In the deep sea, it is unknown how eyes that use concave mirrors to focus can distinguish between the small bioluminescent lights of their prey and those larger lights of more distant predators. Beyond 1000 m depth, where sunlight is no longer perceptable, the deep sea contains a continuous field of (mostly) blue, bioluminescent lights. Here, some predators, such as the ostracods of the genus Gigantocypris, famed for their gooseberry-like appearance, are attracted to their prey through the prey’s bioluminescence. The enigmatic eyes of Gigantocypris spp. focus light using large, parabolic mirrors. Here, I show that the mirrors flex, pulsing continuously, so causing large, distant light sources to pass in and out of focus while small, nearby light sources remain in focus with each pulse cycle. This distinguishes predators from prey and constitutes a new type of eye.