The animal regarded as having the eyes with highest light-collecting ability is the ostracod Gigantocypris which has an f-number of 0.25. By way of comparison, humans have an f-number of approximately 2.55 and a camera lens an f-number of 1.8. Although Gigantocypris is the world’s largest ostracod, or seed shrimp, it only has a diameter of approximately 1.5 cm (0.7 in).

The f-number is a measure of the relative aperture of a lens on a camera or its light gathering capacity. The number becomes smaller as the aperture grows larger. The f-number is equal to the ratio of the focal length of the lens divided by the diameter of its limiting opening (aperture): f-number = focal length/iris diameter.