Liu Tao, Thibos Larry N
School of Optometry, Indiana University, Bloomington, Indiana, USA.
Ophthalmic Physiol Opt. 2017 May;37(3):305-316. doi: 10.1111/opo.12364. Epub 2017 Mar 9.
Oblique astigmatism is a prominent optical aberration of peripheral vision caused by oblique incidence of rays striking the refracting surfaces of the cornea and crystalline lens. We inquired whether oblique astigmatism from these two sources should be expected, theoretically, to have the same or opposite signs across the visual field at various states of accommodation.
Oblique astigmatism was computed across the central visual field for a rotationally-symmetric schematic-eye using optical design software. Accommodative state was varied by altering the apical radius of curvature and separation of the biconvex lens's two aspheric surfaces in a manner consistent with published biometry. Oblique astigmatism was evaluated separately for the whole eye, the cornea, and the isolated lens over a wide range of surface curvatures and asphericity values associated with the accommodating lens. We also computed internal oblique astigmatism by subtracting corneal oblique astigmatism from whole-eye oblique astigmatism.
A visual field map of oblique astigmatism for the cornea in the Navarro model follows the classic, textbook description of radially-oriented axes everywhere in the field. Despite large changes in surface properties during accommodation, intrinsic astigmatism of the isolated human lens for collimated light is also radially oriented and nearly independent of accommodation both in theory and in real eyes. However, the magnitude of ocular oblique astigmatism is smaller than that of the cornea alone, indicating partial compensation by the internal optics. This implies internal oblique astigmatism (which includes wavefront propagation from the posterior surface of the cornea to the anterior surface of the lens and intrinsic lens astigmatism) must have tangentially-oriented axes. This non-classical pattern of tangential axes for internal astigmatism was traced to the influence of corneal power on the angles of incidence of rays striking the internal lens.
Partial compensation of corneal astigmatism by internal optics is due mainly to the highly converging nature of wavefronts incident upon the lens resulting from corneal refraction. The degree of compensation is quadratically dependent on eccentricity but is expected to diminish as the eye accommodates. Neutralising the cornea by index-matching defeats internal compensation, revealing classical, radially-oriented oblique astigmatism in the isolated lens.
斜向散光（oblique astigmatism）是周边视觉中一种显著的光学像差，由光线倾斜入射角膜和晶状体的折射面所致。我们探究了理论上，在不同调节状态下，这两种来源的斜向散光在整个视野中是否应具有相同或相反的符号。
使用光学设计软件，针对旋转对称的示意眼计算整个中央视野的斜向散光。通过以与已发表的生物测量数据一致的方式改变双凸透镜两个非球面的顶点曲率半径和间距来改变调节状态。在与调节性晶状体相关的广泛表面曲率和非球面度值范围内，分别对全眼、角膜和孤立晶状体的斜向散光进行评估。我们还通过从全眼斜向散光中减去角膜斜向散光来计算内部斜向散光。
在纳瓦罗模型中，角膜斜向散光的视野图在整个视野中都遵循经典教科书对径向取向轴的描述。尽管在调节过程中表面特性发生了很大变化，但无论是在理论上还是在实际眼睛中，孤立人眼晶状体对准直光的固有散光也是径向取向的，并且几乎与调节无关。然而，眼斜向散光的大小小于单独角膜的斜向散光大小，这表明内部光学结构起到了部分补偿作用。这意味着内部斜向散光（包括从角膜后表面到晶状体前表面的波前传播和晶状体固有散光）必须具有切向取向的轴。内部散光这种非经典的切向轴模式可追溯到角膜屈光力对入射到晶状体内部的光线入射角的影响。
内部光学结构对角膜散光的部分补偿主要是由于角膜折射导致入射到晶状体的波前具有高度会聚的性质。补偿程度与偏心率呈二次方关系，但预计会随着眼睛调节而减小。通过折射率匹配使角膜中性化会破坏内部补偿，从而在孤立晶状体中揭示出经典的径向取向斜向散光。
