astigmatism

Question 1

mixed astigmatism

Answer 1

occurs when one image focuses in front of the retina and the other image focuses behind the retina. will appear as a + in one meridian and a - in the other on a power cross

Question 2

astigmatism

Answer 2

present in an optical system when the light rays entering the eye are bent unequally due to a non-spherical surface, with the max and min powers (principle meridians) usually 90 degrees apart (regular astigmatism). this causes a point object to be focused as two line images. the separation of these two line images is called the interval of sturm

Question 3

simple astigmatism

Answer 3

one lime image focuses on the retina, and the other line image focuses in front of the retina (simply myopic astigmatism) or behind the retina (simple hyperopic astigmatism). Plano sphere power and some mag of cal

Question 4

compound astigmatism

Answer 4

both lime images focus either in front (myopic compound astigmatism) of or behind the retina (hyperopic compound astigmatism). The powers on the power cross will have the same sign

Question 5

irregular astigmatism

Answer 5

the principle meridians are NOT 090 degrees apart, resulting in a distorted image that CANNOT be corrected with a standard spherocylindrical lens. Occurs as a result of an irregular or distorted cornea (kones)

Question 6

WTR astigmatism

Answer 6

the vertical meridian of the eye has the most plus power and the horizontal meridian of the eye has the least plus power. WTR is corrected by a minus cyl lens with an axis of 180 +/-30

Question 7

ATR astigmatism

Answer 7

the horizontal meridian of the eye has the most plus power, and the vertical meridian of the eye has the least plus power. ATR astigmatism is corrected by a mini Syl lens with an axis 90 +/- 30

Question 8

oblique astigmatism

Answer 8

the most plus power of the eye is between the horizontal and vertical meridians. Corrected by a minus cyl lens with an axis between 30-60 degrees or 120-150 degrees

Question 9

order of types of astigmatism and the amount of blur they cause from greatest to least

Answer 9

oblique
ATR
WTR

Question 10

Javal’s rule

Answer 10

A(RX)=1.25Ac + 0.50 ATR astigmatism

ARX is the refractive astigmatism
Ac is the astigmatism measured on Ks
-0.50DC x 090 is the estimated internal astigmatism

used to predict the amount of astigmatism anticipated in the glasses RX based on the K readings

Question 11

if someones Ks show -5.00x 090 astigmatism, what is the predicted amount of astigmatism in the prescription

Answer 11

A(RX)=1.25Ac + -0.50 WTR astigmatism

1.25(-5.00) + 0.50 ATR=-6.75 x 090

Question 12

K readings with more plus power in the 090 meridian corresponds to ____ astigmatism

Answer 12

WTR

Question 13

K readings with more plus power in the 180 meridian corresponds to ____ astigmatism

Answer 13

ATR

Question 14

1. 50D of cyl power is cut from he OD refraction in the patient’s final specRX in order to improve adaptation, what is the final specRX that will maintain the spherical equivalent of the refraction OD?
   ref: +2.00 -5.00 x 090

Answer 14

+1.25 - 3.50 x 090

Question 15

COLC

Answer 15

dioptrically located halfway between the two line images formed by the principle meridians

Question 16

spherical equivalent

Answer 16

sphere + half of cyl

Question 17

what is the original refraction in plus form

+2.00 -5.00 x 090

Answer 17

-3.00 +5.00 x 180

Question 18

for the patients left eye, a lens rack was used for retinoscopy bc the patient was no longer cooperating. Ret yielded +3.00 in the 180 meridian and a -2.00 in the 90 meridian. assuming the working distance has already been subtracted out, what is the RX OS in minus form?

Answer 18

+3.00 -5.00 x 180

Question 19

what is an example of a Jackson cross cylinder

Answer 19

+1.00 -2.00 x 090

spherical equivalent of 0