FOCUS DETECTOR OF LENS INTERCHANGEABLE TYPE CAMERA

摘要

PURPOSE:To detect the focus of even a low-brightness subject with high precision by preventing projection light from behind a photographic lens from being measured even if the projection light is reflected by a lens surface. CONSTITUTION:Light from a light source LED is collimated by a collimator lens CL into parallel luminous flux, which is reflected by a reflecting prism RP; and only infrared light is made incident to a pentagonal prism PP by a composition surface. The infrared parallel light beam is transmitted through the pentagonal prism PP and a focal plate FP to strike a main mirror MM. Only the S-wave component is projected on the photographic lens TL. The P-wave component is transmitted through the main mirror MM and the absorbed by a light shield member. The S-wave component of the infrared parallel light beam projected from the photographic lens TL upon a subject is reflected and scattered by the subject to obtain the mixed light of both S and P wave components, which reaches the main mirror MM again through the photographic lens TL. The S- wave component is reflected by the main mirror MM and never reaches a focusing detection part FD.