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The process whereby changes in the dioptric power of the crystalline lens occur so that an in focus image of the object is obtained and maintained on the fovea


Proposed lenticular based focusing

Thomas Young

Demonstrated changes in the crystalline lens were responsible for focusing changes

Hermann von Helmholtz

Considered the father of physiologic optics. Advanced the first basic and reasonably accurate explanation of the accommodative process.

Ideas proposed for how we see clearly at different distances:

1) There is no need for an active form of focusing
2) Pupil size changes with the effort to see clearly at near
3) Corneal curvature changes with a change in focal point
4) The anteroposterior position of the lens changes with variation in focal point
5) Changes in the axial length of the eyeball itself account for shifts in the position of the retinal image for objects at varying distances
6) Changes in the shape, and therefore power, of the crystalline lens allow objects at various distances to be focused on the retina -> correct description of accommodation

Reflex accommodation

The automatic adjustment of refractive state to obtain and maintain a sharply defined and focused retinal image in response to blur input.

Accommodation occurs when?

Ciliary muscle constricts -> choroid and posterior zonules stretch -> anterior zonules reduce their tension and relax -> elastic forces cause the crystalline lens becomes more spherical (increasing its dioptric power)

The profile of the accommodative response

1) Linear manifest zone - change in accomm stimulus produces proportional change in accomm response
2) Initial Nonlinear zone
3) Nonlinear Transition zone
4) Nonlinear Latent zone
5) Myopic Nonlinear Defocus zone
6) Hyperopic Nonlinear Defocus region

Crystalline lens is?

A bi-convex structure between the iris and viterous, whose function is to focus light

Suspension Ligament

Holds the crystalline lens in position and enables the ciliary muscle to act on the lens

Optical cues that influence accommodation

Chromatic aberration
Spherical aberration
Blur asymmetry due to fixational eye movements

Nonoptical cues that influence accommodation

Apparent distance
Disparate retinal images
Monocular depth cues

Nonretinal influences on accommodation

Vestibular stimulation
Voluntary effort
Cognitive demand
Visual imagery
Instruction set

Retinal influences on accommodation

Spatial frequency
Retinal eccentricity
Retinal-image motion
Depth of focus
Disparity-driven vergence accommodation

The stimulus to accommodation is?



An objective way to estimate refractive error

Purkinjie-Sanson images

Specular reflections that occur at the interfaces between the transparent media of the eye

The major specular reflection that interferes with objective refraction is?

the corneal reflex

The two components of light reflected from the fundus

1) diffuse component (backscatter)
2) directed component

The most significant sources of diffuse visible light are?

Retinal Pigment Epithelium and choroid
RPE- contains melanin
capillary net directly behind RPE- Hemoglobin
Contribute to red/orange color of fundus reflex

The retinal surface responsible for detection of the image during subjective refraction is the?

outer limiting membrane

Punctum remotum is?

The far point that is optically conjugate to the fovea when accommodation is relaxed.
Myopes - PR is anterior to the eye
Hyperopes - PR is posterior to the eye

With motion of the pupillary streak indicates a PR located?

behind the retinoscope (behind the examiner's eye) OR behind the patient's eye

a PR behind the examiner's eye indicates a patient which could be?

Low myopic

Against motion of the pupillary streak indicates a PR located?

Between the retinoscope (examiner's eye) and the patient's eye.
Only seen with moderate to high myopes

If the examiner observes with motion, neutrailty can be achieved by?

Either adding plus lenses or by moving further away from the patient (approaching the punctum remotum).

If the retinoscope reflex if at neutrality, the far point is located?

at the aperture of the retinoscope

If the PR is behind the examiner's eye, what is the refractive error?

Low myope

If the PR is behind the patient's eye, what is the refractive error?


If the eye is emmetropic?

Light from retinoscope focuses behind the retina
Light reflected from the eye focuses behind the retinoscope
With motion is observed

If the eye is hyperopic?

Light from retinoscope focuses behind the retina
Light reflected from the eye focuses behind the retinoscope
With motion is obeserved

If the eye is moderate to high myopic?

Light from retinoscope focuses before the retina
Light reflected from the eye focuses before the retinoscope
Against motion is observed

Plane mirror mode

Barrel down
The apparent light source is behind the retinoscope
Divergent light leaves the retinoscope
Streak inside the eye moves same as streak on iris (ex: you tilt the retinoscope down the out-of-focus patch of light on the patient's retina moves down)

Concave mirror mode

The apparent light source is between the retinoscope and the patient's eye
When the retinoscope is tilted down the out-of-focus patch of light on the patient's retina moves up

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