A system for measuring the viscosity of printing ink during a printing and ink correction process includes a printing press having an ink supply system, and an optical measuring device for measuring actual optical values of light that has interacted with at least parts of the printing picture. The printing press has an ink mass determination device to determine the weight of at least parts of the ink located in the ink supply system, and a control and evaluation device to receive measured values from the optical measuring device and from the ink mass determination device. The control and evaluation device determines an optical deviation, and, based on the optical deviation and the values from the weighing devices, an amount of corrective ink that is to be fed to the printing press in order to approximate the actual optical values to optical reference values.