1. An apparatus for detecting a movement direction of an object, the apparatus comprising: a lens having an axial chromatic aberration between a first ray and a second ray, the second ray having a wavelength greater than a wavelength of the first ray, the lens configured for converging the first and second rays from the object; an image sensor having a flat sensing surface spaced a distance from the lens for receiving the converged first and second rays, the image sensor being capable of forming the converged first and second rays into a first ray image and a second ray image associated with the object respectively, and converting the first and second ray images into a first electronic image signal and a second electronic image signal respectively; and an image processor electrically connected to the image sensor and configured for comparing image quality of the first and second image signals, and analyzing whether the object is closer to an object plane associated with the first ray or closer to an object plane associated with the second ray when the object moves to different positions, and determining the movement direction of the object based on the analyzed positions of the object relative to the object plane associated with the first ray and the object plane associated with the second ray.