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
Does the lens itself produce increased retinal image in high addition lens? Explain.
No, it acts as a converging system
-as reading material brought closer, retinal image size increases
-lens produces negligible amount of magnification
What is a problem with high addition spectacles and up to what power D is it generally ok? What can be done to minimise this?
Increased peripheral aberrations with increasing power
Up to +8D is ok
Aspheric designs minimise them
What is a doublet spectacle? Does it have the same problems as regular high addition spectacles?
Combination of two convex lenses separated by an airspace
Provides high mag without peripheral aberrations
For high addition spectacles with half-eye lookover designs, what amount of prism is used?
Power of the lens + 2
What is a loupes design for high addition spectacles? What is the tradeoff with these designs?
Allows for slightly extended working distance by extending lens in front of the spectacle plane
-as extension increases field of view decreases
What should you emphasise when a patient is trialling high addition spectacle designs?
Correct working distance
List 5 things the patient must be aware of when high addition spectacles are being considered.
Fatigue - arms/neck
Nausea/dizziness
Eye fatigue
Headaches
No mobility
List 7 advantages of spectacles for magnification.
Acceptance?
Cosmesis
Largest field of view
May be used binocularly
Hands free
Better for prolonged reading
Astigmatism corrected
List 6 disadvantages of spectacles for magnification.
With increasing power:
-working distance decreases
-depth focus decreases
-lighting more critical
-aberrations increase
-lens size decreases
-reduced reading speed
