The Lens

References

Items refer to the human eye unless otherwise specified. This section leans heavily on Wyszecki and Stiles (1982) an excellent book.

The internal focusing element of the eye.
It is curved on both sides.
It is attached to the ciliary muscle which changes the shape of the lens.
The change of the shape of the lens is accommodation.
Relaxed The change of the shape of the lens is accommodation.
The process of changing the shape of the lens (accommodation) is a little peculiar, so here are two examples which I have worked out from looking at three sources 1) Weale's Focus on Vision, 2) Gouras' chapter in Principles of Neural Science 2nd Ed and 3) Fry's chapter in Handbook of Physiology, Section 1: Neurophysiology:
- Focussing on an object involves
  - 1) the lens which, if isolated, would be curved and not flat
  - 2) zonular fibers which are passive and, if left alone, will pull the lens and flatten its surfaces
  - 3) ciliary muscle which can actively contract resulting in relaxed tension on the zonular fibers and a curved lens
- Focus on the horizon (infinity)
  - goal: get lens as flat as possible by tugging on its edges
  - zonular fibers want to tug on the lens to make it flat
  - ciliary muscle should relax so that tension on the zonular fibers is maximized
- Focus on your finger
  - goal: get lens as round as possible by releasing tension on its edges
  - zonular fibers want to tug on the lens to make it flat
  - ciliary muscle should contract so that tension on the zonular fibers is minimized
Accommodation
- Near Point: upper limit of accommodation ~ 10 diopters = ~4 inches (10.2 cm) (Alpern and David, 1958)
- Far Point: lower limit of accommodation ~ 0 diopters = infinity (Alpern and David, 1958)
- Resting Point of Accommodation (RPA), or dark-focus - ~1.5 diopters or focal point of 67 cm (26.4 in) (Leibowitz and Owens, 1978)
- amplitude/range/breadth of accommodation = Far Point - Near Point
- Myopia - near-sighted - can see things close but not far away, image is focused between the lens and the retina. Requires -diopter lens correction.
- Hyperopia - far-sighted - can see things far away but not close, image is focused beyond the retina. Requires +diopter lens correction.
- Gimbel eye-center's helpful "refractive" webpage
- 20/20 Vision on a Snellen Chart means that a letter that subtends 5 minutes of arc at 20 feet is resolvable. see pg 199 of Pedrotti & Pedrotti's Introduction to Optics
- Diopters: the power of a lens as defined by 1 over the focal length in meters (focal length = 20 cm -> 5 diopters)
As the light level of the target decreases, accommodation approaches the resting point of accommodation. Alpern and David (1958) measured accommodation at 910, 154, 18.2, 2.18 and .529 trolands. Accommodation falls appart gradually across this range and is not useful at .529 td.
It absorbs short-wavelength (blue) light more than the rest of the spectrum.
LeGrand's Full Theoretical Eye (from Wyszecki & Stiles, 1982, pg 99)
- Index of Refraction: 1.42 Unaccommodated
- Index of Refraction: 1.427 Accommodated
- Anterior surface
  - Distance from Cornea: 3.6 mm Unaccommodated
  - Distance from Cornea: 3.2 mm Accommodated
  - Radius of curvature: 10.2 Unaccommodated
  - Radius of curvature: 6.0 Accommodated
- Posterior surface
  - Distance from Cornea: 7.6 mm Unaccommodated
  - Distance from Cornea: 7.7 mm Accommodated
  - Radius of curvature: -6.0 Unaccommodated
  - Radius of curvature: -5.5 Accommodated
LeGrand's Simplified Eye (from Wyszecki & Stiles, 1982, pg 99)
- Index of Refraction: 1.4208 Unaccommodated
- Index of Refraction: 1.4260 Accommodated
- Anterior surface
  - Distance from Cornea: 6.3740 mm Unaccommodated
  - Distance from Cornea: 5.7763 mm Accommodated
  - Radius of curvature: 10.2 Unaccommodated
  - Radius of curvature: 6.0 Accommodated
- Posterior surface
  - Distance from Cornea: 6.3740 mm Unaccommodated
  - Distance from Cornea: 5.7763 mm Accommodated
  - Radius of curvature: -6.0 Unaccommodated
  - Radius of curvature: -5.5 Accommodated

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Lance Hahn(lance@retina.anatomy.upenn.edu)