Adaptation to Differences in Light Intensity

Like a camera, the eye's aperture and lens system also automatically adapts to differences in light intensity to avoid "overexposure". This adjustment is effected by two mechanisms.

1. The iris acts as an aperture to control the amount of light entering the eye. This regulation takes about one second and can change the light intensity on the retina over a range of about a power of ten.

2. The sensitivity of the retina changes to adapt to differences in light intensity. The sensitivity of the retina to light is a function of the concentration of photopigment in the photoreceptors and of the neuronal activity of the retinal cells. The change in neuronal activity is a rapid process that takes only a few milliseconds and can alter the light sensitivity of the retina over a range of three powers of ten. The change in the concentration of photopigment takes several minutes but can cover a wide range of retinal light sensitivity, as much as eight powers of ten.

— Range of accommodation in diopters as a function of age.

0 10 20 30 40 50 60 70 Age in years

Fig. 16.5 When the range of accommodation falls below 3 diopters, a previous emmetropic patient will require eyeglasses for reading (adapted from Goersch 1987).

0 10 20 30 40 50 60 70 Age in years

Refraction in the emmetropic eye.

the retina in an unaccommodated eye.

b Accommodation focuses the light rays from a close object on the retina, and the object is visualized with sharp contours.

c Where accommodation is insufficient, as in advanced age, close objects appear blurred.

d A converging lens is required to correct insufficient accommodation for near vision in advancing age.

the retina in an unaccommodated eye.

b Accommodation focuses the light rays from a close object on the retina, and the object is visualized with sharp contours.

c Where accommodation is insufficient, as in advanced age, close objects appear blurred.

d A converging lens is required to correct insufficient accommodation for near vision in advancing age.

Stretch Marks

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