Lecture 7 - Plus lens magnifiers (prescribing and dispensing) Flashcards
Plus lens magnifiers types:
- Hand-held
- Spectacle-mounted
- Stand-mounted
Hand-held magnifiers:
• This is the most common type of magnifier, often bought privately by patients. The main advantage is that it can be held at any desired distance from the eye.
• Low-powered (4x)
• Medium-powered (4x-8x)
• High-powered (>8x)
Low powered Hand magnifier:
- spherical up to 75mm: aspheric up to 100mm
- field of view = 40mm at 25cm eye-to-magnifier distance
- usually single lens
- includes Fresnel sheet
- can be used binocularly if held away from eye
Medium Powered hand magnifier:
• medium-powered (4x - 8x)
- spherical up to 25mm: aspheric up to 50mm
- field of view ~ 10mm at 25cm eye-to-magnifier distance: ~150mm with magnifier close to eye
- internal illumination likely
High Powered hand magnifier:
• high-powered (>8x)
- doublet/triplet lens systems up to 20mm lens diameter
- field of view 25mm with magnifier close to eye
- “folding” design
Advantages of hand-held format
• familiar to all patients, inexpensive easy to prescribe - can be used with D or N Rx
- most are compact, lightweight and portable
- convenient to carry around for short-duration survival reading
• can have long eye-to-magnifier distance
• maybe helpful for patients with reduced peripheral fields
Disadvantages of hand-held format
• can be ineffective if used incorrectly with reading add
• difficult to maintain correct position for extended periods
• poor field-of-view if long eye-to-magnifier distance
• increased distortions as lens held further from eye
Patient instructions - Hand held magnifier:
• lay magnifier on page and slowly pull away from page until image clear
• advise on spex to be worn
- best to use distance Rx
- Near Rx only for short magnifier-eye distance
• obtain largest FoV by holding magnifier as close to eye as possible
• most curved (steeper base curve) towards patient’s eye for least distortion
• hold magnifier parallel to reading material
• move head and magnifier as a unit together (steady eye strategy)
Spectacle-mounted magnifiers (microscopes) formats:
• Edged lenses to “fashion” frame
- SV or Bifocal (need lower “conventional” add for tasks other than reading eg:
eating, pouring drinks, looking at money)
• Paired lenses with base-in prism in standard frame
• Lenses glazed to special mounting or carrier
• Clip-on lenses
• Other LVAs intended for occupational use
Why are spectacle mounted magnifiers Binocular?
Binocular if possible because:
• larger field-of-view
• greater depth-of-field
• better acuity
• depth perception
• monocular overlap in case of field loss (?)
• conventional appearance & psychological preference
Contraindications for binocular magnifiers:
• no evidence of binocularity at appropriate distance
- choose a test appropriate for the patient’s VA greater than 2x difference in acuity
• central distortion in a previously dominant “worse” eye
• binocular acuity worse than monocular VA
• too great a convergence demand, resulting in diplopia or discomfort
- Balancing / occlusion may be necessary
Centring a binocular near correction:
Near PD and Near CD
• When the eyes converge for near viewing, each eye rotates about the centre of rotation (CR*), and the position of the pupil centres changes. The near centration distance (NCD), which projects this distance into the spectacle plane, can be calculated:
NCD = PD x WD/WD + a
Fonda’s Recommendation for NCD:
- Fonda’s recommendation
Total decentration for near = 2 × WD (D)
For the example on the previous page this would give 2*5=10 mm
NCD = PD - Decentration =60 - 10mm = 50 mm
Bailey’s method for NCD:
Total decentration for near = 1.5/ WD (m)
- = 1.5 x WD (D) (+1mm if PD>65)
For the fonda example this would give 1.5*5=7.5 mm
NCD = PD - Decentration =60 - 7.5mm = 52.5mm
Lebensohn’s rule for NCD:
• Total decentration for near = PD(mm)/ WD (inches) + 1
• For the fonda example this would give 60/9=7 mm
- NCD = PD - Decentration =60 -7mm = 53 mm
What needs to be remembered between fonda, bailey’s and Lebensohn’s rule?
Note that all three rules give slightly higher values than the exact calculation. The authors argue that this creates a base-in prismatic effect, which helps to reduce convergence demand.
Advantages of spec-mounted magnifiers:
• hands free, useful for prolonged reading
• widest field-of-view (because of short eye-to-magnifier distance)
• cosmesis - similar appearance to “normal” spectacles
• can incorporate astigmatic correction
• maybe used for patients with hand tremors -e.g. with a reading stand
Disadvantages of spec-mounted magnifiers:
• close working distance
- illumination may be difficult
- fatigue, HA’s, dizziness
reduced reading speed generally occurs with very close WD limited magnification binocularly
Which binocular adds are easy or difficult with NCD?
Because of the problems (convergence demand and viewing distance) associated with high nea adds, binocular adds are:
- easy up to +6.0
- tricky up to +8.0
- risky up to +10.0
- highly unlikely to be successful if +12.0 or greater
Depth of field of plus lens magnifers and accommodation
• patient has to accommodate to compensate for move
• or if amplitude of accommodation is inadequate i.e. presbyopic object will appear blurred
• Consider what happens if the object moves away from the patient.
