This document discusses optical principles of the visual system and different conditions that affect vision. It explains that emmetropia is a normal refractive state where the eye can focus on distant objects without effort. Two main types of refractive errors are discussed - spherical errors like myopia and hyperopia where the optical power is too strong or weak, and cylindrical errors like astigmatism where the optical power differs across meridians. Various conditions that cause blurred vision are described such as presbyopia, different types of astigmatism, and how they can be classified and corrected.

3. *
*In this course, the optical principles
behind the working of the eye and visual
system will be explained .
*Further more descriptions of different
devices used to measure the properties and
functioning of the eye will be described .
*Goals are a general understanding of how
the visual system works and exposure to an
array of different optical instrument .

4. Emmetropia
*Emmetropia is a state in which the eye is
relaxed and focused on an object more than 6
meters or 200 feet away . The light rays
coming from the object are essentially parallel
and the rays are focused on the retina without
effort can emmetropic eye does not corrective
lenses .
*Ammetropic .
*An abnormal refractive condition (as myopia,
hyperopia, or astigmatism ) of the eye in which
images fail to focus upon the retina .

5. Types of Ametropia.
*The word ‘’ametropia’’ can be used interchangeable
with refractive error’’ Types of ametropia include
myopia hyperopia and astigmatism . They are
frequently categorized spherical errors and cylindrical
errors .
*Spherical error:
*Spherical error occur when the optical power of the
eye is either too large or two small to focus light on
the retina .
*Myopia:
*When optics are too power full for the length of eye
ball one has myopia or nearsightedness . This can
arise from a cornea or crystalline lens with too much

6. *Curvature (refractive myopia ) or an eye
ball that is too long (axial myopia) Myopia
can easily be corrected with a concave lens
which causes the divergence of light rays
before they reach the cornea .
*Hyperopia:
*When the optics are too weak for the
length of the eyeball, one has hyperopia of
farsightedness . This can arise from the
cornea or crystalline lens with no enough
curvature ( refractive hyperopia ) or an

7. eyeball that is too short ( axial hyperopia ).
This can be corrected with convex lenses
which cause light rays to converge prior to
hitting the cornea .
*Cylindrical error:
*Cylindrical error causes astigmatism when
the optical power of the eye is too
powerful or too weak across one meridian
and different in other meridian, such as if
the corneal curvature tends towards a
cylindrical shape . The angle between that
meridian and the horizontal is know as the

8. axis of the cylinder .
*Presbyopia:
*When the flexibility of the lens declines
typically due to age . The individual would
experience difficulty in near vision often
relived by reading glasses , bifocal or
progressive lenses .
*Astigmatism:
*astigmatism is an optical defect in which
vision is blurred due to inability of the
optics of the eye to focus a point object
into a sharp focused image

9. on the retina .
This may be due to an irregular or toric
curvature of the cornea of lens.
*Types of Astigmatism :
*Regular Astigmatism :
principal meridian are perpendicular .
With the rule Astigmatism :
The vertical meridian is steepest ( A
rugby ball or American football lying on
its side)
Against the rule Astigmatism :

10. The horizontal meridian is steepest ( A rugby
ball or American football standing on its end
)
Oblique Astigmatism :
The steepest cure lies in between 120 and
150 degrees and 30 and 60 .
*Irregular Astigmatism :
principal meridian are not perpendicular .
*In with the rule astigmatism , a minus
cylinder is placed in the horizontal axis to
correct the refractive error ( or a plus
cylinder in the vertical axis . Adding a
minus

11. Cylinder in the horizontal axis make the
horizontal axis steeper. Which makes both the
axes equally steep.
In against the rule astigmatism a plus cylinder is
added in the horizontal axis ( or a minus
cylinder in the vertical axis )
*Classification based on the focus of the
principal meridian .
With accommodation relaxed
*Simple Astigmatism .
*Simple hyperopic Astigmatism :
first focal line on the retina while second is
located behind the retina .

12. *Simple myopic Astigmatism :
first focal line is in front of retina while
the second is on retina .
*Compound Astigmatism :
compound hyperopic astigmatism both
focal lines are located behind the retina .
Compound myopic astigmatism both focal
lines located in front of the retina .
*Mixed Astigmatism :
focal lines are on both sides of the retina .