Codex @codex  0

Adjustment of the eye's optical power to maintain a focused image on the retina as object distance changes. For near vision the ciliary muscle contracts, tension in the zonular fibres decreases and the elastic crystalline lens becomes more rounded.

A ring of smooth muscle in the ciliary body. Its contraction alters the geometry of the ciliary zonule and permits the crystalline lens to become more curved for near ocular accommodation.

The suspensory fibre apparatus transmitting forces between the ciliary body and the capsule of the crystalline lens. Individual zonular fibres are its components.

The ciliary muscle forms a ring within the anterior ciliary body, deep to the sclera and outside the zonular attachment system of the crystalline lens. Its arrangement connects an anterior scleral attachment to posterior and inward parts of the uveal tissues, allowing contraction to alter zonular tension.

In a meridional section, the anterior ciliary region lies just behind the corneoscleral junction. The scleral spur gives attachment to anterior longitudinal muscle fibres. Posteriorly these fibres extend towards the choroid and its supporting tissues. Between the sclera outside and the ciliary processes inside, oblique or radial and circularly oriented smooth-muscle bundles complete the muscular mass. These fibre directions are regions of one coordinated smooth muscle, not three independently attached external ocular muscles.^[ref]

The ciliary processes project inward from the anterior folded part of the ciliary body, behind the root of the iris and peripheral to the lens equator. Ciliary epithelium covers them; their vascular core and epithelial functions are distinct from the smooth-muscle bundles. The posterior, flatter ciliary region continues towards the ora serrata, where the sensory retina ends. The ciliary body is therefore continuous with choroid posteriorly and related to iris anteriorly, while remaining a separately describable region.

Zonular fibres pass from the ciliary region to the capsule around the equator of the crystalline lens. They cross the circumlental space; the ciliary muscle does not insert directly into the transparent lens substance. The posterior chamber lies behind the iris and in front of the lens and zonular apparatus, and communicates with the anterior chamber through the pupil. The vitreous body lies behind the lens and has its own relation to the posterior zonular region. These spaces must not be drawn as one cavity containing the muscle freely suspended in aqueous fluid.

For near ocular accommodation, contraction reduces the effective diameter of the ciliary ring and changes its position, reducing the zonular forces that flatten the lens. The elastic capsule and lens substance then permit a rounder lens with greater optical power. For distant viewing, relaxation restores greater flattening tension and lower power. Thus ciliary contraction increases lens curvature by relieving a restraint; it does not pull the lens flatter. The conventional mechanism is supported by observations of lens shape and optical changes during accommodation.^[ref]

Parasympathetic motor fibres reach the muscle through the oculomotor nerve, ciliary ganglion and short ciliary nerves. This motor pathway is distinct from the sensory visual input that initiates a near response. Changes in pupil size and convergence often accompany accommodation, but neither should be substituted for an observed change of lens power. With age, reduced lens deformability can impair near focus even though ciliary contraction still occurs. The sketch shows spatial relationships and the direction of the conventional mechanism, rather than a scale drawing or a claim that every historical theory of accommodation was correct.

The cornea supplies much of the eye's converging optical power; the crystalline lens supplies additional, adjustable power through ocular accommodation. The transparent aqueous and vitreous media transmit the light. The iris controls the pupil, acting as an aperture rather than as the focusing element, and the retina is the receiving surface. The posterior optic nerve conveys the resulting neural signals; it does not carry an optical picture out of the eye.

Each sufficiently small object patch sends a pencil of rays through the pupil. Refraction at the corneal and lenticular surfaces brings that pencil towards a retinal image point. Pencils from different object heights meet at different retinal positions, producing a real inverted image. In a paraxial equivalent model,The several physical refracting surfaces have been replaced here by an equivalent thin lens; this is a model, not a claim that all bending occurs at the crystalline lens. For a distant object, approximately parallel incoming pencils focus near the back focal surface. Ocular accommodation increases power for nearer objects while the retinal position remains nearly fixed.

In regular ocular astigmatism, the two principal meridians are perpendicular but have different refractive powers, commonly because the cornea has different curvatures in those meridians. A point object then gives two line foci at different depths rather than a single sharp point. The difference is not merely an overall spherical focusing error. Refraction with differently oriented cylindrical trial lenses can locate the power and axis; keratometry or corneal topography measures corneal shape. Cylindrical lens or toric lens correction supplies the missing meridional power with the proper orientation. Spectacles and appropriately fitted toric contact lenses can provide such correction. A method for measuring corneal curvature should not be confused with a complete measurement of the eye's total refractive error.^[ref][ref]

In the paraxial thin lens approximation, let a converging lens have focal length $f>0$, real-object distance $u>0$, and signed image distance $v$, positive beyond the lens. ThenA ray parallel to the axis emerges through the far focus; a ray through the optical centre is approximately undeviated. Their intersection gives a real, inverted image for $u>f$: reduced for $u>2f$, equal-sized at $u=2f$, and enlarged for $f<u<2f$. At $u=f$ the emerging rays are parallel. For $u<f$ their backward extensions meet on the object side: $v<0$ and the image is virtual and upright.

The retina remains at an approximately fixed distance from the refracting apparatus. Ocular accommodation changes the optical power. A nearer object increases $1/u$, so a fixed retinal image distance requires larger $1/f$.

For near vision, the ciliary muscle contracts and reduces tension in the zonular fibres. The elastic crystalline lens becomes more rounded, increasing its optical power. For distant vision the ciliary muscle relaxes, zonular tension flattens the crystalline lens, and less power is needed. The eye's several refracting surfaces make the single thin lens a qualitative model rather than its full optical prescription.

For regular astigmatism, the two principal meridians of the eye are perpendicular and have different optical powers, commonly because the cornea has unequal curvatures. The crystalline lens can also contribute. A point object consequently does not form one sharp point image: the idealised paraxial optics gives two focal lines at different distances, perpendicular to one another, with an intervening region of blur. An ordinary spherical lens changes the focus in both meridians together and cannot by itself remove their difference. Irregular astigmatism, for example with a scarred cornea, lacks this simple pair of uniform principal powers.^[ref]

Several complementary observations reveal it:

The prescription must specify both the difference in optical power and its orientation. If the required correcting powers in the two principal directions are $R_1$ and $R_2$, choose the cylinder axis along direction 1. In the ideal thin lens model:Here $S$ is the spherical correcting power, $C$ the cylindrical correcting power and $\alpha$ the cylinder axis, all powers measured in dioptres. A cylindrical lens contributes no extra power along its axis and its full power perpendicular to that axis. For example, required corrections of $-1$ and $-3$ dioptres are supplied by a sphere of $-1$ dioptre and a cylinder of $-2$ dioptres, with its axis in the direction requiring the smaller negative correction. These are correcting powers, not the uncorrected eye's intrinsic powers; confusing the two reverses the intended correction.

Spectacles with this sphere-and-cylinder combination are the central historical remedy. Present options also include toric contact lenses, appropriate rigid contact lenses for some irregular cases, and selected refractive surgery. The suitability of these depends on the cause and condition of the cornea; a spectacle cylinder cannot generally cancel all irregularities of an irregular cornea.^[ref]

The opening in the iris through which light enters the eye. Its aperture affects admitted pencils of rays; the cornea and crystalline lens provide focusing power.

A suspensory fibre transmitting tension to the crystalline lens. Reduced zonular tension during near ocular accommodation permits the crystalline lens to become more rounded.