Lecture 8 - Stand magnifiers + flat field

Question 1

What are stand magnifers?

Answer 1

• Maintaining magnifier lens at correct distance from a task is critical. The stand provides a way to achieve this and makes handling easier.
• BUT: Stand magnifiers are more difficult to illuminate and can restrict access to the working plane (e.g. when writing).

Question 2

types of stand magnifers:

Answer 2

1. Fixed Focus
2. Variable Focus

Question 3

key point about stand magnifiers:

Answer 3

• Key Point: When prescribing or dispensing stand magnifiers ensure that the patient has adequate accommodation or suitable reading glasses.

• The magnification achieved varies with the eye - magnifier distance, thus depends on the add (or accommodation) used.

• The higher the near add and the closer the eye to magnifier distance the higher the magnification achieved

Question 4

How can the emergent vergence of a stand magnifier be determined (in practice without calculation?)

Answer 4

1. View object through magnifier and find the maximum plus lens which when held up against the magnifier still gives a clear view
2. As in 1.) Use a telescope to eliminate the effect of accommodation
3. As in 1.) but reverse light path and focus a distant object on the object plane of the magnifier

Question 5

Advantages of stand magnifiers:

Answer 5

• Good for patients with hand tremors
• Most are small and portable
• Relatively inexpensive

Question 6

Disadvantages of stand magnifiers:

Answer 6

• Limited FoV
• Accommodation or add required
• Difficult to determine equivalent power
• Posture & fatigue problems
• Illumination can be problematic unless built-in light source

Question 7

Types of fixed-focus stand magnifiers available

Answer 7

• Power : Low
• Magnification : Up to 4x
• Emergent vergence: Often high
• Lens form : spherical and aspheric
• Typical field of view : e.g 40 mm with 25cm lens to eye distance
• Other : May be used for writing

• Power: Medium/high
• Magnification: 4 to 20x
• Emergent vergence: Usually low (sometimes zero)
• Lens form: Aspheric and compound
• Typical field of view: 100 mm with lens held close to spectacle plane
• Other: Usually internal illumination needed

Question 8

Types of variable-focus stand magnifiers available

Answer 8

• Power: Low
• Magnification: Up to 2.5x
• Emergent vergence: variable
• Lens form: spherical and aspheric
• Typical field of view: e.g 100 mm with 25cm lens to eye distance
• Other: “ Chest magnifers” and “designs for industrial applications)

• Power: Medium/high
• Magnification: 10x and more
• Emergent vergence: variable
• Lens form: Aspheric
• Typical field of view: 30 mm with lens held close to spectacle plane

Question 9

Flat-field and bar magnifers:

Answer 9

• An interesting feature of the design is that it is self-illuminating.

Summary & Conclusions:
• The thicker a flat-field magnifier is (in relation to r), the greater the magnification (usually < 3X)
• Image is formed very close to original object so there is no “relative distance magnification”
• Patient must wear reading correction or accommodate
• Can have bar magnifier with magnification in only one direction (Plus cylinder lens)

Question 10

Advantages of flat-field and bar magnifers:

Answer 10

• Normal reading posture possible
• Very bright - light gathering
• Useful for patients with hand tremor etc.
• Clear image across the whole lens - aberrations have minimal effect
• Combination of bar and spectacle mounted magnifier can increase the magnification while maintaining a long viewing distance

Question 11

disadvantages of flat-field and bar magnifers:

Answer 11

• Large lenses - heavy
• Magnification typically low
• Reading material has to be flat and on firm surface