Optical processing apparatus and operating method of watch

An optical processing apparatus, comprising:

• a light source configured to provide a beam of light to go through a through hole of a rotary shaft to a surface of a rotary part of a button to generate a reflected light beam reflected from the surface of the rotary part of the button;
• an image sensor, configured to capture the reflected light beam to output a first image of the surface of the rotary part of the button by an exposure time length within each frame capturing period;
• and
• a processor, configured to
• calculate an image quality index of the first image,
• compare the image quality index of the first image with at least one press threshold to identify a pressing state of the button within the each frame capturing period,
• calculate a rotational displacement of the button using first image frames of the each frame capturing period, and
• when identifying that the pressing state of the button is between press and non-press by comparing the image quality index with the at least one press threshold, take the rotational displacement calculated between the press and the non-press as undesired movement.