practical 4 - effect of the magnitude of astigmatism on letter acuity

Question 1

what is your independent variable ? - variable that you are controlling

Answer 1

. axis - must no change otherwise you will mix up the effects of the change of axis with changes with magnitude of astigmatism
. sphere power

Question 2

wha is the dependent variable ? thing you are changing

Answer 2

. CYL power - its cyl power that affects magnitude of astigmatism
. a range of cyl powers is required so that the effect of different amounts of astigmatism can be explored
. sign of cyl power affects the type of astigmatism

Question 3

what happens if you use cyls alone ?

Answer 3

. cyls are plano along their axis direction - this means that light entering horizontally 180 axis along direction which its plano is left un affected
. the eye is already corrected ( image on retina ) - therefore cyl leaves one astigmatic image on retina

Question 4

what does +ive cyl produce ?

Answer 4

produce simple myopic astigmatism -
this is because positive power in the direction perpendicular to axis pulls the image forward in eye , therefore pulls image in front of the retina , one astigmatic foci stays on retina because of plano power along axis direction and positive power pulls other axis in front

Question 5

what does -ive cyl produce ?

Answer 5

produce simple hypermetropic astigmatism -

the plano power will leave one focus on retina and negative power pushes other astigmatic focus behind retina

Question 6

what axis should we use ?

Answer 6

. axis is interaction between astigmatism and object you are looking at
. oblique axis 45/135 axis blurs both horizontals and verticals equally but some diagonals will be clear - reduce number of letters that might be correctly identified due to blurring along significant directions in the letter
. must not change axis

Question 7

what happens if you stimulate astigmatism on a letter E along 90 ?

Answer 7

. its not blurry along 90 direction and it’s blurring along 180
. horizontal features are pulled out blurred along their extent( 180 ) and therefore still appear reasonably clear

Question 8

what happens if you stimulate astigmatism on letter E along 180 ?

Answer 8

. blurring is now along 90
. this will blur significantly horizontals
. we can still see vertical limb of letter E which is being blurred along its length

Question 9

why might we not want to use 90 or 180 axis ?

Answer 9

90 or 180 axis cyls blur along common directions in letters- the letter can still possibly be recognised even with significant blurring -
they will bias the data - as some letters are being recognised easily

Question 10

what is the order of presentation of cyls ?

Answer 10

. in general , randomisation is good in that it avoids predictability and any resulting errors are random
. responses from subject maybe more reliable since they can’t predict what should happen ( single blinding )

Question 11

how to stimulate the compound myopic astigmatism ?

Answer 11

. positive cyl( +DC) will produce astigmatic image with one focus on retina and one other astigmatic focus in front of the retina
. positive sph (+DS) - we wil move the whole of that astigmatic image forward in eye , moves everything equally , both astigmatic image on retina and astigmatic image in front retina - can’t change the separation of the astigmatic foci

Question 12

what affects astigmatism ?

Answer 12

the magnitude of the cyl affects the astigmatism - the separation of the astigmatic foci

Question 13

what spheres used for ?

Answer 13

. sphere are needed to shift the astigmatic focus off retina

Question 14

what spheres used for ?

Answer 14

. sphere are needed to shift the astigmatic focus off retina
- this is because on of the astigmatic foci will always stay on retina , this is because along axis of cyl the power is plano and will not shift image

Question 15

how to stimulate compound hypermetropic astigmatism ?

Answer 15

. -DC - negative cyl - one astigmatic focus stays on retina and other astigmatic focus is pushed behind the retina

.-DS - moves both astigmatic focus behind the retina - shift whole astigmatic image behind retina

Question 16

what should you not mix ?

Answer 16

. don’t mix +DS and - DC

Question 17

what is the amount of sphere to add ?

Answer 17

. if you add +1.00 DS blurs 3-4 lines , add some astigmatism on top you will use more lines and will get to top of chart
. swap +1.00DS with +/-0.50DS to generate CMA and CHA this is because we aren’t interested on the effect of sphere on vision

Question 18

why does negative sphere have less effect on vision ?

Answer 18

negative sphere will be compensated by subject’s own astigmatism

Question 19

what is the difference between stimulating spherical refractive error and astigmatism ?

Answer 19

. spherical refractive error - there is a sharp image in eye its just in wrong location
. astigmatism - no part of image that is sharply focused , astigmatism must removed

Question 20

what happens in CHA and SHA ?

Answer 20

. eye can exert accommodation and pull image forward and place it on retina
and disc of least confusion on retina
. in theory this means that there shouldn’t be a difference in results between CHA and SHA?

Question 21

what happens in mixed astigmatism ?

Answer 21

. depends on where the image falls- is least of disc of least confusion in front or behind retina
if behind retina than accommodation can make an impact

Question 22

what happens in SMA AND CMA ?

Answer 22

accommodation will make things worse -
if accommodation acts on SMA it will make it CMA
if accommodation acts on CMA it will make it worse by adding more myopic error

Question 23

where is the best focus in the presence of astigmatism ?

Answer 23

. depends on the interaction of astigmatism with object you are looking at
. e.g. if asked to distinguish set of dark and light lines that are horizontal then axis of 90 which will blur things along horizontal lines, thus will make them clear and easy to see

Question 24

why might you expect SHA and CHA to cause less loss of vision than myopic astigmatism ?

Answer 24

. this is due to accommodation making vision worse in myopia
. in hypermetropia accommodation allows disc of least confusion to be placed in retina

Question 25

would you expect a difference between SHA and CHA ?

Answer 25

. in theory NO because accommodation brings disc of least confusion onto retina in both cases

Question 26

would you predict a difference between SMA and CMA ?

Answer 26

. in CMA there is a positive sphere being added to pull image forward in retina
therefore in CMA result should be 1-2 lines lower than SMA

. trend and reduction for vision should be same