7.2 effect of astigmatism on vision Flashcards
what is the effect of astigmatism on vision compared to spherical ametropia ?
astigmatism has less effect on vision than spherical ametropia
what is the link between spherical ametropia and astigmatism and loss of lines on a chart ?
. twice the amount of astigmatism will produce the same loss in lines as spherical ametropia
e.g in 6/9 spherical ametropia = 0.5
astigmatism = 1
. therefore astigmatism has half the effect of spherical ametropia
what is the effect of axis of astigmatism ?
axis of astigmatism can effect the vision we get , appearance of what we’re looking at can change depending on axis of astigmatism
what happens if axis of astigmatism is horizontal along 180 ?
the meridian that is blurred is 90 ( vertical )
however vertical features appear clear and the reason for that is that blurring is along the same direction as the limbs of the letters hence why they seem sharp
what happens if axis of astigmatism is along 90 ( vertical ) ?
blurring is along 180 so horizontal of letter appears clear
what happens if axis astigmatism is along 135 ?
vertical is no longer clear , blur is along direction 0f 135, therefore blurring is along that meridian
where do letters appear most clear ?
along vertical or horizontal meridian
what is difference in blurring between spherical ametropia and astigmatism ?
. spherical ametropia - blurring is equal in all directions
. astigmatism - blurring is different along each direction
what is the effect of astigmatic axis on an eye whose prescription is ?
0.00DS/-2.00DC X 180
. the eye is myopic along 90
. blurring is along vertical direction
. vertical lines appear sharp
. horizontal lines are blurred
this is because vertical meridian is myopic so focus is in front of retina , rays of vertical meridian have spread out by time they hit retina , focus of a point on retina is therefore a vertical line / elipse
therefore every point on each letter becomes a vertical line in the image.
what happens if we blur each point on a letter along its length ?
apart from top and bottom of letter , middle points will appear dark
depending on what we can actually see might allow us to identify letters even though we can’t see horizontal clearly this is why astigmatism has less effect
-for example it would be easier to see U as vertically would appear more clear as the top point of letter would be blurred but the middle would be clear however horizontally a letter such as E would be more blurry
what is the effect of astigmatic axis on an eye whose prescription is ?
0.00DS/-2.00DC x 90
eye is myopia along 90
blurring is along horizontal direction
horizontal lines appear clear
vertical lines are blurred
what leads to distortion of shapes ?
this is due to different powers acting along two principal meridians this causes different magnifications along the principal meridians which can lead to shape distortion
what happens along oblique axis ?
spectacle magnification along 45 and 135 meridian are different so this pulls shape out one direction more than other- stretching of shape in a parallelogram
-reports of astigmatism are optically based
what is effect of astigmatism dependent on ?
. axis
. magnitude of astigmatism
. nature of the object ( orientation of features )
.
what does astigmatism cause?
cause a more complex blurring of the image than spherical ametropia
