Optical performance is based on acceptable image blur. Blurring occurs due to the movement of the camera relative to the plane of the object. Depth of field determines the maximum allowable detector offset and depends on the F-number (f-number, F / #) value, which determines the amount of light that hits the sensor. The F-number increases as the lens opening (aperture) closes. Closing the aperture (increasing the F-number) increases the depth of field, but decreases the amount of light entering the sensor, so the brightness of the light must be boosted to compensate. The lens specification indicating the depth of focus must also contain information about the value of the F / # parameter at which the measurements were taken.
The value of the depth of field affects the blurring of the image when moving the object. Depth of field is the maximum distance to an object at which it is fully in focus. It also determines the allowed movement of the object (from the best focus). When an object moves closer or further than the working distance, it falls out of focus, and as a result, the resolution and contrast of the image decreases. For this reason, the value of the depth of field is associated with a certain resolution and contrast. Like depth of focus, depth of field can be increased by closing the lens aperture (increasing the F-number); and will also require enhanced lighting.
The depth-of-field range of the lens depends on the effective focal length, the acceptable blur diameter and the nominal back focal length. Some lenses are designed to be hyperfocal, which means that the farthest point in the focusing range is located at infinity. This condition is often fulfilled in lenses with finite focal lengths. The depth of field is large enough and can be changed using the diaphragm.
Telecentric lenses should not be confused with lenses with a large depth of field. Such lenses allow you to adjust the magnification, thereby removing the perspective error and, as a result, all objects of the same size in the image will have the same height, regardless of the distance to them. The scope of such lenses is in computer printed circuit board inspection systems. Telecentric lenses have multiple working distances with a finite depth of field at each point of the working distance, so integrators must take working distance and depth of field into account when choosing telecentric lenses.
In some cases, such as pipe inspection, a greater c can be achieved using multifocal lenses. Multifocal lenses are similar to zoom lenses and are used when the focal length needs to be changed very often. These lenses are very often motorized to smoothly change the focal plane. When searching for defects using such lenses that change the focal length, the pipe can be scanned along its entire length, from sector to sector. Unlike vari-focal lenses, the working distance of the lens also changes, so the instrument may need to be adjusted. That’s all you need to know about depth of field. It might be simple but clear. Thank you for reading.