An input unit for a computer has a housing with a transparent plate at one side thereof over which an input element is movable. The housing contains a camera which identifies the position of the input element and converts this position into a position output signal. An evaluation unit converts the position output signal into control signals for the computer according to a mouse protocol. The input unit can thus be used to control the computer in a manner which mimics the control of a computer using a conventional mouse, but avoids the disadvantages associated with conventional mouse control. The input element may be an element specifically dedicated for that purpose, or may be a tool which the user happens to have available, or may simply be the finger of a user.