculating the MTF curve is a black-and-white line pattern with progressively finer spacing, from which the maximum resolution in lp/mm can be read. Lens resolution helps determine the pixel size that can be resolved. Lens manufacturers often specify the number of megapixels the lens can resolve. For example, a sensor with 5 megapixels has 5 million image points, so a lens capable of resolving all 5 million pixels is required.
System Resolution
The most important parameter for a machine vision system is system resolution. Engineers apply various theories and techniques to improve system accuracy because the higher the system resolution, the more valuable the device. The optical system resolution and the camera's image resolution are like two planks in a barrel; the imaging system resolution equals the shorter of these two planks.
