Most mammals have dichromatic vision. They can see color, but they cannot discriminate along the red-green axis. Humans with this relatively rare type of color blindness have a hard time differentiating between red and green, as well as colors close to them like oranges and browns, as one blogger describes it, depending on how saturated and bright the color is.
In general, mammals don't have the best color vision. In part, that's because our ancestors developed trying to see in the dark, not out in the bright sunlight. "There was a time where to be a mammal was to be a small, nocturnal, rodent-like mammal," said Duke's Sonke Johnsen, author of the book, The Optics of Life. Both humans and whales retain the marks of that evolutionary path. "Our color vision is kind of a kluge," Johnsen continued. "If you look at the color vision of birds and reptiles and fish. It's very well put together, nicely optimized. You look at our trichromatic vision, it's really kind of pieced together."
Whales, unlike nocturnal rodents or ourselves, see the world in monochrome. Leo Peichl at the Max Planck Institute for Brain Research co-authored a paper with the nearly tragic title, "For whales and seals the ocean is not blue." Indeed, the first thing that we can know for sure about how whales see the world is that it exists only in shades of gray. The water we see as blue they would see as black. "They do want to see the background. They want to see animals on the background. And the animals on the background are reflecting light that's not blue," Johnsen explained. If we try to imagine what that might look like, Johnsen said perhaps we could picture a grayscale photograph of people wearing fluorescent clothes under a black light.
When it comes to the optics of whale eyes, the first difference we should note is that its cornea -- the outermost layer of the eyes -- doesn't help it nearly as much as ours helps us. We live in air, which has a different refractive index than the material of the cornea. When light enters our cornea, it bends inward. You know how pencils appear to bend when you put them in a glass of water? That's refraction, and our eyes exploit it to help focus photons on the central part of our retinas. Johnsen told me roughly 70 percent of the work of focusing light on our eyes is done by the cornea before the light even reaches the lens. But that's a clever terrestrial trick. In water, the refractive index of the cornea and water are roughly the same, which means that marine mammals don't get that pencil-going-into-water light bending help. "The lens has to do everything in the whale eye," Peichl said. While our lenses are flattish, theirs are circular in order to provide sufficient focus.
Now, when we talk about the resolution with which whales see the world, it helps to bring back the video camera metaphor for eyes. Whales, like other mammals, are trying to balance the sharpness of their eyes with their sensitivity. Sharp vision requires lots and lots of individual photoreceptors. But in low-lighting conditions, it's hard for the photoreceptors to gather enough photons. The image gets "noisy."
Photographers run into this problem all the time, too. What do you do in lower light settings? The first thing you might do is put a lens with the largest possible aperture on the camera to let in more light. The same goes for eyes: the whale's big cornea and large pupil opening means that it has a huge aperture. It's gathering up a lot of photons.
And it's got a biological mirror at the back of its eye, the tapetum lucidum, which is helping it capture even more light than our own eyes can.
But vision isn't all the optics. Other capabilities matter, too, like the size of the sensor that picks up all that light. We measure that kind of thing in the megapixels of the charge-coupled device, or CCD, in the camera. There's a similar principle at play in biological camera eyes. If an organism wants to see better, it has to have a lot of photoreceptors. More photoreceptors equals more pixels. A big difference here is that CCDs capture what hits them equally. Retinas have areas of greater or lesser rod and cone density that tends to coincide with where the light is being focused. This makes a lot of sense: Evolution has put the most sensors where the most light falls.
The light detecting system, however, is more complex than we find in any digital camera. Photoreceptors send their information to ganglion nerve cells, which integrate them, dynamically increasing the size of the photoreceptor. That increases sensitivity by cutting down on the noise problem, but it decreases the acuity because each "pixel" gets largely, i.e. it has to represent a larger portion of the physical world.
What's fascinating is that by looking at the ganglion cells, researchers can calculate the maximum resolution that a particular eye could have, inferring capabilities from the anatomy alone. That's helpful in species like whales where behavioral tests aren't generally possible.