Chapter 11  (Presbyopia)

1. The onset of presbyopia depends on:

a. the size of the pupil

b. task required

c. sex of the patients

d. the refractive state of the patients

e. amplitude of accommodation
 

Answera.T   b.T   c.F   d.T   e.T
Presbyopia is the age-related loss of the ability to comfortably
sustain the accommodation necessary for clear near vision. 
Its onset is dependent on:
the amplitude of accommodation
task required
refractive state of the patients (earlier with hypermetropia than
myopia)
size of the pupil (small pupil has a pin-hole effect)
latitude where patient lives (onset is earlier in the tropic than temperate countries)
 
 

2. Presbyopia occurs as a result of::

a. loss of elasticity of the sclera

b. sclerosis the lens fibres

c. reduced elasticity of the lens capsule

d. reduced anterior movement of the lens

e. reduced contraction of the ciliary muscle
 

Answera.F   b.T   c.T   d.F   e.T
Presbyopia is multifactorial and is believed to result from poor
ciliary muscle contraction with age and age-related sclerosis
and loss of elasticity of the lens fibres and the lens capsule.
 
 

3. The following are true about accommodation:

a. it decreases with age

b. in order to focus an object at a distance of 25cm, an emmetropic eye needs to use 4D of accommodation

c. in order to focus an object at a distance of 25cm, an eye with -3.00D needs 2D of accommodation

d. in order to focus an object at a distance of 25cm, an eye with +2.00D needs 6D of accommodation

e. for comfortable near vision one-third of the accommodation should be kept in reserve
 

Answera.T   b.T   c.F   d.T   e.T 
A child has about 14D of accommodation which decreases with age.
In order to focus an object at a distance of 25cm, an emmetropic 
eye needs to use 4D of accommodation. For an eye with -3.00D,
only 1D is needed. In an eye with +2.00D, 6D of accommodation
is required (the accommodation is the sum of the patient's refractive 
state + 4D). For comfortable near vision, one-third of accommodation should be kept in reserve.
 

4. The following are true about an emmetropic patient has an amplitude 
     of accommodation of 6D.:

a.  the near point is 33 cm

b. in order to achieve comfortable reading he will need to keep 2D of 
    his accommodation in reserve

c. if the patient is emmetropic, he will require glasses for comfortable 
    reading at 25cm

d. if the patient is myopic, he will require glasses for comfortable reading 
   at 25cm

e. if the patient is hypermetropic, he will require glasses for comfortable 
    reading at 25cm
 

Answera.F   b.T   c.F   d.F   e.T
The near point for this patient is 1/6 = 0.17m=17cm. For comfortable 
reading, one third of his accommodation should be kept in reserve
which is 2D in this patient. Therefore, he has 4D of accommodation
for use. If he is emmetropic, the 4D of accommodation is adequate 
for an object at 25cm. If he is myopic, the amount of accommodation 
required will be less but if the he is hypermetropic, the amount of accommodation needed will be greater than 4D and plus lens will be needed. 
 
 

5. With regard to bifocal glasses:

a. Franklin design involves attaching a supplementary lens to the surface 
    of a distance lens of the same refractive power

b. fused bifocals involves the use of lenses with different refractive indexi.

c. it used for children with accommodative esotropia

d. the problem of prismatic jump is related to the power of the lenses

e. the near visual point of the bifocals is usually inferior and nasal to the 
    distance visual pint.
 

Answer
The Franklin design is a split bifocals in which a distance lens is mounted 
on a near lens. The cemented designs involves attaching a supplementary lens to the surface of a distance lens of the same refractive power. Fused bifocals is made by heat-fusing a near portion made of flint glass into a depression on a crown glass with a lower refractive index. 
Bifocals are used for children with convergence excess esotropia.
The problem of prismatic jump is related to the power of the lenses as well as the distance between the distance of the interface from its optical centre.
a

6. The following can be used to reduce prismatic jump in bifocal glasses:

a. adding base down prism to the distance portion

b. adding base-up prism to the reading section

c. moving the optical centres of the lens near the junction of the two portions

d. using executive glasses

e. pantoscopic tilt
 

Answera.F   b.T   c.T   d.T   e.F
Image jump can be reduced by moving the optical centres towards 
the junction of the two portions as in the executive glasses. Alternatively,
a base up prism can be incorporated into the reading section. 
Pantoscopic tilt is used to reduce astigmatic aberration.

a
a
7.Varifocal lenses:

a. are also called trifocal lenses

b. have no visible interface between the distance and near portions

c. do not produce the prism jump seen in conventional bifocal glasses

d. often produce significant aberration at the periphery of the intermediate 
    portion

e. of the soft designs are better than the hard designs for prolonged VDU 
    works

 
Answera.F   b.T   c.T   d.T   e.T
Varifocal glasses has three sections: distance, intermediate (for VDU or working at arm length) and near (for reading).
Varifocal or progressive lenses have no visible interface between the 
distance and near portions unlike bifocal or trifocal lenses. There is
a power progression corridor (intermediate portion) which reduces
the image jump seen in bifocal glasses; however on either side of
the corridor aberration or astigmatism can become intolerable. 
The soft designs have wider progressive corridor and therefore bigger intermediate portion for VDU work.