LV - Near Low Vision Aids I - Week 4 Flashcards
How would you estimate the magnification a patient may require for near?
Measure near acuity at 25cm with a +4 addition over the distance prescription
What print size should be aimed for when prescribing magnification?
Two lines spare
-if N8 required, aim for N5
Give the number of reserve lines for the following intensities of reading the patient intends to do:
Spot reading
Fluent reading
Maximum/near maximum
Spot reading - 1 line reserve
Fluent reading - 3 lines reserve
Maximum/near maximum - 4 to 5 lines reserve
Give the formula for the magnification ratio. Give the approximation based on logMAR lines.
mag ratio = size letters read / size letters wish to read
-i.e. reading N20, wanting to read N5 = 20/5 = 4x
Every 3 lines is 2x mag
Describe the purpose of magnification and how it achieves this. List three ways this can be achieved.
To increase retinal image size by increasing angular subtense of the object
- relative size
- relative distance
- angular
Define relative distance magnification.
Distance between the eye and object is reduced
Define angular magnification.
Optical devices used to increase visual angle
When combining magnification, is it additive or multiplicative?
Multiplicative
List the two types of magnification.
Relative/effective
Rated/conventional
Which type of magnification is most used by clinicians?
Relative
Define relative magnification.
Angular size image through the magnifier / angular size of the object at a standard reference distance
What is usually the standard reference distance?
25cm
-sometimes 40cm
What is the formula for relative magnification and how is rated magnification different? What does it assume?
Relative: M = F/4
Rated: 1 + (F/4)
-assumes lens will be held close to the eye and the eye will exert 4D accommodation
List 6 near magnification options.
High additions Hand magnifiers Stand magnifiers Near telescopes Handheld electronics Desk electronics
Name two advantages and disadvantage of high addition spectacles.
A - hands free, large field of view
D - short working distance