Focus detection apparatus

A focus detection apparatus comprising:

• an image forming optical system which forms an image of an object;
• photo-electric conversion means comprising a first pixel group which receives a light beam passing through a first pupil area of said image forming optical system, and a second pixel group which receives a light beam passing through a second pupil area of said image forming optical system;
• focus detection means for detecting a focus state of said image forming optical system;
• a correction calculator for performing, based on the output from said focus detection means, correction calculation for a first object image generated by said first pixel group of said photo-electric conversion means, and a second object image generated by said second pixel group of said photo-electric conversion means;
• signal processing selection means for selecting, based on the outputs of the object images, optimum signal processing from a plurality of types of signal processing having different characteristics;
• signal processing means for performing the signal processing selected by said signal processing selection means for the object images;
• and
• determination means for determining, based on the output from said focus detection means, whether to perform the correction calculation again,
• wherein the signal processing is selected based on signals before the correction calculation, before said determination means determines that the correction calculation is not to be performed again.