Accommodation by the eye

1. Dr Zaid Azhar
M.B.B.S
Ophthalmology Resident SIH
ACCOMMODATION BY THE
EYE

2. • Reduced eye image schematic is used in clinical practice
• Fa = anterior focal point
• P= Principle plane in the anterior chamber
• N= Nodal point at the posterior capsule of the lens
• F2= Second principal focus on the retina
RECAP: REDUCED EYE SCHEMATIC

3. • If the refractive power of eye was fixed, only objects at infinity would
be clear.
• Viewing objects nearer than infinity would be focused beyond F2
and hence produce a blurry image.
ACCOMMODATION BY THE EYE

4. • Thankfully, our eyes have the ability to increase their
dioptric power.
• Accommodation: Ability of the eye to increase it’s
dioptric power is known as accommodation.
• Accommodation is made possible by the ability of the
crystalline lens to change it’s shape.
• A thicker lens bends light more, hence it increases the
eye’s dioptric power.
• A thinner lens bends light less, hence it decreases the
eye’s dioptric power.
ACCOMMODATION BY THE EYE

5. • Ciliary muscles contract
• Ciliary body pulls
forward and inward
• Tension on suspensory
ligaments of lens is
decreased
• Lens becomes thicker
(rounder) due to its
elasticity
• Pupils constricts
ACCOMODATION FOR NEAR

6. • Ciliary muscles relaxes
• Ciliary body returns to
its resting state,
backward and outward
• Tension on suspensory
ligaments of lens is
increased
• Lens becomes thinner
(flatter) due to its
elasticity
• Pupils dilate
ACCOMODATION FOR FAR

7. • Far point: Position of an object when its image clearly falls on retina with no accommodation.
(infinity)
• Near point: Nearest point clearly seen with maximum accommodation.
• Range of accommodation: Distance between far point and near point.
• Amplitude of accommodation: Dioptric power difference between rest and fully
accommodated eye.
A=P-R (near point) or A= V-R (intermediate point)
( A: amplitude of accommodation; P: dioptric value of near point; V: dioptric value of intermediate
point and R: dioptric value of far point.)

8. • Accommodation and convergence go
hand in hand; accommodation for
clear retinal images, & convergence
for binocular single vision.
• Accommodative
Convergence/Accommodation
Ratio: The number of prism dioptres
of convergence which accompanies
each dioptre of accommodation is
(AC/A) ratio.
• The normal range for the AC/A ratio is
3:1 to 5:1
ACCOMMODATIVE CONVERGENCE/ ACCOMMODATION
RATIO

9. • IPD = Inter pupillary distance in
cm
• Dd = Ocular deviation for distance
• Dn: Ocular deviation for near
• D = Near fixation distance in
dioptres
ACCOMMODATIVE CONVERGENCE/ ACCOMMODATION
RATIO
Calculated using:
1) Heterophoria method: ocular
deviation for distance and near
measured with full spectacle correction
• Simple
• Use Prism and alternate cover test
at point of distance fixation i.e 6m
with refractive correction
• Use prism and alternate cover test
at a point of near fixation e.g 33cm
with refractive correction
• + sign = eso deviation
• -ve sign= exo deviation

10. • Dd = Ocular deviation for
distance
• Dn: Ocular deviation for near
• D = power of minus lens
ACCOMMODATIVE CONVERGENCE/ ACCOMMODATION
RATIO
Calculated using:
• 2) Gradient method: uses a minus
lens rather than a near object
• Change in stiumulus produced by
ophthalmic lenses
• -1D lens induces accommodation of
1D
• +1D Lens reduces the
accommodation by 1D
• Full optical correction is carried out
and then other lenses are introduced
and deviation measured

11. • Used to differentiate between
convergence excess esotropia
and esotropia
• Convergence excess
esotropia: AC/A ratio is
abnormally high, eyes are
straight for distance but break
down to convergent squint for
near accommodation. Can be
corrected using bifocal glasses
and surgery
AC/A RATIO CLINICAL USES
Representative photographs of a high accommodative
convergence/accommodation (AC/A) ratio consecutive
esotropia patient.

12. • Images formed in the eye
as a result of reflection from
each refracting surface of
the eye.
• Also known as Purkinje-
Sanson images.
CATOPTRIC IMAGES

13. • P1 (First Purkinje image): It is coming from the anterior surface of cornea. It is virtual and erect because anterior surface
of cornea act as a convex lens.
• P2 (Second Purkinje image): It is coming from the posterior surface of cornea. It is virtual and erect because posterior
surface of cornea act as convex lens.
• P3( Third Purkinje image): It is coming from anterior surface of crystalline lens and it is virtual and erect because anterior
surface of crystalline lens act as convex lens.
• P4 (Fourth Purkinje image): It is coming from posterior surface of crystalline lens and it is real and inverted because
posterior surface of lens acts as concave mirror.

14. CATOPTRIC IMAGES

15. • All images are used for various optical measurements of the eye
• Image I: Used in keratometry, corneal topography and in Hirschburg’s test
(squint)
• Image I and II: Corneal Thickness
• Image III, IV: Used to study changes in lens during accommodation, IOL
centration, to check for lens subluxation, these images are absent in aphakia,
Image IV absent in mature posterior subcapsular cataract
CATOPTRIC IMAGES