What does the back of the middle frame mean? Like mamia, like faith. ...

Have you ever seen a medium format camera? Its film-loading part can be taken off as a whole.

To put it simply: to the three independent parts of the medium format camera: lens, body (including shutter, pentaprism, reflector, metering system, main operation buttons, etc. ) and the back (mainly used for loading films). When you want to shoot color, stick color film on the back, and black and white film on the back.

The medium format camera uses 120 film, which is troublesome to install. The number of shots can be small, and multiple backs can be prepared. After shooting a roll, take it off and put another back on it immediately to continue shooting.

Digital back: there is no film, only the back of the photosensitive element (and of course the screen). If you wear it, it will become a medium format digital camera.

You can't take pictures on your back, and you can't get a camera.