Traditional Culture Encyclopedia - Photography major - Classification and selection of focusing screens for split-image focusing screens

Classification and selection of focusing screens for split-image focusing screens

The focusing screen is divided into a viewfinder screen and a manual focusing screen according to its functions. Generally, our cameras are equipped with a typical viewfinder screen, which is only used to compose pictures and observe photographic objects, and cannot accurately determine whether the focus is accurate. In addition to the viewfinder function, the manual focus screen can also be used to determine whether the focus is accurate.

Common manual focus screens include split image matte screen, micro diamond screen and split image micro diamond screen. In the center of the split-image screen are two wedge-shaped prisms with crossed bevels. Since the prism bevels intersect at different angles, the refraction angles of light are different. When the focus is behind the split-image prism, a pair of real images is formed, and when the focus is in front of the split-image prism, a pair of real images is formed. For virtual images, the position of the image is opposite to the real image. When focusing, the two images are misaligned and shifted with each other. Only when the focus is at the intersection of the wedge inclined plane, the split images overlap (Figure 2), so the focusing accuracy is very high (Figure 3). The micro-diamond screen is composed of a group of tiny pyramid-shaped prisms. When it is in focus, it forms a clear image (Figure 4). When it is out of focus, the points quickly turn into discrete spots, and the boundaries of the scene appear in a zigzag shape. The principle of microprism is similar to that of a split-image screen. The focusing accuracy is greater than that of a matte screen but smaller than that of a split-image screen. At the same time, there is no semi-darkness phenomenon under a small-aperture lens.

Generally speaking, photographers often choose split-image, micro-crystal, or matte three-in-one screens when modifying their digital SLRs (Figure 5). After trial by the author, I found that Mingjia MCK1000 focusing screen adopts three focusing methods: horizontal split image, annular micro-edge and frosted, and has a Fresnel focusing ring to enhance the brightness of the focusing screen. The dimensions are 28mm×40mm×1.3mm. It is larger than the original focusing screen of all DSLRs and is cheaper (about 20 yuan per piece). It is very suitable for modifying the split-image focusing screen of DSLRs.

Figure 2 Focusing principle of split-image screen

Figure 3 Split-image frosted screen

Figure 4 Micro-diamond frosted screen

Figure 5 Split image micro diamond frosted screen