We present a practical real-time approach for rendering lens-flare effects. While previous work employed costly ray tracing or complex polynomial expressions, we present a coarser, but also significantly faster solution. Our method is based on a first-order approximation of the ray transfer in an optical system, which allows us to derive a matrix that maps lens-flare-producing light rays directly to the sensor. The resulting approach is easy to implement and produces physically-plausible images at high framerates on standard off-the-shelf graphics hardware.