TOPIC 12: STAND AND BRIGHTFIELD MAGNIFIERS

Question 1

How to calculate equivalent power and magnification if the object is closer than the focal point of the plus lens?

Answer 1

Feq = FM + FA – zFMFA
M = Feq/4

Question 2

How to read the markings on a magnifier (e.g. 4x / 16 D / 150. What do they mean)

Answer 2

4 times magnification
16
The last number (15cm) is the working distance that the patient should use with a standard +2.50D reading addition

Question 3

Advantages and disadvantages of stand magnifiers

Answer 3

Advantages of Stand magnifiers
1. Good for elderly with trembling hands as the lens-to-task distance is maintained
2. Fixed focused – not as tired compared holding a hand magnifier – results in less image movement
3. Moderately priced

Disadvantages of Stand magnifiers
1. Low powered SM are generally large because to maintain a long working distance
2. No access to the reading material in high magnification
3. Some designs may limit the incoming light
4. Need a flat surface to support reading material

Question 4

How do bar magnifiers produce magnification?

Answer 4

REAL IMAGE/ TRANSVERSE MAGNIFICATION.
The amount of magnification produced is limited by the refractive index of the material
The diameter of the lens determines the field of view (not the eye-to-magnifier distance).
As image formed is very close to the original object, patient need to wear READING correction to focus image position

Question 5

What are the advantages/disadvantages of brightfield / bar magnifiers

Answer 5

Advantages of Bright field and Bar magnifiers
1. Excellent light gathering (hence the name bright field!)
2. Few reflections
3. BINOCULAR viewing at normal working distance
4. MINIMAL aberrations in the lens periphery
5. UNAFFECTED by hand tremor as rests on page
6. Available in range of diameters to suit everything from large desktop paperweight to small coin-sized pocket version

Disadvantages of Bright field and Bar magnifiers
1. Limited magnification (not available in high mag)
2. Needs a firm flat surface
3. Only a limited horizontal FOV with flat field magnifiers due to the weight and bulk of larger hemisphere. Plastic ones are lighter but more susceptible to scratches
4. Enlargement in only ONE direction with bar magnifiers
5. NO access to material underneath (it rests flat on the object, so it is not suitable for writing, painting)