Subjective Refraction: Principles and Techniques for the Correction of Spherical Ametropria

• Subjective Refraction

• Objective Refraction

Two methods of evaluating the refractive error of an eye?

Subjective Refraction

Where the result depends on the patient’s ability to discern changes in clarity.

Objective Refraction

Where the result depends purely on the examiner’s judgement to determine the optimum optical correction.

1. Determine BVS

2. Determine astigmatic error

3. It involves the balancing and/or modification of the refractive correction to ensure optimal visual performance and patient comfort.

Subjective Refraction consists of three distinct phases?

• Blurred distance vision

• HA from screwing up the eyes to try to obtain clearer vision by the pinhole effect.

• Clear near vision

Symptoms of uncorrected myopia may include? (3)

• Poor distance vision on a letter chart.

• Good near vision on a near test chart.

Signs of uncorrected myopia may include? (2)

• Eyestrain, especially for close work, caused by the accommodative effort to form a clear image.

• Blurred vision with medium-to-high amounts of hypermetropia and in advancing age (blurred vision is not usually a problem with low amounts of hypermetropia).

Symptoms of uncorrected hypermetropia may include? (2)

• Usually no signs in low hypermetropia – screwing up the eyes and wrinkling of the brow may be a sign of high amounts of uncorrected hypermetropia

• A nasalward deviation (esotropia) of one eye in high amounts of uncorrected hypermetropia.

Signs of uncorrected hypermetropia may include? (2)

Objective Refraction

Often used to determine the initial spherical element of refraction.

Fogging Technique

Employed whereby the spherical element is deliberately over-plussed and then reduced to find the final spherical power.

Simultan Technique

This technique relies on the sequential presentation of plus and minus spheres, which are usually mounted together in a ‘twirl’ with a handle, although individual trial lenses may be used.

Most plus or least minus that does not blur the VA.

End-Point in the Simultan Technique?

Point of Focus

When the BVS has been reached in the case of spherical ametropia?

Disc of Least Confusion

When the BVS has been reached in the case of astigmatic ametropia?

Maximum plus or minimum minus that the patient will tolerate without causing blurring of the retinal image.

Endpoint of the Best Vision Sphere?

Diffraction effects and also a reduction in retinal illumination.

A pinhole with an aperture smaller than 1mm?

It approaches the size of some human pupils and so might not significantly reduce the blur circle produced by an uncorrected refracted error.

A pinhole aperture larger than 2mm?

Duochrome Test

The eye, in common with most optical systems, displays a certain amount of axial chromatic aberration (ACA).

Axial Chromatic Aberration

ACA =

Approximately 2.50D

The total amount of ocular chromatic aberration present has been estimated as?

570nm

Yellow light with a wavelength of?

535nm

Green light with a wavelength of?

620nm

Red light with a wavelength of?

Scheiner Disc

The earliest instrument employed to asses refractive error.

• 0.75mm in diameter

• 2-3mm apart and equidistant from the centre in opposite direction along a common meridian.

(Schener Disc) This trial case accessory is an opaque disc with:

• Diameter?

• How many mm apart?

Scheiner Disc

Somewhat outdated technique that is rarely used in optometry practice.

In the one-position keratometer and some autorefractors

Scheiner Disc can be used to demonstrate the existence of accommodation and is also used as the focusing mechanism in? (2)