The present invention relates to a single-lens reflex camera having an automatic focus detecting element such as a self-scanning type photoelectric element hereinafter referred to as "an image sensor" when applicable. More particularly, the invention relates to a light shielding device for an automatic focus detecting device in such a single-lens reflex camera.
First, an example of a focus detecting principle, a so-called "contrast method" will be described. The contrast of light and shade of an optical image of an object which is formed by a lens becomes a maximum when the image is precisely focussed. This is due to the fact that the power spectrum of the object's image becomes a maximum when the lens is focused on the object. A variety of devices for automatically detecting a focal position utilizing this phenomenon have been proposed in the art. In one of automatic focus detecting elements employed in such an automatic focus detecting device utilizing the contrast of an object, an image sensor is used to electrically scan an object's image to thereby obtain a time-series signal. The time-series signal is electrically processed to dynamically detect the contrast information of the object's image. The image sensor is used is most commonly a photoelectric element in which a plurality of microphotoelectric elements and a scanning circuit are built. The image sensor is called "a MOS-FET type image sensor" or "a CCD type image sensor" depending on the type of scanning circuit. These devices are commercially available.
An automatic focus detecting device utilizing the image sensor may be made quite compact because it can be constructed with an electrical circuit and a simple mechanism in which the image sensor is arranged at a position equivalent to the plane of the film.
In applying the automatic focus detecting device using the image sensor to a single-lens reflex camera it is necessary to simultaneously introduce a light beam forming the image of an object to the finder system and the focus detecting system. For this purpose, for instance, a technique has been employed in which the beam is divided into two parts using a half-silvered mirror and a total reflection mirror so that one of the two parts is utilized to form the image of the object on the focusing plate in the finder system while the other is used to for the image of the object on the image sensor in the focus detecting system.
In this connection, it is required that the aforementioned mirrors be disposed away from the optical path of the image thereby to prevent the occurrence of vignette on the film surface and to eliminate the occurrence of ghosts or the like on the film surface which may be cause by unwanted light which is reflected by a required protecting plate, such as a glass plate, of the image sensor in the focus detecting system or by the focusing plate in the finder system.
It is well known in the art to provide a frame holding the mirrors which is lifted to cover the focusing plate to block the light reflected by the focusing plate and to prevent the leakage of light from the finder to the film surface. It is absolutely necessary to eliminate the unwanted light reflected by the protecting plate of the image sensor since it adversely affects the picture.
In view of the foregoing, an object of the invention is to provide a light shielding device for an automatic focus detecting element such as an image sensor in a single-lens reflex camera.