lecture 8

• Hand magnifiers (HM)

  A plus lens held in the hand, mounted in a handle, but NOT in a stand
• Hand magnifiers (HM)

  • Provide relative distance and angular magnifications
  • It is assumed that the object is held at the focal distance of the lens
  • Light emerging from the magnifier is parallel & accommodative demand is zero

  
• Hand magnifiers (HM)

  • The magnifier can be held at any distance from the eye & there will be no change in accommodative demand (it remains zero)
  • Both the distance between the object & the HM, and the distance between the eye & the magnifier are variable
  • HM can be used with or without a reading add or accommodation
• Trade magnification

  Assumes hand magnifier is held in contact with a reading addition of +4.00DS (or accommodation of 4D is exerted) & the comparison is made with a reference working distance of 25 cm
• Magnification of HM used with bifocal or accommodation

  • Object is not always held at the focal point of the plus-lens
  • Pre-presbyopic can converge & accommodate for the physically near location of the objects
  • Presbyopic PXs using HM (or MS) are expected to wear reading glasses
  • Divergent rays will be leaving the magnifier and a converging effect of accommodation or reading glasses will make rays parallel

  
• Advantages of HM

  • Convenient for short term reading "spot or survival reading"
  • Allow more flexibility of WD
  • Can have internal illumination
  • Psychologically acceptable (over the shelf)
  • Usually compact, lightweight and portable
• Disadvantages of HM

  • Require good manipulation & steady hand
  • One hand is occupied
  • Reading speed usually slower than microscope with equivalent power
  • Field of view smaller than equivalent MS
• Stand magnifiers (SM)

  • A plus lens or lens combination, mounted in a housing, the base of which sits on the material to be viewed
  • Fixed distance form the page (fixed focus)
  • Magnifier to object distance (stand height) is LESS than focal length of the lens
• Stand magnifiers (SM)

  • Image formed is NOT at infinity but within infinity & the light emerging from the magnifier is, divergent
  • As the rays of light leaving the lens are divergent, the PX has to accommodate/wear an add to neutralise the divergence so that parallel light enters the eye
  • The image will be larger than the original object

  
• Stand magnifiers (SM)

  • For the two-component system (magnifier lens + reading add), a higher mag. will be achieved with: a higher add & a close distance between add & magnifier lens
  • Reading cant be bigger than the divergence of light leaving the magnifier lens, or a clear retinal image CANNOT BE FORMED

  
• Determining the emergent vergence

  • Manufacturers label magnifiers according to M = Fm/4 or M = Fm/(4 + 1)
  • The position of the image created by the SM & the way the magnifier is positioned will affect the magnification obtained
  • This is rarely taken into account by the manufacturers when labelling SM
  • No STANDARD reading addition or accommodative effort
• Advantages of SM

  • Accurate working distance, beneficial for PXs with hand tumor or weakness
  • Hands free
  • Available with built-in illumination
• Disadvantages of SM

  • PXs may require reading spectacles to neutralize the emergent divergence
  • Not flexible distances
  • May be heavy to carry around
  • Need a flat stable surface
• Some examples of stand magnifiers

  • Ruler magnifier
  • Round the neck magnifier
  • Dome magnifier