This is a mirror, so it is called a SLR camera because of it. Usually, when the optical viewfinder is used, the light from the lens is reflected through it, so that the light enters the metering screen and the focusing screen, enters the optical five-mirror and finally appears in the optical viewfinder. When the shutter is pressed, it will lift upward, allowing the light to directly hit the photosensitive element to complete the exposure. That's why when it is exposed, nothing can be seen in the viewfinder.