This app allows the user to explore the effects of the different lens distortion parameters (3 radial and 2 tangential) of a common camera lens distortion model.
This is model is used in the MATLAB Computer Vision System Toolbox:
http://www.mathworks.com/help/vision/ug/camera-calibration.html

and OpenCV:

This tool allows the modification of the 5 distortion parameters (k1, k2, k3, p1, p2), the 2 focal lengths (fx, fy), and the camera center (cx, cy). It allows the user to use one of two built-in images or to load a custom image.