Low vision magnification Flashcards
What is low vision?
a person with impairment of visual function. VA of less than 6/18 with correction. Affects everyday tasks that you would normally do
What is image magnification/enlargement?
compares the size of the magnified image size to the original retinal image size. the ratio is known as retinal image magnification
What are the four ways you can enlarge an image in low vision?
relative distance enlargement
relative size enlargement
electronic (transverse) enlargement
lens vertex (angular) enlargement
What is equivalent viewing distance?
distance at which the image subtends the same angle as the object. the resolution limit is proportional to the viewing distance (Mag=F/4)
What is the relative distance enlargement?
RDM = old object distance/new object distance
What is relative size enlargement?
using the same diatnce but increasing the image size
RSM = new object size/old object size
How do you predict the required magnification?
M = present acuity/goal acuity
What are the advantages of prescribing higher spectacles?
cosmetically and psychologically acceptable
hands free to perform tasks
large fields of view
may incorporate correction for ametropia
What are the disadvantages of prescribing higher spectacles?
shorter working distance than other low vision aids that are of the same equivalent power
What are the advantages of prescribing simple hand-held magnifiers?
convenient and discreet
socially acceptable
generally inexpensive
light weight
internal illumination possible
can be used further from eye
What are the disadvantages of prescribing simple hand-held magnifiers?
too easily purchased and used inappropriately
require a steady hand and ability to maintain fixed working distance
