10.2 basic principles of accommodation

Question 1

what is near point (Mp )?

Answer 1

• point conjugate with the retina when the eye is in its fully accommodated state
• subscript p- punctum proximum

Question 2

what happens when eye accommodates ?

Answer 2

power of eye increases- thus allowing eye to focus at near objects

Question 3

what is the equation of unaccommodated eye ?

Answer 3

K’=K+Fe

Question 4

what is the equation for accommodated eye ?

Answer 4

K' = B + Fe + Amp
B= near point distance in D

Question 5

what is the amplitude of accommodation equation ?

Answer 5

Amp = K - B 
therefore 
B ( near point distance ) = K - Amp

Question 6

why is it important when using RAF rule that subject distance must be fully corrected ?

Answer 6

• if subject is distance corrected the K= 0 then we can measure amplitude of accomodation directly from near point distance

Question 7

what is difference between Amp s and Amp o ?

Answer 7

Amp s - distances are referenced to the spectacle plane

Amp o - distances are referenced to the eye

Question 8

what does accommodation allow us to do ?

Answer 8

• accommodation allows us to focus over a range of distances

Question 9

what is range of accommodation r acc ?

Answer 9

• linear distance from far point to near point

- range of distances which we can see things clearly

Question 10

what is the equation for r acc ?

Answer 10

r acc = b - k ( lower case )

Question 11

what is range of accommodation for emmetrope ?

Answer 11

far point distance ( k) = ∞

r acc = ∞

Question 12

what are the two parts in the range of accommodation for hypermetropia ?

Answer 12

there are two parts to the range of accommodation : virtual and real

Question 13

what is the virtual and real part for a px with
Amp  +12D
K = +4D 
k = + 250 mm  ( behind eye )
b= -125 mm ( in front eye ) ?

Answer 13

virtual part : the first 4D of accommodation will act to reduce the blur of a distance object and bring it into sharp focus
- amount of power needed to over come hypermetropia and allowed us to see things at infinity

real part : the remaining 8D of accommodation can help to focus on near objects - the useful part

Question 14

what is range of distinct vision ?

Answer 14

• the real part of the range of accommodation
• same as r acc for emmetropes and myopes
• always at infinity or zero in hypermetropia

Question 15

what happens if amplitude of accommodation is bigger than ocular refraction ?

Answer 15

Amp>K

near point will be real and the furthest real point of distinct vision will be at infinity

Question 16

what happens if amplitude of accommodation is smaller than ocular refraction ?

Answer 16

Amp < K
near point is virtual and therefore there is no part of the range of accommodation that is real
- range of distinct vision is zero

Question 17

what is the range of distinct vision in hypermetropia ?

Answer 17

is either always at infinity or zero depending on the relationship between the amplitude of magnification and hypermetropia

Question 18

what are the two types of hypermetropia ?

Answer 18

• facultative - amount of hypermetropia that can be compensated by accommodation to produce clear distance vision
• absolute - remaining hypermetropia that can not be neutralised by accommodation

Question 19

what happens at age of 20 ?

Answer 19

• accommodation is around 12D
• high amount of facultative hypermetropia
• little absolute hypermetropia

Question 20

what happens at age of 40 ?

Answer 20

• 2-3D of accommodation
• facultative hypermetropia decrease
• absolute hypermetropia increases

Question 21

what is amplitude of accommodation ?

Answer 21

maximum increase in power