Every photographer has had a moment when they wished their eye was a camera. It seems that no matter how hard we try, there are always some images and experiences that just look better through our own eyes, than through our camera. There are a lot of factors that go into how the eye takes in and processes information, but the F-number for your eye can easily be calculated using the human eye’s focal length (~22mm) and physical aperture size.
The eye’s pupil can be as large as 6–7 mm wide open, which translates into the maximum physical aperture, meaning the f-number of the human eye varies from about f/8.3 in a very brightly lit place to about f/2.1 in the dark. According to the incoming rays of light (what we actually see), the focal length of the eye is a bit longer, resulting in minimum f-number of f/3.2, but again, that’s different that f/3.2 on your DSLR, because the eye cannot be considered an ordinary air-filled camera since it’s filled with light refracting liquid.
Still interesting to think about the next time your images fall short of what you saw with your eyes.