Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.

  • The lens in each eye is located just behind the pupil. The shape of the lens is controlled by the ciliary muscle which forms a ring of flexible tissue around the edge of each lens.
  • The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance between the centre of the lens and the retina always remains the same.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • Lens with a longer focal length bring objects at a greater distance from the eye into focus and has a narrower field of view.
  • Because the image distance is fixed in human eyes, they accommodate for this by using the ciliary muscle to alter the focal length of the lens.
    • To accommodate nearby objects the ciliary muscle contracts, making the lens more convex with a shorter focal length.
    • To accommodate distant objects the ciliary muscle relaxes, causing the lens to adopt a flatter and less convex shape with a longer focal length.