How the Human Eye Focuses

Multifactor Determinants of Visual Accommodation as a Critical Intervening Variable in the Perception of Size and Distance: Phase I Report

“…Paradoxically, as detailed by Koretz and Handelman (1988), the lens also becomes more rounded with age for any given object distance. This rounding normally would be associated with an increase in power.…”

Polymeric nanolayered gradient refractive index lenses: technology review and introduction of spherical gradient refractive index ball lenses

“…The GRIN decreases from a value of 1.42 at the lens center to n ¼ 1.37 at its surface. 11,31 The ability of the human eye lens protein layers to vary its refractive index results from a dynamic chemical make-up, i.e., volumetric ratio of protein to water concentration proportional to the material refractive index, within the layers. Although the shape and GRIN distribution of the human eye lens has been extensively studied, 12,32,33 traditional interdiffusion 19 and sol-gel GRIN fabrication techniques 15,16 have not produced equivalent refractive index distributions.…”

“…Without GRIN optics, spherically curved homogenous lenses could still focus light and offer large fields of view, however, they would exhibit significant spherical aberrations and focused light would not intersect at one point along the lens's optical axis, causing image blur. [1][2][3] By contrast, air-dwelling creatures, [9][10][11][12] such as the lion, cow, rat, and human, utilize GRIN lenses to correct for significantly larger geometric aberrations stemming from a larger difference in environment (air n ¼ 1.0) to lens material (n ¼ 1.33 to 1.43) refractive index as well as contributions from aspheric-shaped lenses (Table 1). A bi-product of nature's incorporation of GRIN into eye lens optics has resulted in most biological imaging systems containing a low, typically one to three, number of lenses, which minimizes the size necessary for an organism's eyes to exhibit a powerful accommodating image system.…”

Inverse elastographic method for analyzing the ocular lens compression test

“…1 This process uses biomechanical deformation of the lens to achieve a change in optical power. 2 Presbyopia is the progressive loss of accommodation amplitude with age and appears to arise due to an alteration in the biomechanical coupling of the lens and its capsule. 3,4 The clinical presentation of presbyopia coincides with the massive sti®ening of the lens¯ber cells, but it is unclear whether this is purely correlation, cause, or e®ect.…”