Image pickup apparatus

An image pickup apparatus, comprising:

• a lens for focusing light beams from at least one subject to form an optical image;
• an image sensor for receiving the optical image and converting the optical image into at least one electric signal;
• an optical filter disposed in a light path of the light beams and between the lens and the image sensor, wherein the optical filter comprises:
• a first plate disposed between the lens and the image sensor;
• and
• a second plate disposed between the first plate and the image sensor, wherein the second plate has substantially the same index of refraction as the first plate, the second plate has a first surface and a second surface opposite the first surface, the second surface forms an angle with the first surface, and a thickness of the second plate is continuously decreased or increased across the second plate;
• and
• a driving device for moving the second plate, wherein a total optical path length between the lens and the image sensor varies according to a position of the second plate relative to the first plate.