8.1 - effect of astigmatism on vision 2

Question 1

what effect does astigmatism have on vision compared to spherical ametropia

Answer 1

. astigmatism has half the effect that spherical ametropia has on vision
. to lose 3-4 lines on 6/18 line we would need 1D of sphere but to get the same loss with astigmatism we need twice the amount 2D

Question 2

what does the size DLC depend on ?

Answer 2

. size of DLC depends on the pupil size , magnitude of astigmatism
. it’s then divided by sum of the principal meridian powers of the astigmatic eye

Question 3

where does the DLC lie ?

Answer 3

. since the DLC lies midway dioptrically between the astigmatic foci
. we can take half the sum of the principal meridian powers of the astigmatic eye and equate that to K’ because disc of least confusion fall on retina

Question 4

explain the difference between spherical ametropia and astigmatism using DLC (z) and blur circle equation ?

Answer 4

. z= p x ast / 2K’
. b= p x k / K’

. therefore the blur produced by 1D of astigmatism is half that of the equivalent amount of spherical ametropia - this is due to that in the equation it is divided by 2K’ and blur circle is divided by K’.
. this assumes that DLC can be placed on retina but if placed on retina DLC (astigmatism) will have half the effect as spherical ametropia because blurring is half

Question 5

what is the best vision sphere ( BVS ) ?

Answer 5

. it’s the spherical correction in the spectacle plane that places the disc of least confusion on the retina for distance vision

Question 6

how to determine the BVS ( best vision sphere ) ?

Answer 6

. from sph/cyl prescription take the sphere and add half the cylinder power:
-1.00/-2.00x30
BVS = -1.00+(-2.00/2)=-2.00DS
so -2.00DS will push DLC on retina

-can also do for principle meridian powers
. -1.00x30 and -3.00x120

BVS = ( -1.00 + ( -3.00 ))/2 = -2.00DS will push DLC on retina

Question 7

what are the classification of astigmatism ?

Answer 7

. compound hypermetropic astigmatism - both astigmatic foci fall behind the retina

. simple hypermetropic astigmatism - one focal line falls on retina and the other falls behind the retina
- in theory this could be corrected by single positive CYL but in practise we correct most positive , least negative prescription will be written +sSPH / - CYL

. mixed astigmatic - astigmatic foci lie either side of retina
this doesn’t mean that DLC lies on retina but rather close

. simple myopic astigmatism
one astigmatic foci fall infront retina
other foci falls on retina

. compound myopic astigmatism
both foci falls in front retina

Question 8

what is the effect of accommodation on CHA and SHA ?

Answer 8

. accommodation can improve vision by placing the DLC on the retina which there will be a circular blur path and equal blurring in all directions
- this is because both astigmatic foci lie behind retina

Question 9

what is the effect of accommodation on mixed astigmatism ( MA ) ?

Answer 9

. the DLC can lie in front or behind the retina - it is not always possible to improve vision

Question 10

what is the effect of accommodation on SMA and CMA ?

Answer 10

. there is no scope for improving vision with accommodation for SMA or CMA
. this is because astigmatic foci lie behind the retina

Question 11

what is the effect of axis on astigmatism when the magnitude is the same ?

Answer 11

. at different axis different features of the letter appear clearer depending on what axis it is
. with horizontal axis 180 it’s the verticals on the letters that appear clearer
. with 90( vertical axis ) the horizontal appear clear
. in between we get different diagonals clear

Question 12

what lines predominate in test letters ?

Answer 12

. verticals and horizontals lines predominate in test letters - particularly verticals

Question 13

what do oblique axis cause ?

Answer 13

. oblique axis - these have axis values that are away from horizontal and vertical 45 +/- 15 deg 135 +/- 15deg
. oblique axis cause a greater reduction in letter acuity because those oblique axis values don’t don’t give us clear vision in any of the directions on the letters apart from 1 or 2 diagonals
. in oblique axis astigmatism blurs both horizontal and vertical limbs

Question 14

what happens to axis directions close to 90 or 180 ?

Answer 14

blur the letters along one of their common directions

Question 15

how does the distortion of shapes occur in oblique axis ?

Answer 15

. letters appear to be distorted - pulled out into a slightly different shape
. if we take a square object and look at it through a +3.00DC axis 45 , this distorts the shape and the shape pulls into a parallelogram
. this distortion is caused by the difference in spectacle magnification along two principle meridian
. along 45 power is plano
. along 135 power is 3.00D

Question 16

how is the spectacle correction of astigmatism carried out ?

Answer 16

. must work with principal meridian powers
. determine ocular refraction for each meridian
. adjust for vertex distance to give spectacle refraction
. write in sph-cyl

Question 17

how can you go from spectacle prescription to ocular refraction ?

Answer 17

. work backwards

. work out cross cyl which power of spectacle lens along two different meridians

Question 18

what is the effect of astigmatism with oblique axis ?

Answer 18

. astigmatism with oblique axis values has a greater deficit on vision because the dominant directions in letters are vertical and horizontal lines

Question 19

why does distortion occur ?

Answer 19

distortion occurs due to difference in spectacle magnification - shapes are pulled out by different amounts in different direction

Question 20

why is loss in hypermetropic astigmatism less than myopic astigmatism ?

Answer 20

because in hypermetropic astigmatism accommodation can bring different features of astigmatic image on retina while in myopic astigmatism accommodation makes blurring worse