Axial elongation is the elongating of the eyeball. Speculation abounds as to the causes, though the most accepted is that axial elongation is just part of the whole story. Axial change, rather than axial elongation […]
Axial elongation is the elongating of the eyeball. Speculation abounds as to the causes, though the most accepted is that axial elongation is just part of the whole story. Axial change, rather than axial elongation alone should be considered as a mechanism by which the eye compensates for focal plane error.
“The homeostatic control of eye growth functions to keep images sharply focused on the retina. Therefore, if the eye length increases more slowly than does the focal length, the focal plane will be behind the retina, creating hyperopic defocus on the retina. The same occurs if one puts a negative lens over the eye (Figure 2A). To regain sharp focus, the retina needs to be displaced backward to where the image is. This is done in two ways: the eye is lengthened by increasing the rate of growth or of remodeling of the sclera at the posterior pole of the eye Gentle and McBrien 1999 and Nickla et al. 1997, and the retina is pulled back within the eye by the thinning of the choroid, the vascular layer between the retina and sclera ( Figure 2B; Wallman et al. 1995 and Wildsoet and Wallman 1995); once distant images are again focused on the retina (emmetropia), both the rate of ocular elongation and the choroid thickness return to normal.
Conversely, if the eye length increases more quickly than the focal length does, the image will be formed in front of the retina, creating myopic defocus. The same occurs if one puts a positive lens over the eye (Figure 2A). The eye compensates first by expanding the choroid, which pushes the retina forward toward the image plane, and then by slowing ocular elongation, which causes the continuously increasing focal length of the eye to move the image plane back to the retina (Figure 2B; Hung et al. 2000 and Wildsoet and Wallman 1995).
The range of lens powers compensated for is greater in chicks than in monkeys, although monkeys can also compensate for stronger lenses if the lens power is stepped up gradually Irving et al. 1992 and Smith and Hung 1999. The greater range of compensation in chicks may be due to the chicks’ viewing objects at closer distances (reducing the effective power of the positive lenses), to their visual acuity being lower, and to their amplitude of choroidal compensation being greater (Flitcroft, 1999).
In addition to these changes in eye length and choroid thickness that occur over days or weeks, the eye can change the focal length of its optics in a fraction of second (ocular accommodation). These three processes all act to put the image onto the retina (Figure 3).”
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