The power of accommodation is the ability of the human eye to adjust its focal length so that objects at different distances are all brought to a sharp focus on the retina. Since the image distance inside the eye (from lens to retina) is essentially fixed, the only way to keep the image on the retina as the object distance changes is to change the focal length (and hence the converging power) of the eye lens.

The correct statement is that the eye lens changes its focal length by altering its shape through the action of the ciliary muscles. The mechanism is:

• Near object: the ciliary muscles contract, which slackens the suspensory ligaments and lets the flexible lens bulge and become thicker/more curved. This gives a shorter focal length and greater converging power, needed to focus close objects.
• Distant object: the ciliary muscles relax, the ligaments pull the lens thinner/flatter, giving a longer focal length and less converging power, suitable for parallel rays from far objects.

The other statements are incorrect:

• The cornea does provide a large, fixed portion of the eye's total refracting power, but it cannot change — 'all the focusing power' and the ability to adjust it come only from the lens, so the cornea alone cannot explain accommodation.
• The iris merely controls the size of the pupil to regulate the amount of light entering the eye (like a camera aperture); it does not focus the image or change focal length.
• The retina is a fixed light-sensitive screen at the back of the eye; it does not move forward or backward to adjust focus.