Visual Impairment 2- Magnification Flashcards
What are the 4 ways to enlarge retinal image size?
Relative Size magnifiction
Relative Distance Magnification
Angular Magnification
Real image/ Transverse Magnification
What are the advantages to making objects bigger? (RSM)
No training required
Less conspicuous
No change in habitual posture
Patient can plan themselves
What are the downsides to enlarging objects?
More cumbersome
Limited mag
Loose contrast i.e. enlarging by photocopying
How do we work out magnification when changing the relative distance?
M= OLD obj dist/ NEW obj dist
What does angular magnification refer to?
Telescopes
Mag= instrument image/obj (s telescopes)
What are some examples of real/transverse imaging?
CCTV
TV screens
What formula would be used if different magnification were used with one another?
Total mag= relative SIZE mag X relative DIST mag
What are the 2 different types of telescopes?
Keplerian/ astronomical
Galilean
Which telescope uses prism to create correct inverted image?
Keplerian
Which telescope uses 1 convex and 1 concave lens?
Galilean
How high can the magnification go on a keplerian compared to a galilean?
Keplerian upto 10x
Galilean up to 4x
Where is the exit pupil located in a keplerian compared to a galilean?
Keplerian external
Galilean internal
What is the field size on keplerian compared to a galilean?
Keplerian larger
Galilean smaller
Between an eye piece and objective lens which is stronger powered in astronomical?
Eye piece
How would you work out the magnification in a telescope
M= diam of OBJ LENS/ diam of EXIT PUPIL
Which telescopes would you find markings?
Kelperian
What would you do to get the maximum field of view in a telescope?
Objective lens would need to as large as possible
What formula is used to predict the DIST mag?
Required/Actual— snellen as decimal
Actual/Required— looking for a denominator
What is the formula to predict near mag?
Mag required= actual VA/required VA
Whats another way to find what VA is required for a specific task?
Nomogram
What is the typical routine to determine how much mag is needed?
- ID specific and amoun
- Bino or Mono
- Select appropriate LVA
- Perform trial of suitability
- Determine specific correction necessary
- Loan device after training
- Follow up visits
Which is the only useful LVA to use for distance?
Telescopes
What would a telescope commonly be used for?
Outdoor use
Whats the main problem with telescopes?
FOV!!
Increased mag= SMALLER FOV
Decreased mag= LARGER FOV
What is the best LVA to use for manipulative task?
Telemicroscopes
What rx is used with a telescope used for dist?
Dist rx
When is a distance rx is used for near?
When using a hand held magnifier
What are the advantages of telescopes
Alter size with altering working dist
Px refractive error incorporated
Focused D+N
What are the disadvantages of telescopes?
Restricted FOV Not suitable while moving EXPENSIVE Speed also magnified Often requires training
What formula is used to find out the accommodation that is required for near vision telescopes?
Accom(n) = WD (diopter) x mag(2)
Why use telescopes for near?
Increase WD compared to spectacle magnifier
What is the formula used to workout telemicroscope mag?
Mag total= mag telescope x F(power of reading cap) /4
What are the disadvantage of telemicroscopes?
Heavier if spec mounted
Worse FOV
Cosmetically poor
Costly
What 2 types of task could telescopes be used for?
Sedentary tasks
Spotting tasks
