A sharp image requires more than just a good camera; it also takes the right lens. Several aspects must be considered to make sure that the camera and lens work perfectly together and fit a specific application.
The sensor size is a decisive factor in selecting the right lens. In the past, high-resolution area-scan and line-scan cameras had larger sensors than cameras with lower resolution. Today’s features have changed. Since the sensors have become ever smaller, it is more difficult to resolve the tinier pixels. The dimensions of sensors are not determined by any standard, but are defined by the resolution and the pixel size of the sensor. Theoretically, anything is possible here; it is only a question of price.
The camera mount is the second factor to consider when choosing a lens. Mounts come in standard sizes and are labeled according to the screw-threaded type of the camera body. Your camera and the lens should therefore have the same mount. A C mount, for example, is the most common mount in machine vision cameras and is appropriate for a sensor diagonal of up to 20 mm – corresponding to a sensor size of 1.5 inches. Larger sensors often require a larger mount, like the F mount. The size, however, is not very common in industrial machine vision applications; CS and S mount lenses are more commonly used.
Although machine vision sensors – and their pixels – have grown smaller over recent years, the image circle sizes on the matching lenses for those cameras have not changed. The majority of sensors used today are smaller than 1⁄2", but must work with lenses for machine vision applications with an image circle, or circle of light transmitted by the lens, of 2⁄3", as this is the most common type for machine vision lenses. As a result, a large section of the lens’s image circle is not used.
Ideally, a 1⁄3" C mount lens should be mounted on a camera with a 1⁄3" large sensor. The available image circle is then optimally used. If the same lens were attached to a camera body with a 1⁄2" sized sensor, a defect called vignetting would occur.
Vignetting is a reduction of an image’s brightness from the center to the edges. Assuming that a 2⁄3" lens with the same focal length is used with a 1⁄3" sensor, vignetting will not occur. The angle of view, however, will change.
In principle, the angle change is an advantage; by using the bigger lens, a larger image circle is created, which means that the brightness of the image remains more consistent from the center to the edge. A large portion of the image circle, however, is not used, which is a waste of money.
The size of the lens’s image circle does not matter; the object size is determined by the sensor size and the focal length of the lens. The larger the image circle of the lens, the more expensive it is. For a smaller sensor, including ones smaller than 1⁄2", you should use an appropriate lens with a smaller image circle.
Resolution, Pixel Size
A high-resolution image can only be created if a high-resolution lens is used. In order to get a first-rate highresolution image, you need more than a high megapixel count. The lens must also be capable of resolving the pixel size. The resolution of a lens is given in line pairs per millimeter and specifies how many line pairs on a millimeter appear as separate from one another. The more line pairs appear as differentiated, the better the resolution of the lens.