Originally posted by violini brofkand, thanks for the link about hyperfocal distance.
The lens in the eye is a soft lens; the muscle can change the focal length instantly. That's why we can see objects near and afar.
That's true, but, especially in low light (just like a camera), the iris in your eye is wide open. It is possible to focus your eye on a close object, while mentally concentrating on another, far-away object, which will be out of focus. In your example, you are simply changing focus, which a camera lens can do, as well, albeit perhaps not as quickly.
The human eye can't focus a 3D object onto a 2D surface, either. The retina of your eye is two-dimensional, although it is not a plane. In order to focus on objects at different distances, the lens in the eye, as you point out, changes focal length (rather than focal distance) rapidly, but it is still true that not all 3D objects in your field of view are in focus simulataneously.
Try Adam's experiment in a dimly lit room and then again, outside in bright sunlight. Remember, do not allow your eye to change focus. Keep it focused on your finger in both cases, but mentally concentrate on the background. You should find that, outside, the background will be more in-focus than indoors.
To say that the human eye doesn't have bokeh (a term I personally hate) is hogwash.
And, in a camera lens, bokeh is not the result of a "flaw" in the lens. It is a fact of physics. To say it is a flaw implies that a really high-quality lens could achieve more DOF at the same aperture than a poor lens. That is simply not true. A good lens can have a more PLEASING bokeh than a cheap lens, but DOF is a mathematical function of the focal length, aperture and focus distance.
Just shoot at the smallest aperture you can, to achieve the greatest DOF. Of course, then, you risk diffraction. Again, diffraction isn't really a result of a flaw or defect, either. It happens because different light wavelengths bend differently as they pass the edge of the iris. This, too, is a fact of physics.
And, Rawhead, a pinhole is a lens, if you accept the definition of lens to be a device that focuses light. It is not a GLASS lens, but it still accomplishes the same function.