A user input device to generate a shutter threshold adaptive to ambient light conditions. The user input device includes a sensor array, a shutter controller, and a threshold generator. The sensor array obtains image data in response to light incident on the sensor array. The shutter controller is coupled to the sensor array. The shutter controller controls a shutter time during which the sensor array is responsive to the light incident on the sensor array according to a shutter value. The threshold generator is coupled to the shutter controller. The threshold generator dynamically generates a shutter threshold corresponding to the shutter value. The shutter threshold defines a transition of a finger-on/off state.