Camera Lens Parts: White Button
Camera lens parts on an SLR film camera can get a little confusing because it's a complex instrument. On these cameras the aperture is controlled at the lens with an aperture ring. All you need to do is set the aperture at it's desired size and it would lock into place. If you wanted to change the setting then you need to press down on a white button on the right to release the lock.
If you want to put the camera into shutter priority mode, a function that allows the user to set the shutter speed with the camera automatically choosing the best aperture size for exposure then you have to adjust the ring so that the white button is positioned for the aperture setting. This allows you to surrender control to the camera.
White buttons are not found on the lens for digital cameras because the aperture is controlled through the camera and not the lens. Whether you're in manual or auto mode you cannot control the aperture through the lens. The shutter is also no longer controllable through an external control. Everything in regards to exposure is controlled digitally on a digital camera. The only manual functions one can control through a ring are the zoom and focus.