LV: Prescribing +Lens Magnifiers

Plus lens magnifiers are either hand-held, spectacle mounted or stand mounted.

• Hand-helds can be held at any dist from the eye
• Most common, often bought by patient
• Low Powered <16d fov =" 40mm">
• Med Power = 16-32D 4x-8x spherical up to 25mm size, 50mm size if aspheric, FOV = 10mm at 25cm from eye, 150mm with magnifier close to eye, likely to have internal illumination
• High Power = >8x, doublet/triplet lens up to 20mm lens diam, 25mm view with magnifier close to eye, have 'folding' design

Hand Held Advantages

• Familiar to all px, cheap, easy to prescribe
• Good to carry around for short duration survival reading
• Can have long eye to magnifier distance (although this gives poor FOV)
• Could help for px w/reduced peripheral fields
• Most are compact, lightweight and portable
• Can have internal illumination
• Equivalent power varies depending on how it's used

Hand Held Disadvantages

• Can be ineffective if used with reading add
• Difficult to maintain correct position for long periods
• Requires steady hand eye coord
• FOV is poor w/long eye to magnifier distance and more distortion as lens held further from eye

H-H Instructions

• Lay magnifier on page and pull away til clearest image
• Use distance Rx or add depending on preferred magnifier-eye distance
• Get biggest FOV by holding magnifier as close to eye as possible
• Most curved surface towards patient's eye for least distortion
• Hold parallel to reading material and move head and magnifier and heed as a unit

Spec Mounted

Type

• Edged lenses to fashion frame
• Paired lenses w/base in prism in standard frame
• Lenses glazed to special mounting or carrier
• Clip on
• Other LVA for occupational use

Binoc If Possible

• larger FOV, greater depth of field, better acuity, depth perception
• More conventional appearance, psychological preference/advantage
• Can provide w/clip on occluder if they want

Contraind.

• No evidence of binocularity at appropriate distance - use worth 4-dot, mallett, bar reading
• Don't use if more than 2x difference in acuity
• No good if central distortion in a previously dominant 'worse' eye
• If binoc VA worse than monoc
• If there's too great a convergence demand and the px experiences diplopia or discomfort

Calculating Near Centration

• NCD = PD x WD / WD + a
• a = vertex distance PLUS half axial length of eye (eg axial length 24mm = 12mm)
• eg axial length 24, PD 65, vert dist = 12mm, working dist 10cm
• NCD = 65 x 100 / 100 + 24 = 52.41mm
• SO lenses need to be decentered by a much larger amount and this must be done when testing patient
• Even if eyes look thru optical centre the convergence requirement is considerable eg 1D base out for each mm of PD to view at 10cm
• Therefore base in prism may be required

Adv of Spec Mounted

• Hands free, widest FOV due to short eye to mag distance
• Similar appearance to normal specs
• Useful for prolonged reading
• Can incorporate astigmatic correction
• Good if px has hand tremors

Disad

• Close working distance, illumination could be difficult
• Limited magnification if binocular
• Blurred vision if looking up into distance altho bifs and half eyes are available
• inc power = inc working distance which could lead to fatigues, headaches and dizziness
• Reading speed is reduced
• Add-wise, +6 = easy, +8 = tricky, +10 tricky, +12.00 highly unlikely