brief description of the optics of an ametropic eye namely myopia, hypermetropia, astigmatism and aphakia.

1. OPTICS OF AMETROPIA
Presenter: Dr.Shamika
Moderator: Dr.Amit C.
28/1/2016

2. Ametropia is defined as a state of
refraction wherein the parallel rays of
light coming from infinity are focussed
either in front or behind the retina which
therefore receives a blurred image.
It includes-
Myopia
Hyperopia
Astigmatism
Aphakia

3. HYPERMETROPIA  Long sightedness.
 Images are focussed behind retina.
ie. Posterior focal point is behind retina.
Mechanisms-
1. AXIAL: 1mm shortening of AP diameter of eye causes 3D of
hypermetropia.
2. CURVATURAL: curvature of cornea or lens or both is flatter
than normal. 1mm decrease in radius of curvature causes
6D of hypermetropia.
3. INDEX: due to change in refractive index of lens in old age
and in diabetics.
4. POSITIONAL: posteriorly placed lens.
5. ABSENCE OF CRYSTALLINE LENS.

4. CLINICAL TYPES:
1) SIMPLE HYPERMETROPIA: due to biological variation in size and
shape of eyeball. Could be axial or curvatural.
2) PATHOLOGICAL:
A) CONGENITAL: Associated with microphthalmos, micro cornea
congenial posterior subluxation of lens or congenital aphakia.
B)ACQUIRED:
i. SENILE: could be curvatural or index (due to cortical
sclerosis)
ii. POSITIONAL: subluxation of lens
iii. APHAKIA.
iv. CONSECUTIVE:
v. ORBITAL MASS: tumors or edema may push the retina
forward.
C) FUNCTIONAL: due to paralysis of accommodation as in 3rd
nerve palsy.

5. COMPONENTS OF HYPERMETROPIA:
TOTAL
MANIFEST
FACULTATIVE
ABSOLUTE
LATENT
Total- after complete cycloplegia with atropine.
Latent- about 1D of hyperopia that is corrected by
inherent tone of ciliary muscles.
Manifest- remainder of hyperopia.
Facultative- can be corrected by patient’s accommodative
effort.
Absolute- residual of manifest hyperopia.

6. MYOPIA  Short sightedness.
 Parallel rays coming from infinity focus in
front of retina with accommodation at rest.
MECHANISMS:
1. AXIAL: increased axial length of the eye ball.
Commonest form.
2. CURVATURAL: increased curvature of cornea or lens
or both.
3. INDEX: increased refractive index of lens with nuclear
sclerosis.
4. EXCESSIVE ACCOMMODATION: in spasm of
accommodation.

7.  In myopia the image of a distant object is formed of the
divergent beam.
 Far point of the myopic eye is at a finite point in front of
the eye.
 Nodal point of the eye is further away from the retina.
Hence the image of the object formed is larger than
that of the emmetropic eye or spectacle corrected eye.
This compensates for visual acuity to some extent.
 They do not need to accommodate. Hence it is not well
developed and they may suffer from convergence
insufficiency, exophoria or early presbyopia.

8. ASTIGMATISM
Astigmatism is a refractive error in which the
refraction varies in different meridia.
ETIOLOGY:
 CORNEAL: due to abnormalities in the curvature of cornea.
Maybe congenital or acquired (often irregular).
 LENTICULAR:
 CURVATURAL: due to abnormal curvature of the lens. eg- lenticonus.
 POSITIONAL: due to oblique placement or tilting of the lens eg-in
subluxation.
 INDEX: due to difference of refractive index of the in different
meridia.
 RETINAL: due to oblique placement of macula.

9. ASTIGMATISM Regular astigmatism – principal meridians 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 – 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.

10. 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 (straddling the retina)

11. FARPOINT
Far point of the eye is the position of
an object such that its image falls on
the retina of the relaxed eye (ie. With
accommodation relaxed).
For emmetropia it is at infinity.
For myopia it lies at a finite
distance in front of the eye.
In hypermetropia it is virtual (as
only converging light can focus on
the retina in hyperope).

12. OPTICALCORRECTIONOF
AMETROPIA
The purpose of optical correction is to deviate the rays of
light so that they appear to come from the far point.

