The present invention relates to a focus detecting optical system that is installed in a film camera or a video camera that exposes or captures a subject image formed by an objective lens for detecting a focusing condition of the objective lens with respect to the subject to be taken.
An autofocus single lens reflex camera is provided with a focus detecting optical system. FIG. 5 shows an optical system of a conventional single lens reflex camera. Light from a subject of a photograph is incident on an taking lens 1, a major portion of the incident light is reflected by a main mirror 2 to form an image of the subject on a focusing screen 3. A user observes the image of the subject via an eyepiece lens 4 and a pentagonal prism 5 that constitute a finder optical system.
The main mirror 2 is provided with a half mirror portion in the center thereof. The transmitted light through the half mirror portion of the main mirror 2 is reflected by a sub mirror 6 toward a focus detecting unit 8.
The main mirror 2 and the sub mirror 6 move to the side out of the optical path such that an image of the subject is formed on a film 9 at the time of the exposure.
The conventional focus detecting unit 8 includes, as shown in FIG. 6, a field mask 8a, a condenser lens 8b, a separator mask 8g, a pair of separator lenses 8c and 8d, and a pair of line sensors 8e and 8f that are arranged in this order from the side of the sub mirror 6. The field mask 8a is substantially coincident with an equivalent film plane F on which a primary image of the subject is formed through the taking lens 1. The separator mask 8g has a pair of openings to limit the light to be incident on the separator lenses 8c and 8d. The separator lenses 8c and 8d re-form the primary image into a pair of secondary images. The line sensors 8e and 8f capture the secondary images.
The positional relationship between the secondary images formed on the line sensors 8e and 8f varies in accordance with the focusing condition of the taking lens 1 with respect to the subject. Therefore, the focusing condition can be detected on the basis of the relationship that is calculated from outputs of the line sensors 8e and 8f.
However, since the conventional focus detecting unit 8 detects the focusing condition of the taking lens only at the center of the taking area, it cannot meet requirement of users to detect the focusing condition of the taking lens with respect to a subject at any point in the taking area.
In order to meet the requirement, the focus detecting optical system must be provided with area sensors as image sensors and must expand the range of the light taken into the focus detecting unit 8. However, since the expansion of the range of the taken light with the above descried optical system increases the sizes of the sub mirror 6 and the condenser lens 8b, it is hard to install such a focus detecting optical system in a limited space of the single lens reflex camera.
The focus detecting optical system disclosed in the Japanese Patent Provisional Publication No. Hei 9-184965 uses area sensors to enable focus detection at any chosen point in the taking area and a concave mirror as the sub mirror to expand the range of the taken light without upsizing of the focus detecting optical system.
However, the convergent mirror is tilted with respect to the optical axis in the optical system disclosed in the publication, which causes distortions in the secondary images. This may cause a mismatch between the intended focus detecting point in the taking area and the actual detecting point on the area sensor, which may obstruct the accurate focus detection with respect to the intended subject.