Single lens reflex camera comprising a focus detecting apparatus and method of photographing

An auto focusing device usable in a camera, comprising:

• an optical unit configured to receive an optical input and to separate the received optical input into a first optical input and a second optical input;
• an AF sensor unit having a first sensor and a second sensor, the first sensor having a first plurality of pixels each configured to receive from the optical unit the first optical input, the second sensor having a second plurality of pixels each being configured to receive from the optical unit the second optical input, each of the first and second plurality of pixels being configured to output an electrical signal that represents a sampling of respective corresponding one of the first and second optical inputs, a first group of pixels including select ones of the first plurality of pixels that collectively output the electrical signals of a first period and select ones of the second plurality of pixels that collectively output the electrical signals of the first period, a second group of pixels including select ones of the first plurality of pixels that collectively output the electrical signals of a second period and select ones of the second plurality of pixels that collectively output the electrical signals of the second period, the first group of pixels having a first arrangement common to its member pixels, the second group of pixels having a second arrangement common to its member pixels but different from the first arrangement;
• and
• a control unit configured to control a focusing of the camera based on a correlation between at least respective electrical signals output by ones of the first plurality of pixels belonging to the first group of pixels and by ones of the second plurality of pixels belonging to the first group of pixels.