Refractive Error and Spectacle Lens Conventions

absence of a refractive error; NORMAL vision, 20/20, light focuses on the fovea within the retina's macula like it's supposed to

emmetropia

refers to parallel light rays at a distance greater than or equal 20 feet

optical infinity

condition that exists when parallel light rays do not focus on the retina, resulting in a blur; in this instance, correction is needed

ametropia

What is a diopter (D)?

1/focal length in meters

units for lens power

diopters (D)

What is the focal length of a 5 D lens?

20 cm

(D=100/f, so 5=100/f, so f=100/5=20)

condition in which the eye has too much refracting power and parallel light rays focus anterior to the retina; use CONCAVE lenses (with MINUS power) to correct this condition

myopia

If the optical system of the eye has too much refracting power, do parallel light rays focus anterior or posterior to the retina?

anterior

What kind of lenses are used to correct myopia: convex or concave? Minus or plus?

concave, minus

condition in which the eye has too little refracting power and parallel light rays focus posterior to the retina; use CONVEX lenses (with PLUS power) to correct this condition

hyperopia

If the optical system of the eye has too little refracting power, do parallel light rays focus anterior or posterior to the retina?

posterior

What kind of lenses are used to correct hyperopia: convex or concave? Minus or plus?

convex, plus

natural method of providing more refracting power (PLUS power) to the eye in which the ciliary muscles contract, zonular fibers relax, and the lens thickens and becomes more convex

accommodation

In accommodation, to provide more refracting (PLUS) power to the eye, do the ciliary muscles contract or relax? Do the zonular fibers tighten or relax? Does the lens thin or thicken out?

ciliary muscles contract, zonular fibers relax, lens thickens out

What are the two broad categories of ametropia?

spherical errors, astigmatism errors

Eyes with which kind of error have UNIFORM power: spherical or astigmatism?

spherical

refractive error in which eyes bring light to focus at a single point anterior or posterior to the retina

spherical

refractive error in which the curvature of the cornea and/or the lens may vary and provide two different focal points

astigmatism

In astigmatism, the curvature of the cornea and/or the lens may vary across the surface(s) and provide two different focal points. These two focal points represent the maximum and minimum amount of refractive power of the eye and are the ____ ____ of an eye with astigmatism.

principal meridians

refers to astigmatism in which the two meridians (on which the two focal points of the eye lie) are perpendicular to one another and thus separated by 90 degrees

regular

normal age-related decrease in accommodation that occurs over time as the lens slowly loses its ability to change its shape when the ciliary body relaxes and contracts

presbyopia

Presbyopia usually starts occurring around/over what age?

40

maximum amount of accommodation available to a patient

1/d[m] or 100/d[cm]

amplitude of accommodation

refers to the additional power added to supplement the patient's natural accommodation, found on bifocals; amount of power needed to make up for lack of accommodation

**the plus power you must add to a distance correction to provide clear near vision

ADD

In bifocals, the near prescription is a sum of the distance correction and the amount of power you add to the distance correction in the form of a bifocal.

For example, if you have the Rx +3.00 -1.00 x 180 / +2.00, what is the distance correction? What is the near correction?

distance is +3.00-1.00x180, near is +5.00-1.00x180

lens that has uniform curvature that may cause light to converge or diverge; parallel light passing through any portion of it comes to focus at a single point because power is the same all throughout it

spherical

What is the designation for a sphere lens?

DS (diopters sphere)

lens that has a single curved surface and flat surface perpendicular to the curved surface; curved surface contains power and causes light to converge or diverge

cylinder

The flat surface of a cylinder lens contains NO power, so it is the what for the lens?

axis

What is the range of possible axis values?

001-180

The focal line of a cylinder lens is ____ to the axis of the lens.

parallel

The power of a cylinder lens is ____ to the axis of the lens.

perpendicular

lens that has one spherical surface and one cylinder surface; the portions of the lens that are most curved and least curved are the major refracting surfaces of the lens, which are separated by 90 degrees

spherocylinder

In an optical cross, the power at the axis location represents what power?

sphere

In an optical cross, the power 90 degrees from the axis location represents what power?

sphere + cylinder

If you look at something through a + lens, does it move in the same or opposite direction as you're moving it?

opposite

If you look at something through a - lens, does it move in the same or opposite direction as you're moving it?

same

type of refractive error in which the eye focuses at a single point anterior to the retina

simple myopia

type of refractive error in which the eye focuses at a single point posterior to the retina

simple hyperopia

What kind of lens do we use to correct simple myopia?

concave spherical

What kind of lens do we use to correct simple hyperopia?

convex spherical

type of refractive error in which the eye focuses at two points: one anterior to the retina, and the other on the retina

simple myopic astigmatism

What kind of lens do we use to correct simple myopic astigmatism?

spherocylinder

(with one plano meridian and one minus meridian)

type of refractive error in which the eye focuses at two points: one on the retina, and the other posterior to the retina

simple hyperopic astigmatism

What kind of lens do we use to correct simple hyperopic astigmatism?

spherocylinder

(with one plano meridian and one plus meridian)

type of refractive error in which the eye focuses at two points: both anterior to the retina

compound myopic astigmatism

What kind of lens do we use to correct compound myopic astigmatism?

spherocylinder

(with both meridians at different minus powers)

type of refractive error in which the eye focuses at two points: both posterior to the retina

compound hyperopic astigmatism

What kind of lens do we use to correct compound hyperopic astigmatism?

spherocylinder

(with both meridians at different plus powers)

type of refractive error in which the eye focuses at two points: one anterior to the retina, and other posterior to the retina

mixed astigmatism

What kind of lens do we use to correct mixed astigmatism?

spherocylinder

(with one meridian at a plus power and the other at a minus power)

category of astigmatism in which the orientation is within 30 degrees of 180, so the minus cylinder axis is 0-030 or 150-180

**most common

with the rule (WTR)

category of astigmatism in which the orientation is within 30 degrees 0f 090, so the minus cylinder axis is 060-090 or 090-120

against the rule (ATR)

category of astigmatism in which the minus cylinder axis is 031-059 or 121-149

**least common, has the biggest impact on vision when not corrected

oblique

Which category of astigmatism is the most common?

WTR

Which category of astigmatism is the least common?

oblique

If a patient's axis is 0-030 or 150-180, what kind of astigmatism do they have?

WTR

If a patient's axis is 060-090 or 090-120, what kind of astigmatism do they have?

ATR

If a patient's axis is 031-059 or 121-149, what kind of astigmatism do they have?

oblique

What is the correct order of values in a prescription?

sphere cylinder axis

What do you ALWAYS have to be sure to include when writing a prescription?

proper signs

(whether it's + or -)

How do you represent zero sphere power in a prescription?

plano

In optometry, do we use plus or minus cylinder?

minus

How many digits should there be in the cylinder axis value?

3

(030, 074, 150)

How many decimal points are included in the sphere and cylinder values?

2

If there is no astigmatism, how do you represent a spherical refractive error?

DS