TheND filter on the Sony RX100 IV is physical (or mechanical), you can also hear the mechanical noise in the camera when it is activated and it is designed to allow you to use the camera in situation where it would be otherwise too bright for the electronics to handle. One example is when shooting video where you want a particular aperture and shutter speed for depth field and motion blur effects. On a bright sunny day, there might be more light than the sensor can handle at it’s lowest ISO seeing… that’s where the physical ND filter is useful.
