Focus detecting device

A focus detecting device comprising:

• image forming lens means for forming an object image;
• an image forming surface, behind said lens means;
• two light source means for producing light, respectively arranged in two optical paths different from each other both of which pass through said image forming lens means and intersect with each other in front of and behind said lens means, said behind intersect being separated from said lens means by a distance equivalent to a distance between said lens means and said image forming surface;
• means for alternately energizing said light source means to alternately project light on an object through said different optical paths;
• photoelectric converting means, arranged in a position optically conjugate with said image forming surface with respect to said image forming lens means and having a single light receiving surface, for receiving the light emitted from said light source means, reflected by the object and returned through said image forming lens means, and for converting the light into a signal indicative thereof;
• position detecting means, coupled to said photoelectric converting means, for calculating a position of incidence of returned light which is incident on said light receiving surface based on said indicative signal, to issue an output signal indicating the position thereof;
• and
• signal processing means coupled to said position detecting means for detecting a displacement of the object image from a predetermined image forming position on the basis of a relative position relationship between the positions of light incidence as indicated by two output signals obtained from said position detecting means.