Optics of ametropia

1. Optics Of Ametropia
Dr.Mohammad Dmour- PGY1
Supervised by : Dr.Yehya AL-Zorqan
Ophthalmology Department- Islamic Hospital

2. Introduction
• In contrast to emmetropia
the ametropic eye fails to
bring parallel light to a focus
on the retina, i.e. the second
principal focus of the eye
does not fall on the retina.

3. Myopia
• In the myopic eye, the second principal focus lies
in front of the retina .

4. What is the difference between axial and refractive myopia?
In axial myopia , the refractive power of the eye is normal (about 60 D),
but the eye is too long. In refractive myopia , the refractive power of the
eye is too strong (more than 60 D), while the length is normal. Both
situations create a focal point in front of the retina.

5. Hypermetropia
• In the hypermetropic eye, the second principal
focus lies behind the retina.

6. Is there a difference between refractive and axial hyperopia ?
Yes, in axial hyperopia , the refractive power of the eye is normal (about 60
D), but the eye is too short. In refractive hyperopia , the refractive power of
the eye is too weak (less than 60 D), while the length is normal (aphakia is
the extreme example). Both situations move the focal point behind the
retina.

7. Hypermetropia
• Phakic patients can overcome some or all of their
hypermetropia by using accommodation for
distance vision. They then have to exercise extra
accommodation for near vision.
• Because the amplitude of accommodation declines
with age , these patients require reading glasses at
a younger age than emmetropic patients.

8. Hypermetropia
TOTAL
MANIFEST
FACULTATIVE
ABSOLUTE
LATENT

9. What are four types of hyperopia ?
• 1. Manifest hyperopia : Without cycloplegia , this is the most
plus correction the eye can accept without blurring of vision.
the ( strongest convex lens correction accepted for clear
distance vision) .
• 2. Absolute hyperopia : Without cycloplegia , this is the least
amount of plus correction required for clear vision at distance
• 3. Latent hyperopia : is the remainder of the hypermetropia
which is masked by ciliary tone and involuntary
accommodation. ( the difference between manifest hyperopia
and hyperopia measured with cycloplegia)
• 4. Facultative hyperopia : This is the difference between
absolute and manifest hyperopia.

10. A patient requires +1.00 D to see at distance. Manifest
refraction reveals she will tolerate up to +2.00 D. Cycloplegic
refraction is +5 D sphere. What are the absolute, manifest,
cycloplegic, facultative , and latent hyperopia ?

11. w Absolute = +1 D
w Manifest = +2 D
w Cycloplegic = +5 D
w Facultative = 2 – 1 = +1 D
w Latent = 5 – 2 = +3 D

12. Astigmatism
• The refractive power of the astigmatic eye varies
in different meridians. The image is formed as a
Sturm's conoid.

13. • The conoid of Sturm is the three-dimensional envelope
of light rays formed by an astigmatic lens acting upon
the rays of light from a point object.
• Instead of single focal point there are two focal points
separated by focal interval. The distance between two
focal points is called sturms conoid interval.

15. astigmatism
• Regular astigmatism – principal meridians
are perpendicular (at 90° to each other)
▫ With-the-rule astigmatism – the vertical meridian is
steepest (a rugby ball or American football lying on
its side).
▫ Against-the-rule astigmatism – the horizontal
meridian is steepest (a rugby ball or American
football standing on its end).
▫ Oblique astigmatism – the steepest curve lies in
between 120 and 150 degrees and 30 and 60
degrees.
• Irregular astigmatism – principal meridians
are not perpendicular (are not at 90° to each
other ) and cannot be corrected by
spectacles.
Based on axis of the principal meridians

16. astigmatism
• Simple astigmatism
▫ Simple myopic astigmatism – first focal
line is in front of the retina, while the
second is on the retina.
▫ Simple hyperopic astigmatism – first focal
line is on retina, while the second is
located behind the retina.
• Compound astigmatism
▫ Compound myopic astigmatism – both
focal lines are located in front of the
retina.
▫ Compound hyperopic astigmatism – both
focal lines are located behind the retina.
• Mixed astigmatism – focal lines are on
both sides of the retina.
Based on focus of the principal meridians

17. Astigmatism

18. Anisometropia
• When the refraction of the two eyes is different, the
condition is known as anisometropia.
• Small degrees of anisometropia are commonplace.
Larger degrees are a significant cause of amblyopia.
• A disparity of more than 1 D in the hypermetropic
patient is enough to cause amblyopia of the more
hypermetropic eye because accommodation is a
binocular function, i.e. the individual eyes cannot
accommodate by different amounts. The more
hypermetropic eye therefore remains out of focus.

