Abstract: Installation to measure the limit of visibility in a fog, comprising a motor of a control device effecting the setting of the variable focus of the optical apparatus (1) and simultaneously driving one or several moving assemblies (8) displaceable with respect to one or several stationary assemblies (9). In each combination of a moving assembly (8) and the corresponding stationary assembly (9), one of the two assemblies comprises at least one detector (11) or sensor (12) of numerical and/or analog information consigned to the other assembly and having as its object the light of the series of lights upon which the variable focus is set.