Automatic focus apparatus

A combination comprising optical means for focusing an image and further means for providing an output signal representative of the direction said optical means is displaced from a predetermined position of optimum focus, said further means comprising spatial filtering means arranged to transmit spatial frequency components of said image to provide recurring fluctuating light flux patterns that are characteristic of the displacement of said optical means from said predetermined position of focus, said spatial filtering means including an interposed rotatable disc and means for rotating the latter, said disc having alternate relatively transparent and opaque radial sectors, the widths of said relatively transparent sectors being randomly different to keep said patterns characteristic of said displacement in the presence of images which tend to produce misleading out of phase effects in said patterns, said randomly different widths lying in a range which permits adequate transmission of said spatial frequency components for the spatial frequencies present in the images typically encountered in general photography, and means responsive to said patterns for producing said signal.