An image sensor, an electric device using the same and a focusing method of the electric device are provided. The image sensor comprises a plurality of image capturing units and a detecting unit. Each image capturing unit includes an image capturing pixel and a first micro lens. The first micro lens is disposed in front of the image capturing pixel. The detecting unit has a focus detecting pixel and a second micro lens. The second micro lens is disposed in front of the image detecting pixel. A length of the second micro lens is different from a length of the first micro lenses.