An augmented reality, virtual reality, or other wearable apparatus comprises an eye tracking device comprising an image sensor, a lens, and one or more processors. In some embodiments, the lens comprises a marker, and the one or more processors are configured to receive an image from the image sensor, wherein the image shows the marker, determine a distance from the image sensor to the marker based on the image, and change a calibration parameter of an eye tracking algorithm based on the distance. In some embodiments, the one or more processors are configured to receive image data from the image sensor, wherein the image data corresponds to an image as observed through the lens, determine a level or pattern of pincushion distortion in the image based on the image data, and change a calibration parameter of an eye tracking algorithm based on the level or the pattern of pincushion distortion.