Here are some shots from today. Top shot of Bald Eagle with clear eye and then shot of Bald Eagle with nictitating membrane closed. Nice to see the cropping ability of K-1.
Some birds, amphibians, reptiles, fish, and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris, or to moisten the eye.
Shot with K-1 and SMC 500mm f/4.5
Normal clear eye
And now with nictitating membrane closed. You can see the veins in it.