19. Anisometropia
• The myopic patient with anisometropia is less
likely to develop amblyopia because both eyes
have clear near vision. However, when one eye is
highly myopic it usually becomes amblyopic.
• However, myopic patients who have been
anisometropic all their lives may tolerate higher
degrees of anisometropia and achieve binocular
vision with more than 2 D difference between the
two eyes.

20. Far Point
• The far point (FP) of an eye is the position of an
object such that its image falls on the retina of
the relaxed eye, i.e. in the absence of
accommodation.
• The distance of the far point from the principal
plane of the eye is denoted by r, which according
to sign convention carries a negative sign in
front of the principal plane and a positive sign
behind the principal plane.

21. The far point in emmetropia is infinity

22. The far point in myopia lies a finite distance in front of the eye.

23. The far point in hypermetropia is virtual, as only
converging light can be focused on the retina.

24. Optical Correction of Ametropia
• The purpose of the correcting lens in ametropia
is to deviate parallel incident light so that it
appears to come from the far point in myopia or
to be converging towards the virtual far point in
hypermetropia.
• The light will then be brought to a focus by the
eye on the retina. Thus the far point of the eye
must coincide with the focal point of the lens.

25. Optical Correction of Ametropia
• The focal length, f, of the correcting lens is
approximately equal to (») the distance, r, of the
far point from the principal plane when the
correcting lens is close to the principal plane
• Thus the power of lens, F, required is

26. Optical Correction of Ametropia
• where F is the power of the lens in dioptres; f is
the focal length of the lens in metres; and r is the
distance of the far point from the principal plane
in metres.

27. Effective Power of Lenses
• The power of the correcting lens must be adjusted to
take into account its position in front of the eye.
• Suppose a lens of focal length f1 at a given position
in front of the ametropic eye corrects the refractive
error; then a different lens of focal length (f1 – d) is
required when the correction is moved a distance d
towards or away from the eye.
• The value of d is positive if the lens is moved
towards the eye, and negative if moved away from
the eye. The usual sign convention applies to the
lens.

28. Effectivepoweroflenses
Formula to calculate the new focal length of lens at the
new distance:
F2= 1/ f1- d or
F2= F1/ 1- Df1
Where, F1= power of the original lens in diopters
F2= power of lens in diopters at new position
f1= focal length in meters of original lens
d= distance moved in meters. It is taken positive if
moved toward the eye and negative if moved
away from the eye.

29. Effectivepoweroflenses
In uncorrected hyperopia the
image of an object falls behind the
retina.
The purpose of convex lens is to
bring the image forward.
If the correcting lens is itself
moved forward the image will
move still forward.ie- the
effectivity of the lens is increased.
Thus a weaker lens is required to
project the image onto the retina
in uncorrected myopia the image
falls in front of the retina.
The purpose of the concave lens is
to bring the image behind.
If the correcting lens is itself
moved forward the image moves
still forward.ie- the effectivity of
the lens is reduced.
Thus a stronger lens is required to
project the image onto the retina
MyopiaHypermetropia

30. Correction of hypermetropia.

31. Correction of myopia.

32. Practical Application:
Back Vertex Distance
• For any lens of power greater than 5 dioptres, the
position in front of the eye materially affects the
optical correction of ametropia. This is especially true
in aphakia where high power lenses are prescribed.
• For this reason the refractionist must state how far
in front of the eye the trial lens is situated so that the
dispensing optician can adjust the lens power if a
contact lens is to be used, or if spectacles are to be
worn at a different distance, e.g. because of a high-
bridged nose or deep-set eyes.

33. Practical Application:
Back Vertex Distance
• Therefore any high powered lens should be placed in the
back cell of the trial frame and the distance between the
back of the lens and the cornea measured. This is called
the back vertex distance (BVD and must be given with all
prescriptions over 5 dioptres.
• The measurement may be made with a ruler held parallel
to the arm of the trial frame. Other means include a small
rule which is slipped through a stenopaeic slit placed in
the back cell of the trial frame until it touches the closed
eyelid. Two millimeters must be added to the
measurement to correct for the thickness of the lid.

35. Example 1
• Refraction shows that an aphakic patient
requires a +10.0 D lens at BVD 15 mm. He
needs a contact lens (F2)

37. Example 2
• Likewise a high myope whose spectacle
correction is –10.0 D at BVD 14 mm
requires a contact lens (F2)

38. Happily, tables exist which give the value of F2 when F1 and d are known !