Back in the days of 35mm film, a 50mm prime was considered a 'standard' lens. That's because it gives pretty much the same perspective as viewing a scene with the human eye, without the magnification of a telephoto lens or the shrinkage a wide-angle lens uses to squeeze more into the frame.
The flexi disc (also known as a phonosheet or Soundsheet, a trademark) is a phonograph record made of a thin, flexible vinyl sheet with a molded-in spiral stylus groove, and is designed to be playable on a normal phonograph turntable.
Generally, writable DVD discs come in 12 cm (120 mm) and 8 cm (80 mm) diameter sizes. The most commonly used is the larger 12 cm type which has the same physical dimension as most commercial video, audio, computer software and game console DVDs. 8 cm discs are less common and are typically used in portable consumer electronic devices such as digital video camcorders.