We present CamTrackPoint, a new input interface that can be controlled by finger gestures captured by front or rear cameras of a mobile device. CamTrackPoint mounts a 3D-printed ring on the camera's bezel, and it senses the movements of the user's finger by tracking the light passed through the finger. The proposed method provides mobile devices with a new input interface that offers physical force feedback like a pointing stick. The cost of our method is low as it needs only a simple ring-shaped part on the camera bezel. Moreover, the ring doesn't disturb the functions of the camera, unless a user uses the interface. We implement a prototype for a smartphone; two CamTrackPoint rings are made for the front and rear cameras. We evaluate its performance and characteristics in an experiment. The proposed technique provides smooth scrolling and would give better game experience on the available smartphone.