Been asking myself the same question. Then I asked Google but couldn't get a definitive answer.
Here's a website:
"What is the focal length of the eye? I did a google search and found many "answers" ranging from 17mm to 50mm (50 is totally absurd). For the correct answer, is Reference: Light, Color and Vision, Hunt et al., Chapman and Hall, Ltd, London, 1968, page 49 for 'standard European adult'":
Object focal length of the eye = 16.7 mm
Image focal length of the eye = 22.3 mm
The object focal length is for rays coming OUT OF THE EYE. But for an image on the retina, the image focal length is what one wants….
So this explains the commonly cited ~17mm focal length, but the correct value is ~22 mm focal length."
EDIT: Oops, didn't check the original post date. Oh well, might be of use to somebody.