ABO Review Induced Prism Working with Oblique Axis

ABO Review Induced Prism Working with Oblique Axis 1 California State Society for Opticians

Degree from Axis 00/180 30/120 2 45/135 60/150 90/90 % of Cylinder Power . 25*. 50*. 75* 100* 135 What is Oblique Axis? 120 90 60 45 30 150 180 0 Oblique Axis – all numbers not 90 or 180

Calculating Oblique Axis 3 Before calculating induced prism first find cylinder power at 90 degrees. The following is how to find cylinder power at 90 degrees. Degree from Axis 00/180 30/120 45/135 60/150 90/90 % of Cylinder Power . 25*. 50*. 75* 100* Example: • -1. 50 +1. 00 x 045 Total Power @ 90 degrees – 45 = 45 degrees Locate 45 degrees on the chart . 50 Important to memorize the above. Multiply cylinder +1. 00 x. 50 = +0. 50 • -1. 50 +0. 50 @ 90 • Total Power is -1. 00 @ 90 degrees

Prentice’s Rule 4 Calculating Unwanted Prism (also called Induced Prism). Prentice Rule formula written below Prentice’s Rule Formula Prism = Power x Decentration (cm) / 10 Example How much prism is created when a 1. 50 D lens is moved 5 mm? Answer: 1. 50 D x. 5 cm =. 75 of prism Example How much prism is created when a 4. 00 D lens is moved 3 mm? Answer: 4. 00 D x. 3 cm = 1. 20 of prism Apply Prentice Rule d. D P = ------- 10 Also written; d. D = ------- 10 Notice: Multiply by. 3, because when we divide 3/10 =. 3 When taking the exam just move the decimal to the left.

Direction of Prism 5 Direction of Prism Base IN; Base OUT; Base UP; Base Down There are exam questions asking for the direction of the prism after we calculate induced prism, use the following notes to help with the direction of the prism. When the Rx is written with Negative Sphere, example: -1. 50, -2. 50, -5. 50 the Direction of Prism is OPPOSITE When the Rx is written with Positive Sphere, example: +2. 50, +0. 25, +6. 50 the Direction of SAME

is e: Notic ue Ax q i l b O OD -1. 00 – 1. 00 x 045 OS -1. 75 – 1. 00 x 135 P. D. 64 mm O. C. 72 mm (also called the frame PD) 72 – 64 = 8 mm 8/2 = 4 mm (Prentice Rule move decimal left =. 4 cm) Right -1. 50 so you take 1. 50 x. 4 = . 6 prism BI Left -2. 25 so you take 2. 25 x. 4 = . 9 prism BI Total Unwanted Prism = 1. 5 prism BI 90 Axis – 45 = 45 135 on chart =. 50% -1. 00 x. 50% = -0. 50 Add -1. 00 -0. 50 = -1. 50 Calculated Oblique Axis Bring Axis to 90 degrees Prentice Rule 90 Axis – 135 = 45 45 on chart =. 50% -1. 00 x. 50% = -0. 50 Add -1. 75 -0. 50 = -2. 25

7 Practice Oblique Axis OD -2. 00 – 1. 00 x 045 OS -1. 50 – 1. 50 x 135 P. D. 62 mm O. C. 65 mm (also called the frame PD OD -1. 50 – 1. 25 x 030 OS -2. 75 – 1. 00 x 150 P. D. 64 mm O. C. 66 mm (also called the frame PD OD -2. 50 – 1. 50 x 045 OS -3. 75 – 1. 75 x 120 P. D. 66 mm O. C. 70 mm (also called the frame PD OD -1. 00 – 1. 50 x 135 OS -1. 75 – 1. 00 x 120 P. D. 66 mm O. C. 60 mm (also called the frame PD For answers to the above contact the California State Society for Opticians at email: California. Society. Opticians@gmail. com