Want to know why all the photos taken by SLR cameras are black?

Overexposure can lead to black films. That means you can't use it, but you can't see where the exposure parameters may be wrong. At this time, the most direct way is to set the exposure mode to green automatic file, which is equivalent to a stupid camera. Everything is set automatically by the machine, avoiding the mistake of human setting. However, the advanced professional SLR may not have a fool file, so you can try it with the program exposure file (P file).