Plus Lens Magnifiers

Question 1

What is the formula for field of view

Answer 1

• FoV = D/dFm

Question 2

What is the formula for handheld magnifiers

Answer 2

• Feq = F1 + F2 - d/n F1F2

Question 3

3 types of plus lens magnifiers

Answer 3

• hand held
• spectacle mounted
• stand mounted

Question 4

Most common forms of hand held magnifiers

Answer 4

• low powered 4x
• medium powered 4-8x
• high powered >8x

Question 5

Advantages of hand held magnifiers

Answer 5

• familiar to all px
• inexpensive
• easy to prescribe
• compact
• convenient to carry
• can have long eye-to-magnifiers distance
• helpful for px with reduced peripheral fields

Question 6

Disadvantages of hand held magnifiers

Answer 6

• can be ineffective if used incorrectly with reading add
• difficult to maintain correct positions for extended periods
• poor field of view if long eye tot magnifier distance
• increased distortions

Question 7

Px instructions for hand held magnifiers

Answer 7

• lay magnifiers on page and pull away until clear
• advise to wear spex
• obtain largest FoV by holding magnifier as close too eye as possible
• most curved side towards px
• hold parallel to reading material
• move head and magnifier as a unit together

Question 8

Available types of spectacle mounted magnifiers

Answer 8

• edged lenses
  
  • SV or Bifocal
• Paired lenses with base in prism
• lenses glazed to special mounting or carrier
• clip on lenses
• other LVAs intended for occupational use

Question 9

Advantages of binocular viewing with spectacle lens magnifiers

Answer 9

• larger FOV
• increased depth of field
• better acuity
• depth perception
• monocular overlap in case of field loss
• conventional appearance

Question 10

Contraindications for binocular magnifiers

Answer 10

• no evidence of binocularity at appropriate distance
• greater than 2x difference in acuity
• central distortion in a previously dominant worse eye
• too great a convergence demand, resulting in diplopia or discomfort

Question 11

Formula to calculate near centration distance

Answer 11

• NCD = PD X WD/WD + a

Question 12

Advantages of spec mounted magnifiers

Answer 12

• hands free, useful for prolonged reading
• widest field of view
• cosmesis - similar appearance to normal specs
• can incorporate astigmatic correction
• maybe used for px with hand tremors

Question 13

Disadvantages of spec mounted magnifiers

Answer 13

• close working distance
  
  • illumination may be difficult
  • fatigue, HAs, dizziness
• reduced reading speed
• limited magnification

Question 14

Advantages of a stand magnifier

Answer 14

• maintains magnifier at correct distance
• easier handling
• inexpensive

Question 15

Disadvantages of a stand magnifier

Answer 15

• difficult to illuminate
• can restrict access to the working plane
• limited field of view
• posture and fatigue problems

Question 16

2 types of stand magnifier

Answer 16

• fixed focus
• variable focus

Question 17

How can the emergent of a stand magnifier be determined

Answer 17

• view object through magnifier and find the maximum plus lens which held up against the magnifier

Question 18

What are flat-field magnifiers

Answer 18

• self illuminating
• hemi-spherical or hemi-cylindrical
• bar magnifier
• thicker magnifier = more magnification
• image formed very close to actual object