Joe focuses his attention on the tree. The focal length which is of the cornea-lens system in his eye must be less than the distance between the front and back of his eye. The focal length can be of any distance only that it is not greater than space which is within the eye.
The distance which is between the front and fovea or back of the eye will remain constant. We focus by adjusting the curvature of the lens which is nearer to the fovea than the one which is in the front eye.