Focimetry

Question 1

What is another name for the focimeter?

Answer 1

lensometer

Question 2

What does a focimeter measure?

Answer 2

• axis and power of spectacles and CL
• can find optical centre and power and base direction of any prism in unknown lenses

Question 3

How is the vertex power of the lens usually measured with manual focimetry?

Answer 3

by taking the inverse of the focal length of the unknown lens

Question 4

How can chromatic aberration be eliminated in manual focimetry?

Answer 4

green light used

Question 5

What are 3 components of the manual focimeter?

Answer 5

1. moveable illumination target
2. viewing telescope
3. fixed collimating lens (renders light parallel)

Question 6

What is the overall method of using the manual focimeter? 3 key points

Answer 6

1. ensure eyepiece is focused and target seen sharply focused
2. insert unknown lens
3. spectacles should be mounted with back surface of lens against rest to measure BVD

Question 7

What part of spectacles should be mounted against the rest of a manual focimeter?

Answer 7

back surface - to measure back vertex power

Question 8

How is manual focimetry performed for simple spherical lenses?

Answer 8

dial (this moves target backwards or forwards) until the graticules are sharp, and read off the power

Question 9

How is manual focimetry performed to calculate cylindrical power?

Answer 9

• target is rotated, as well as dialled, until **one set of lines is sharp. **
• the reading is noted.
• the target is then dialled again until the **other lines are sharp. **
• the difference in these two readings is the cylindrical power
• the axis of the cylinder is read fom the dialling wheel

Question 10

How can prismatic power be seen from manual focimetry?

Answer 10

• can be seen fom deviation of the light target from the central measurement position within the focimeter

Question 11

How will a lens with a ground prism displace the cross image of a manual focimeter for base up prism?

Answer 11

displaces image downwards

Question 12

How will a lens with a ground prism displace the cross image of a manual focimeter for base down prism?

Answer 12

displaces image upwards

Question 13

How will a lens with a ground prism displace the cross image of a manual focimeter for base in prism?

Answer 13

displaces image outwards

Question 14

How will a lens with a ground prism displace the cross image of a manual focimeter for base out prism?

Answer 14

dispalces image inwards

Question 15

How can bifocal addition be measured using a manual focimeter?

Answer 15

turn spectacles around to measure front vertex power; difference between the front vertex power of the distance and near portions is the bifocal add

Question 16

What exists to help measure distance and near powers on multifocal lenses with a manual focimeter?

Answer 16

templates from the lens manufacturer to line up with discrete markings on the lens

Question 17

What would be the prescription for the following finding from the focimeter?

Answer 17

+1.0 / +3.0 x 60degrees

Question 18

What is the broad principles of atomated focimetry based on?

Answer 18

work in similar manner to autorefractors, but isntead of having to pass light through the optics of th eye and assess the deviation or distance of peak focus in multiple locations across the pupil on its return, they simple image light frmo the other side of the lens (i.e. single pass)

Question 19

What is the set up like of automated focimetry?

Answer 19

• support frame for the spectacles
• changing the lever on the unit above the support frame will automatically read either the right or the left lens, as required