9 - LV/Magnification

Question 1

Describe angular magnification

Answer 1

Result of an incr in retinal image size of an object by introducing an optical system (i.e. LENSES USED) in bw the object and the eye

• hand-held/collimating
• stand/non-collimating

Question 2

Difference bw lateral and angular magnification

Answer 2

L: compares object size to image size

A: compares original retinal image size to retinal image size when viewed thru an optical system

Question 3

Hand-held magnifiers

-describe

Answer 3

High plus-powered lens

Puts object of interest at the primary focal point of the magnifier

• this creates an image at infinity by collimating outgoing light
• no need for accommodation when using

Question 4

Hand-held magnifiers

-an object at the primary focal point

Answer 4

Once there, the HHM-object system can be moved anywhere in front of the eye and total magnification will remain the same
-as lens-eye distance changes, the spectacle mag of the lens changes, however there’s an equal but opposite change in RDM = 0

Question 5

Hand-held magnifiers

-what changes/doesn’t change when HHM is moved closer/farther from eye

Answer 5

Changes: linear FOV (incr when brought closer to eye)

Does not change: angular/total mag, accommodation (not needed still)

Question 6

Stand magnifiers

-describe

Answer 6

High plus-powered lens mounted such that object to lens disparity is FIXED

• for most SMs, the object is located INSIDE the primary focal point
• upright, magnified, virtual image
• need for accommodation (light is still diverging as it leaves)

Question 7

Telescopes

-describe

Answer 7

For magnifying distance objects

Both light in and out is parallel/plane = no need for accommodation

Question 8

Telescopes

-entrance and exit pupils

Answer 8

EnP = objective lens

ExP = image of objective lens thru ocular lens

Question 9

Telescopes

-meaning of label A x B

Answer 9

A = magnification
B = diameter entrance pupil (dent) in mm

E.g. 4 x 45 = 4x mag, 45mm dent

Question 10

Telescopes

-field of view dependent on (2)

Answer 10

Diameter of EnP - incr dent will incr exit pupil = incr FOV

Diameter of ExP - will continue to incr FOV by incr dex until dex becomes larger than the pt’s pupil, at which point the pupil is the limiting factor

Question 11

Telescopes

-Keplarian

Answer 11

Both plus lenses

Inverted, real, magnified image
-need another lens to upright -> longer tube, heavier

AS and EnP are the objective
ExP is behind ocular (outside/in free space) = LARGER FOV bc can align with pupil
-typically very small = DIM image

Question 12

Telescopes

-Galilean

Answer 12

Positive objective, negative ocular lens

Upright, magnified image
-objective lens forms real image that becomes a virtual object for ocular lens

AS and EnP are the objective
ExP is inside telescope = SMALLER FOV
-typically larger = BRIGHT image

Limited to 4x mag

Question 13

Telemicroscopes

-describe

Answer 13

Telescope + reading cap

Used for viewing near object
-reading cap acts as a HHM - object located at primary focal point -> plane waves enter TS

Question 14

Spectacle-mounted telescopes

• center fit
• bioptic fit

Answer 14

C: in LOS = continuous viewing, requires minimal training, shouldn’t walk with

B: in upper portion of spectacle lens, used for spotting, significant training required
-uses include classroom work, driving, traveling, grocery stores

Question 15

Reverse telescopes

• use
• describe
• alternative

Answer 15

Expand FOV - pts with RP, advanced glaucoma, etc.

Pt looks thru objective lens = minifies objects

• must have good central acuity
• most often hand-held, 2.5-4x Galilean

Minus lenses - esp successful with -5 to -10D

Question 16

VFD

• most effective method
• prism orientation

Answer 16

Scanning techniques

Base toward defect

Question 17

Classifying vision

• normal
• near-normal
• moderate LV
• severe LV
• profound LV
• near-blind

Answer 17

Snellen:
12 to 25
30 to 60
70 to 160
200 to 400
500 to 1000
Worse than 1000

Question 18

M notation

Answer 18

1M

• subtends 5 arcmin at 1m
• equals 20/20
• linearly = 1.45mm (or 1/16th of an inch)

Question 19

Just noticeable difference

Answer 19

Smallest lens power change the pt can detect

JND = snellen ÷ 100

E.g. 20/400 pt’s JND is 4D
-when doing trial frame use +/- 2.00 D flippers

Question 20

For a pt that uses eccentric viewing, the point located adjacent to the scotoma that is used for viewing is called

Answer 20

Preferred retinal locus (PRL)

Question 21

Examples of contrast sensivity charts (3)

Answer 21

Pelli-Robson: large letters, uniform size

Vistech system: sine-wave gratings

Bailey-Lovie: 3 charts, each at diff level contrast

Question 22

Filters

• neutral density filters
• blue blockers/amber tints

Answer 22

NDF: reduce glare, good for pts with photophobia, all wavelengths are transmitted equally so no effect on contrast

BB/AT: reduce transmission of short-wavelengths -> reduced glare and enhanced contrast

Question 23

VF testing

-central scotomas are detected most effectively with

Answer 23

Scanning laser ophthalmoscope