Nowadays more than two billion people own smartphones, without necessarily realizing they have a mobile physics lab that can be used to study acoustics, mechanics, astronomy, optics, fluids mechanics, or even serves as an oscilloscope. All smartphones have at least one camera, which is probably the most well-known and used sensor of this device. It allows us to take pictures, videos, to scan QR codes, etc. In this article, we show how to study simple ray optics with smartphones and how to determine the focal length of the smartphone camera, which could be an interesting and interactive way to introduce ray optics concepts.