LENS POWER AND LENS THICKNESS

Question 1

Power of a thick lens depends on the following:

Answer 1

• Index of refraction
• Surface powers
• Thickness of the Lens

Question 2

the power of a lens that is derived from the combination of its front and back curves

Answer 2

Nominal Power

Question 3

Nominal power is measured with a ______

Answer 3

Lens Clock

Question 4

is the power that the lens have if it was reduced to a thin lens

Answer 4

Equivalent Power/ True Power

Question 5

is the power specified for ophthalmic lenses as the eye is usually positioned behind the back vertex of the lens

Answer 5

Effective Power / Back Vertex power

Question 6

the reciprocal of the reduced distance from the primary focal point to the front vertex of the lens

Answer 6

Neutralizing power / Front Vertex power

Question 7

is the change in vergence of light that occurs at different points along its path

This is related to vertex distance

Answer 7

Lens effectivity / Effective power of lens

Question 8

Lenses in general should be placed before the eyes at a distance of ___mm to ___mm from the corneal vertex to coincide with the eyes anterior nodal point

Answer 8

12mm to 14mm

Question 9

The vertex distance for contact lens is?

Answer 9

Zero

Question 10

used to measure the vertex distance from the back surface of the spectacle to the cornea through a close eyelid

Answer 10

Vertexometer or Distometer

Question 11

Effective power of PLUS lens when Moved Towards the eye:

Answer 11

Decreased Effective power

LESS PLUS

Question 12

Effective power of PLUS lens when moved AWAY from the eye:

Answer 12

Increase Effective power

MORE PLUS

Question 13

Effective power of MINUS lens when moved TOWARDS the eye:

Answer 13

Increased effective power

MORE MINUS

Question 14

Effective power of MINUS lens when moved AWAY from the eye:

Answer 14

Decreased effective power

LESS MINUS

Question 15

results in shifting the optical axis away from the center of rotation

it means that the line of sight is at an angle to the lens in its primary position of gaze

Answer 15

Lens Tilt Induced Power

Question 16

When we tilt the lens either horizontally or vertically, we are potentially inducing ______ in the lens

Answer 16

astigmatism

Question 17

The induced astigmatism when tilting the lens is called _______ or sometimes called as ______.

Answer 17

Marginal astigmatism

Oblique Astigmatism

Question 18

• a lens tilt about the horizontal axis with respect to primary gaze
• tilt ranges from 8-12 degrees

Answer 18

Pantoscopic Tilt

Question 19

achieved by angling the bottom rim of the frame/bottom of the lens closer to the face than the top of the lens

Answer 19

Pantoscopic Tilt

Question 20

Spherical and Cylindrical power changes caused by Pantoscopic tilting can be corrected by changing the position of the _______

Answer 20

Optical Center

Question 21

For every 1 degree of Pantoscopic tilt, lower the optical center by _______.

Answer 21

0.5mm (½)

Question 22

is achieved by angling the bottom rim of the frame front away from the face

Answer 22

Retroscopic tilt

Question 23

The purpose of Retroscopic tilt is to remove the lower rim of the from from a ______

Answer 23

Protruding Cheek

Question 24

tilt in which the frame follows the natural form of the face

Answer 24

Positive Face form tilt

Question 25

tilt used when the patient’s PD is narrow than the frame PD

Answer 25

Positive face form tilt

Question 26

a tilt usually employed when the patient’s PD is actually wider than the frame PD

Answer 26

Negative face form tilt

Question 27

• this tilt should be avoided
• instead find a suitable frame size

Answer 27

Negative face form tilt

Question 28

Pantoscopic/Retroscopic tilt will increase a lens spherical power and cylinder power along the _____ degree axis

Answer 28

180 degrees

Question 29

Face Form tilt will increase a lens spherical and cylindrical power along the ______ degree axis

Answer 29

90 degrees

Question 30

Formula for lens tilt:

Answer 30

Martin’s Formula for tilt