An optical method of inspecting containers comprises taking an image of each container and determining a search zone in each image of the container, a visible pattern appearing in the search zone. A digital mask is prepared for a treatment zone of the images including the visible pattern and at least each pixel of the treatment zone of the images is compared with a digital mask. A visible pattern is selected belonging to the container and the position and the orientation of the selected visible pattern in said search zone is determined. A geometrical transformation is applied to the digital mask or to the treatment zone to place the mask and the treatment zone in a position in which they coincide. Image treatment is applied to each pixel of the treatment zone, which treatment depends on the intensity value of the coincident pixel of the digital mask.