1. What amount of prism is present 2 mm below the optical center of a +8.50 D lens?

   d = 2
   D = +8.50
   P = ?    d D
P = -------
   10
P = (2)(8.50)/10 = 1.7 

The wearer is looking below the optical center. The optical center is UP. It is a plus lens, so the base is where the optical center is. The base is UP. The answer is 1.7  BU






2. What amount of prism is the wearer experiencing if the optical center is moved 5 mm out toward the temple on a -3.25 DS lens?

   d = 5
   D = -3.25
   P = ?    d D
P = -------
   10
P = (5)(3.25)/10 = 1.6 

The optical center is OUT. It is a minus lens, so the apex is where the optical center is, and the base is in the opposite direction. The base is IN. The answer is 1.6  BI






3. The optical center is 4 mm below the pupil on a +7.75 DS lens. What prism is the wearer experiencing?

   d = 4
   D = +7.75
   P = ?    d D
P = -------
   10
P = (4)(7.75)/10 = 3.1 

The optical center is DOWN. It is a plus lens, so the base is where the optical center is. The base is DOWN. The answer is 3.1  BD






4. The Rx is +5.00 -1.50 x 090. What amount of prism is present 8 mm below the optical center?

 

 
 
 

First we need to know what the power is. The wearer is looking below the optical center, so we need the power vertically or on the 90th meridian of the lens. On the 90th meridian for this Rx there is no cylinder power present, and the power is +5.00.
   d = 8
   D = +5.00
   P = ?

   d D
P = -------
   10
P = (8)(5.00)/10 = 4.0 

The optical center is UP. It is a plus lens, so the base is where the optical center is. The base is UP. The answer is 4.0  BU






5. The Rx is +5.00 -1.50 x 090. If the optical center is decentered out 5 mm from the wearer's pupil, what amount of prism is present?

 

 
 
 

First we need to know what the power is. The wearer is looking to one side of the optical center, so we need the power horizontally or on the 180th meridian of the lens. On the 180th meridian for this Rx there is 100% cylinder power present, and the power is +3.50.
   d = 5
   D = +3.50
   P = ?

   d D
P = -------
   10
P = (5)(3.50)/10 = 1.75

The optical center is OUT. It is a plus lens, so the base is where the optical center is. The base is OUT. The answer is 1.75  BO






6. The Rx is -10.00 -2.75 x 045. What prism is present when the wearer looks down through the lens at a point 6 mm below the optical center?

 

 
 
 

First we need to know what the power is. The wearer is looking below the optical center, so we need the power vertically or on the 90th meridian of the lens. On the 90th meridian for this Rx there is 50% cylinder power present, and the power is -11.37.
   d = 6
   D = -11.37
   P = ?

   d D
P = -------
   10
P = (6)(11.37)/10 = 6.8 

The optical center is UP. It is a minus lens, so the apex is where the optical center is, and the base is in the opposite direction. The base is DOWN. The answer is 6.8  BD

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