TOPIC 11: SPECTACLE MOUNTED AND HAND MAGNIFIERS

Question 1

What is the formula to calculate magnification of a plus lens

Answer 1

Magnification = Feq / 4
where Feq is the equivalent power of the plus lens

Question 2

What factors affect field of view with a magnifier?

Answer 2

1. Magnifier DIAMETER:
   the diameter of a magnifier is limited by aberrations (eg coma, distortion), weight (the higher the power, the thicker and heavier the lens), and manufacturing ability
2. Magnifier POWER:
   give minimum magnification that achieves comfortable reading (account for acuity reserve, but don’t prescribe extra magnification in anticipation of future vision loss)
3. EYE-TO-MAGNIFIER distance:
   halving eye-to-magnifier distance will double the field of view. This is a factor that the patient may have some control over!

Question 3

4 Advantages and 4 disadvantages of spectacle mounted magnifiers

Answer 3

Advantages:
1. Hands free. Great if the activity requires both hands eg handicraft work
2. Widest field-of-view because of short eye-to-magnifier distance
3. Looks like ‘normal’ spectacles
4. Can incorporate ametropic correction

disadvantages:
1. Close working distance, which can block external lighting from reaching the object (may need to use additional lighting)
2.Difficult to maintain binocularity due to close working distance
3. Blurred vision if the patient looks up from the near task (similar problem as reading glasses), but bifocal and half-eye design can be made
4. Expensive to customise to individual prescription

Question 4

Advantages and disadvantages of hand magnifiers

Answer 4

Advantages of handheld magnifiers
1. Inexpensive
2. Convenient to carry around for use on the go (e.g. at supermarket to read food labels)
3. compact, lightweight, portable
4. Can have long eye-to-magnifier distance (eg turning on the stove)
5. comes in internally-illuminated options to provide additional light to the object (good as LV patients frequently need brighter lighting)

disdvantages of handheld magnifiers
1. can be ineffective if used with reading addition
2. difficult to maintain the correct position (at focal point) for extended durations of time (the magnifier must be held above the object)
3. Poor field-of-view if the patient maintains a long eye-to-magnifier distance