practical 1 summary

Question 1

what is the effect of putting a positive trial lens on an emmetrope or corrected ametrope ?

Answer 1

. positive lens increases convergence at the eye
. the image moves in front of the retina
. stimulates myopia in an emmetropic or corrected eye

Question 2

what is the effect of putting a negative trial lens on an emmetrope or corrected ametrope ?

Answer 2

. negative lens creates divergent light at the eye
. the image moves behind the retina
. stimulates hypermetropia in an emmetropic or corrected eye

Question 3

what is the x-axis ( independent variable ) ?

Answer 3

stimulated refractive error(D) but must swap the signs
. minus lens power become positive stimulated hypermetropia .
. the negative stimulated myopia using positive lens is to the left of zero on x-axis

Question 4

why don’t we use snellen fraction to label y-axis ?

Answer 4

. need to scale 2/36 , 3/60 to a fixed testing distance
. steps are not all equal ( ordinal scale )
. can’t score 6/6 + 2

Question 5

why don’t we use decimal V to label y-axis ?

Answer 5

. its reciprocal scale instead of interval
. vision is worse for lower number
. can’t score 6/6 - 2

Question 6

why do we use MAR ( min of arc ) to label y-axis ?

Answer 6

. 1/decimal V
. scaling ok
. increases as vision/VA gets worse
. still can’t score 6/6 -1

Question 7

what was the effect of myopia when stimulated with positive lens ?

Answer 7

when stimulating myopia with positive lens vision got worse rapidly

Question 8

what was the effect of hypermetropia when stimulated with negative lens ?

Answer 8

. when increasing the amount of hypermetropia , subject was not suffering a big reduction in vision compared to when stimulating myopia and this is due to accommodation
. vision starts to get worse for high levels of stimulated hypermetropia

Question 9

what is accommodation ?

Answer 9

. the ability of eye to increase power to focus on near object
. the negative lens that stimulates hypermetropia pushes the image behind the retina , if the eye can increase its power that has the ability to move the image forward in the eye and put image on retina , keeping image sharp
. in young px accommodation is particularly high , subject is able to compensate for negative lens that you put up

Question 10

why shouldn’t you leave hypermetropia un corrected ?

Answer 10

this only works in our experiment because we are only using one eye, when you get accommodation for two eyes when you looking into distance you risk squints happening for children which can effect development of eye

Question 11

summary of results ?

Answer 11

. vision gets rapidly worse in myopes
. accommodation can compensate for hypermetropia in young subjects although hypermetropia should not be left uncorrected
. a pinhole improves vision when there is significant optical blurring
. a pinhole can reduce vision if the retinal image is already sharp
. vision falls 3-4 lines per D - but note accommodation in young hypermetropes
. use 1.00 DS check test - must loose 3-4 lines to make sure image is on retina and we are not using accommodation