The same is true of eyes. In all eyes, the pupil functions as the aperture through which photons reach the light-sensitive cells of the retina. On a moonless starry night, human pupils reach a maximum width of 8 millimeters. With such a diameter, we are able to discern stars with a magnitude as faint as 6.5, allowing us to see over 9,000 stars in the entire night sky.
What about whales, basking on a starlit ocean surface at night? Remarkably, despite the often gigantic sizes of whales, their eyes are not as large as one might think. Even the largest whale eyes studied are only around 70 millimeters across (compared to around 25 millimeters for the human eye), but these eyeballs have thick outer layers of muscles and insulating fat. The actual eye itself, embedded within these layers, is typically only around 40 millimeters across. The pupil is only half as large again as our own pupil — around 12 millimeters wide in the dark (as measured in southern right whales, gray whales, and bowhead whales), so nothing near as large as the objective lenses of typical binoculars! Assuming similar exposure times and that the photoreceptors of whales are as efficient at absorbing photons as our own eyes are (which is amazingly only around 5 to 10 percent), whales would be able to see stars almost one magnitude fainter than we can (around 7.4).
This would allow them to see some 2½ times as many stars, in fact around 22,500! While this number does seem impressive and would give whales a superior view of the night sky compared to our own, imagine the number of stars that would be seen by the giant deep-sea squid Architeuthis dux if it were ever to come to the surface. With eyes almost 30 centimeters across and with pupils 9 centimeters wide, this squid could potentially see stars as faint as magnitude 12, which is truly staggering!
University of Lund, Sweden