Quote: http://blog.sina.com.cn/s/blog_77cb9a050102vrp9.html
The lens separation phase detection method is currently used by most AF SLR cameras, and it is also a method of passing through the photographic lens. In order to distinguish it from Honeywell's TCL components, the same name is no longer used. The lens separation detection device consists of a set of separation lenses and one or more sets of ranging components (or AF sensors) composed of photosensitive elements. At present, there are three categories of photosensitive elements: the first category is CCD photosensitive elements, which are used by many manufacturers; the other two categories are BASIS (BAse-Store Image Sensor, base storage image sensor) and CMOS BASIS developed by Canon. The photosensitive elements on the ranging assembly are arranged in an array (usually in a row). The above-mentioned photosensitive element is characterized in that a certain electrical signal will be generated under the irradiation of light.
The function of the split lens is to split the light passing through the photographic lens into two beams, adjust the focus, and project them onto the ranging component respectively.
The working principle of the phase detection method is described below with the distance measuring component of the CCD photosensitive element. The working principle is shown in Figure 2-11. When the focusing is accurate, the distance between the two beams of light generated by the split lens projected on the CCD array is constant, so that the pair of CCD elements on the CCD (remember that this is an array) is irradiated by the beam of light. The location is also fixed. The distance between the pair of CCD elements has been set during the design of the camera as a benchmark for focus detection.
When focusing is accurate, see Figure (a), we use AB to represent the distance between a pair of CCD elements as a reference. When the focus is not accurate, there are two possibilities. One is that the focus of the lens is in front of the subject, as shown in (b). At this time, the distance between the two light-receiving CCD elements is shorter than AB; in another case, the focus of the lens is behind the subject, as shown in (c), and the distance between the two light-receiving CCD elements is longer than AB. , According to the distance between a pair of CCD elements receiving light, it can be identified whether the focus is accurate. The electrical signals generated by the two CCD elements pass through the conversion circuit and the analog/digital conversion circuit, and then are sent to the CPU (Central Processing Unit) in the camera. The difference value can be used to calculate the defocus amount (ie the difference between the actual focus and the exact focus) and the defocus direction.