An optical apparatus includes a photosensitive element, a lens and a microstructure unit. The microstructure unit is arranged between the photosensitive element and the lens. After plural light beams passing through the lens are received by the microstructure unit, travelling directions of the plural light beams are changed. Consequently, at least a portion of the plural light beams is guided to the photosensitive element. In such way, the light collecting efficacy of the photosensitive element is enhanced.