Originally posted by BigDave You can beg to differ, and the one web page does mention the change in focal length. If the focal length changes, then you can see from wide angle to telephoto, or something similar, hence changing the focal length. Your "field of view" and "angle of view" does not change. The focus
distance does change as you look at subjects closer to you. The same thing happens with your camera as you focus on closer subjects. But instead of the lens moving, as happens in your camera, in the eye, the image plane moves, and the same effect is achieved.
You can calculate focal length at any distance, but typically the focal length is calculated when the eye/imaging system if focused at infinity (or some distance where it does not matter anymore as algebraic calculations do like infinity). By using a standard distance like infinity, the effect the imaging lens system has on the imaging system can be compared. However, I was mistaken about the actual focal length it is not 17mm, but about 22.3mm. See also the following:
Focal Length of a Human Eye
BD
From your link: "The lens flexes to change its curvature and finish the focusing process. When an object is located at infinity, the focal length, or the distance from the cornea to the retina, of a normal relaxed eye is about 1.7 cm (17 mm).
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The author specifically says at infinity, the focal length is about 17mm. If you change the curvature of a lens, you change its focal length. He doesn't explicitly state that the FL doesn't change. Unfortunately, in his explanation of how the eye focuses, he doesn't use the phrase "changing focal length" or anything like that. He does however, refer to changing the curvature of the lens. If this doesn't change the curvature of a lens, what does it do?
Focus is affected by the focal length, the image distance and the object distance. Since the eye itself does not change shape, the lens remains the same distance (the image distance) from the retina. If neither the focal length nor the image distance change, there is no change in focus. Sort of like a Brownie box camera. Since we all know that the human eye does, in fact, change focus, it must change focal length.
As for changing the field of view, the human eye has a FOV of almost 180 degrees. The small change in FL that I calculated would not significantly affect the FOV, so we wouldn't notice it. Besides, we hardly notice the things at the periphery of our vision anyway.
The relationship between FL, image distance and object distance is given by the formula in my last post. I have no reason to believe that it is invalid the human eye, just because it is organic, rather than glass. If the image distance remains constant and the object distance changes, the only variable left to change is FL.
Next time I go to my opthomologist, I'll ask her.
I just re-read your post before hitting submit. Are you saying that the
retina (the image plane) moves? I agree that that would have the same effect, but I don't see anything in any of the links you posted, or any others I could find, that indicates that the retina moves.