15. 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.
Similarly 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

16. EFFECTIVEPOWEROFLENSES Thus the convex lens in
hypermetropia has to be made
weaker and the concave lens in
myopia has to be made stronger
when the lens is moved further
away from the eye
Hence aphakics or high
hyperopes pull their glasses
down their nose to read.
While myopes do not like their
glasses slipping down.

17. 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.

18. BACKVERTEXDISTANCE
For any prescription greater than 5D especially in aphakics the
refractionist must state how far the trial frame was placed, to
adjust the power of contact lens is used or if the glasses are to
be worn at a different distance.
The distance between the back of the lens and the cornea
must be measured.
Measurement can be made with a ruler held parallel to the arm
of the trial frame or slipped through a steanopic slit till it
touches thee closed lid. 2mm should be added to correct for
the thickness of the lid.

19. BACKVERTEXDISTANCE
Example 1: A patient has been prescribed glasses
with +16.00D sphere at a BVD of 14mm. He selects a
frame that fits him at a BVD of 16mm. What is the
power of the new lens?
Ans: +15.50D

20. BACKVERTEXDISTANCE
Example 2: A aphakic patient requires a +10.00D lens
at BVD 15mm. He now wants a contact lens. What
should be the power of the contact lens?
Ans- +11.75D

21. BACKVERTEXDISTANCE
Example 3: A patient was given a prescription of -
16.00D at a BVD of 14mm. He selects a spectacle
frame of BVD 16mm. What will be the power of the
new lens?
Ans- -16.50D

22. BACKVERTEXDISTANCE
Example 4: A high myope whose spectacle correction
is -10.00D at BVD 14mm requires a contact lens. What
is the power of the contact lens?
Ans- -8.75 D

23. SPECTACLEMAGNIFICATION The optical correction of ametropia is associated with in a
change in the retinal image size.
Spectacle magnification = corrected image size
uncorrected image size
Relative sp. magnification = corrected image size
emmetropic image size

24. SPECTACLEMAGNIFICATION
In axial ametropia, if the correcting lens is placed at the anterior focal
point of the eye then the image size is same as that of emetropia.
But in refractive ametropia the image size differs even if lens is placed at
the anterior focal point.
In refractive hypermetropia the image size is increased. RSM>1
While in refractive myopia the image size is reduced.RSM<1
As the distance of the lens approaches the eye the image size
approaches the emmetropic size

25. SPECTACLEMAGNIFICATION RSM= 1.36 for aphakia with lens at anterior focal point ie 23.2mm
RSM= 1.33 for aphakia with lens placed at 12-15mm
RSM= 1.1 for contact lenses.
normal
contact lens
spectacles

26. OPTICALPROBLEMSINAPHAKIA
1. SPECTACLE MAGNIFICATION:
The spectacle magnification produced by
aphakic glasses is 1.33. thus the image is one
third times larger than emmetropes.
The patient thus tends to misjudge distances.
Objects appear closer to the eye than they are.
Leads to enhanced performance in visual acuity
tests.

27. OPTICALPROBLEMSINAPHAKIA
2. DISTORTION OF IMAGES DUE TO
ABERRATIONS:
Straight lines appear curved except
through a small central portion of the lens.
At the periphery of the lens the lines
appear to be more curved- pincushion
effect.
Thus the environment appears as curves as
the patient moves his eyes across different
parts of the lens. Patients adapt to this by
moving their head rather than eyes.

28. OPTICALPROBLEMSINAPHAKIA 3) PRISMATIC EFFECT OF LENS:
The prismatic effect increases towards the
periphery of the lens.
It produces a troublesome ring scotoma at
the edge of the lens. Hence they can trip
over unseen objects.
The direction of the ring scotoma changes
and objects disappear into the scotoma and
appear to reappear out of it- jack in the box
phenomenon.

29. OPTICALPROBLEMSINAPHAKIA
4) DUE TO WEIGHT OF THE GLASSES:
Aphakic glasses are very heavy and tend to slip
down the nose.
Plastic glasses are lighter but less scratch resistant.
Lenticular form of lenses reduce weight but also
reduce field of vision.

30. OPTICALPROBLEMSINAPHAKIA
5) UNILATERAL APHAKIA WITH
NORMAL FELLOW EYE:
The image in aphakic eye is one third larger hence
causes aniseikonia. Patient is unable to fuse these
images and hence suffers from diplopia.
The use of contact lenses and intra ocular
implants reduce this effect.
Aniseikonic glasses though available are very
heavy and costly.

31. THANK YOU