Plus Lens Magnifiers Flashcards
1
Q
What is the formula for field of view
A
- FoV = D/dFm
2
Q
What is the formula for handheld magnifiers
A
- Feq = F1 + F2 - d/n F1F2
3
Q
3 types of plus lens magnifiers
A
- hand held
- spectacle mounted
- stand mounted
4
Q
Most common forms of hand held magnifiers
A
- low powered 4x
- medium powered 4-8x
- high powered >8x
5
Q
Advantages of hand held magnifiers
A
- 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
6
Q
Disadvantages of hand held magnifiers
A
- 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
7
Q
Px instructions for hand held magnifiers
A
- 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
8
Q
Available types of spectacle mounted magnifiers
A
- 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
9
Q
Advantages of binocular viewing with spectacle lens magnifiers
A
- larger FOV
- increased depth of field
- better acuity
- depth perception
- monocular overlap in case of field loss
- conventional appearance
10
Q
Contraindications for binocular magnifiers
A
- 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
11
Q
Formula to calculate near centration distance
A
- NCD = PD X WD/WD + a
12
Q
Advantages of spec mounted magnifiers
A
- 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
13
Q
Disadvantages of spec mounted magnifiers
A
- close working distance
- illumination may be difficult
- fatigue, HAs, dizziness
- reduced reading speed
- limited magnification
14
Q
Advantages of a stand magnifier
A
- maintains magnifier at correct distance
- easier handling
- inexpensive
15
Q
Disadvantages of a stand magnifier
A
- difficult to illuminate
- can restrict access to the working plane
- limited field of view
- posture and fatigue problems
