Jackson cross cylinder practical 6

Question 1

what is the JCC lens made of ?

Answer 1

• combination of two cylinders with their axis crossed at right angle to each other

Question 2

how far are the axis in the JCC to the handle ?

Answer 2

• axis of the two crossed cylinders are 45deg to
  handle
• axis always at 45 deg to handle

Question 3

what are the two types of JCC ?

Answer 3

astigmatic lens : +0.25/-0.50
comes in other powers
higher powers= bigger changes in astigmatism and as you alter axis - test if subject prefers steeper or shallower axis

Question 4

what does axis depend on ?

Answer 4

axis depends on orientation - how you hold the handle

Question 5

how do you hold JCC ?

Answer 5

• when checking power axis marking of cross CYL are along your cylinder axis
• when checking axis of correcting cylinder you hold with the handle along axis direction

Question 6

what is BVS of JCC ?

Answer 6

0D
we are not changing sphere component
we are changing the magnitude and axis of astigmatism

Question 7

why is JCC preferred method for refining cylinder power and axis ?

Answer 7

. can change correcting cylinder axis and power without changing lenses
. provides quicker and easier way to carry out testing

Question 8

what do you have on the cross-cyl ?

Answer 8

. you have axis marking not directions which power acts

Question 9

how does JCC work ?

Answer 9

. JCC work with standard cyl notation that the axis is the direction along which the power does not acts
. along positive axis we get negative cyl power and along negative axis we get positive cyl power

Question 10

how to investigate cyl power ?

Answer 10

• assumes emmetropic or corrected eye
• put -1.00DC x 45 we create with this lens simple hypermetropic astigmatism and with axis at 45 both horizontals and verticals will be blurred
• accommodation should bring DLC on to the retina -
• we would expect a loss of 1-2 lines based on what we know about the effect of astigmatism on vision

when we use the cross-cyl

• hold 0.50 cross cyl , such that negative cyl axis is along the 45
• effect of cross cyl is to add 0.50 of power along 45 - remember negative cyl axis is where positive power is acting
• the negative power of cross cyl acts along 135 -
• 1.00 becomes -1.50

astigmatism is +0.50 - -1.50 = 2.00D

Question 11

what happens when you twirl the cross cyl ?

-1.00DC x 45

Answer 11

.negative cyl axis along 135 and positive cyl axis along 45

• along 45 where negative cyl power is acting it goes from 0.00 to -0.50
• along 135 where we have to + 0.50 to -1.00 = -0.50
• difference is zero

Question 12

what is the conclusion observed when changing cyl power ?

Answer 12

• cross cyl when held with its axis aligned with the axis of correcting cylinder has the ability to increase or decrease its power
  . 0.50 cross cyl will increase or decrease a power by 1D
  . 0.25 cross cyl will increase or decrease power by half D

Question 13

how does cross cyl change the axis of correcting cyl ?

Answer 13

• assumes emmetropic eye
• prescription in trial frame generates 0.75 D of simple myopic astigmatism axis 180
• advantage of creating simple myopic astigmatism we can generate one clear direction
• must hold cross cyl with handle along the axis of correcting cyl
• in theory vertical direction of fan chart must appear clearest - line blur as you move away - if no then possible uncorrected astigmatism , misalignment of cyl axis

Question 14

what happens when you hold cross cyl along the axis of cyl in trial frame ?

Answer 14

• the effect is to rotate the axis of the astigmatism

- flipping the cross-cyl alternately steepens and flattens the axis of the astigmatism

Question 15

what does the magnitude of the axis shift depend on ?

Answer 15

• the magnitude of the axis shift depends on the magnitude of the cross-CYL and the magnitude of astigmatism
• higher power of cross CYL = greater axis shift

Question 16

what happens if you have weaker correcting cyl in the trial frame ?

Answer 16

• bigger axis shift

Question 17

what is is the effect of lower cyl powers compared to higher cyl powers ?

Answer 17

• lower CYL powers you make relatively large changes in axis but for higher CYL powers you make small
• visual deficit for incorrect axis when cylinder power goes up is much tighter - people respond much more quickly to visual deficit if there is an error in axis

Question 18

what does obliquely crossed cylinder show ?

Answer 18

• SPH and CYL and axis are not completely independent

- when make a change for one of them you will get changes for all of them

Question 19

how determine unknown astigmatic error with the help of cross CYL ?

Answer 19

• subject should’ve had astigmatic error
• must find BVS using ret or a letter chart
• we now have mixed astigmatism and DLC fairly close to the axis
• ask subject to look at target
• we try to determine astigmatic axis for correction - hold cross cyl with horizontal axis and ask subject to look at target and ask ( is it more clear in position1 , position 2 or same )
• if position 1 which is when negative axis of JCC is along 135 - this will tell us that negative correcting cyl for that astigmatic prescription is in the quadrant 90-180
• if position2 which is when negative axis of JCC is along 45 - this will tell us that negative correcting cyl for that astigmatic prescription is in the quadrant 0-90
• same - will tell us that axis is close to the boundary
  which is 90 or 180
• in the next stage hold the JCC handle along the oblique axis 45 or 135
• ask px to look at target again ( position 1 or position 2 or same )
• if position 1 ( quadrant 90-180 ) vertically - negative correcting CYL in the middle of 90-135
• if position 2 ( quadrant 90-180 ) horizontally - negative correcting cyl axis is in the middle of 135 and 180

Question 20

how to estimate power of correcting CYL ?

Answer 20

. DLC is on retina so can use our knowledge that astigmatism has half the effect of sphere
. 1D sphere loses you 3-4 lines , the 1D of astigmatism is half that 1-2 lines
. use negative cyl

Question 21

what is refining axis based on ?

Answer 21

• the basic rule - if there is a preference from your subject in one of the positions of the cross cyl you rotate the axis negative correcting cyl towards that direction
• must hold cross cyl handle along the axis of correcting cyl even when it moves
• when you change direction you halve the step you are making in terms of axis
• refining axis is based on a staircase approach - at each reversal halve the change to home in on your target value

-change the axis by an amount appropriate to the x- CYL power and correcting cylinder power

Question 22

what is summary of experiment ?

Answer 22

• magnitude of the cross-cyl affects the change in astigmatism when checking DC
• magnitude of cross-cyl and correcting cyl affect axis change when refining axis
  higher correcting cyl = smaller axis change
• axis change is based on obliquely crossed-cyls
• the JCC is normally used following ret although it is neat to see that we can start without a ret result by using our knowledge of visual optics
• in this experiment we haven’t changed SPH to keep DLC on retina because accommodation will do that for us