Detection mechanism and image pickup apparatus having the same

An image pickup apparatus comprising:

• a detection mechanism;
• and
• an image sensor,
• wherein the detection mechanism includes:
• a rotational operation member rotatable around a rotation axis,
• a phase plate that includes a plurality of reflective portions disposed on a plurality of concentric circles around the rotation axis and configured to reflect light, and a non-reflective portion configured to reflect less light than each of the plurality of reflective portions, and is rotatable together with the rotational operation member,
• a plurality of detection elements arranged on the plurality of concentric circles facing either one of the plurality of reflective portions or the non-reflective portion, and configured to output a signal according to an intensity of light reflected by either one of the plurality of reflective portions or the non-reflective portion,
• one or more memories storing instructions, and
• one or more processors that, upon execution of the instructions, operate to determine a rotation position of the rotational operation member according to an output of each of the plurality of detection elements,
• wherein the plurality of detection elements are arranged on different straight lines that pass through the rotation axis and extend in a radial direction perpendicular to the rotation axis.