prism

Question 1

how much vertical imbalance is induced when the patient looks down 10mm to read?
OD: +3.00 DS
OS: -3.00DS
ADD: +2.50 lined bifocal

Answer 1

6BU OD
OD: 3(1cm)=3PD BU
OS: 3(1cm)=3PD BD
add together=6PD BU OD or 6PD BD OS

Question 2

if the vertical imbalance is corrected with reverse slab off, which lens will have induced prism, and what is the base direction of the induced prism?
OD: +3.00 DS
OS: -3.00DS
ADD: +2.50 lined bifocal

Answer 2

OD, BD

Question 3

vertical imbalance

Answer 3

• diff prism induced where looking down between 2 eyes
• looking down
• prentices rule
• need: diet from OC that eyes move, power of lens in vertical meridian (do power cross if cyl involved)
• dont need: add power or type

Question 4

how much vertical imbalance isa problem

Answer 4

> 1.5PD

Question 5

things that can correct for vertical imbalance

Answer 5

dissimilar segments
separate single vision reading glasses
slab off
contact lenses

Question 6

dissimilar segment for vertical imbalance

Answer 6

two different bifocal designs are used in each lens; this method reduces vertical imbalance because each bifocal segment will have a different distance from the top of the segment to the segment optical center

Question 7

slab off for vertical imbalance

Answer 7

induces BU prism in the most minus lens

BUMMM=BU most minus meridian

Question 8

reverse slab off

Answer 8

another method to correct vertical imbalance. induces prism on the most plus lens in the 90 meridian in order to counteract the BU prism induced by the plus lens

Question 9

what are PALs good for fixing

Answer 9

image jump

Question 10

each diopter of anisometropia produces ___ difference in the size of the retinal image

Answer 10

1%

Question 11

aniseikonia > ____ prevents the patient from fusing

Answer 11

10%

Question 12

aniseikonia > ___ is bothersome to the patient

Answer 12

3%

Question 13

Knapps law

Answer 13

tells us whether glasses or CL are best for axial vs refractive anisometropia

Question 14

Knapps law and axial anisometropia

Answer 14

correct with spectacles

give them an “axis”

Question 15

knapps law and refractive anisometrpia

Answer 15

correct with CL

“refractive are active” give them CL

Question 16

decreases SM in plus lenses

Answer 16

decrease vertex distance
decrease thickness
decreased BC
increase n

Question 17

decreases SM in minus lenses

Answer 17

increases vertex distance
decreases thickness
increase n
decrease BC

Question 18

spec mag

Answer 18

Power Factor= (1/(1-(h+3))(P))

shape factor= (1/1-(t/n)(F1))

Question 19

the patient notes her vision is distorted when she looks through the peripheral portion of the lens. which of the following factors is least likely contributing to he complain?

• CA
• incorrect BC
• SA
• radial astigmatism
• curvature of field

Answer 19

SA

Question 20

chromatic aberrations

Answer 20

shower wavelengths (blues) are refracted more than longer wavelengths (reds) as they pass through an optical system, causing blue waves to focus closer to the cornea and red waves to focus closer to the retina. this results in color fringes around objects that is more pronounced when viewing through the periphery of the lens. higher powered lenses will also have more chromatic aberrations

Question 21

monochromatic aberrations

Answer 21

SA
coma
radial astigmatism 
curvature of field
distortion

Question 22

what monochromatic aberrations distort image quality

Answer 22

SA
coma
radial astigmatism

Question 23

what monochromatic aberrations distort image plane

Answer 23

curvature of field

distortion

Question 24

spherical aberrations

Answer 24

marginal rays bent more
on and off axis points
blur circle
pupil fixes, corneal fixes

Question 25

night myopia

Answer 25

SA + pupil size

Question 26

coma

Answer 26

monochromatic abberation. occurs only for off-axis point sources due to a difference in magnification for incident rays of different heights

Question 27

radial astigmatism

Answer 27

monochromatic Aberration
light rays can strike a lens surface obliquely, resulting in a distorted image due to induced cylinder power. can be reduced by selecting the optical base curve from the Tscherning ellipse (pick BC to decrease marginal astigmatism. always use Oswald curve)

Question 28

distortion

Answer 28

monochromatic aberration
caused by differences in magnification of a point object based on the distance of the object from the optical axis. it causes a straight line object to form a curved line image

Question 29

barrel

Answer 29

minus lenses

Question 30

pincushion

Answer 30

plus lenses

Question 31

practical application of SA and coma

Answer 31

generally don’t affect vision because the pupil is small enough to block marginal rays that cause these aberrations. high plus lenses (>+7.00D) would be deigned as aspheric lenses in order to minimize these aberrations

Question 32

practical application of radial astigmatism

Answer 32

minimized by selecting the optional base curve for the specific lens power using the T curve

Question 33

curvature of field practicality

Answer 33

can be minimized by using the manufacturers recommended BC

Question 34

CA can be reduced by

Answer 34

using a shorter vertex distance and having sufficient panto tilt

Question 35

point focal lens corrects for

Answer 35

oblique astigmatism

Question 36

Percival form lens corrects for

Answer 36

curvature of field

Question 37

field of view

Answer 37

increased with decreased vertex distance
with plus lenses, the higher power, the smaller FOV
with minus lenses, the higher power, the larger the FOV