Spectacle Magnification Flashcards
What is anisometropia?
two eyes with unequal refractive power
What is antimetropia?
subclass of anisometropia - one eye is myopic, one eye is hyperopic
What could occur from anisometropia and antimetropia?
aniseikonia
spectacle magnification compares what?
size of image seen by person when wearing glasses with the size of the image seen when that same person is not wearing glasses
SM=
retinal image size in corrected eye/retinal image in same eye uncorrected
normal image size is taken as image size for the standard emmetropic eye with _ refractive power
+60.00 (magnification of 1.00)
What 2 factors contribute to magnification or minification of an image?
power of lens
shape of lens
Power factor includes? (2)
vertex distance, back vertex power
Shape factor includes? (3)
thickness, index of refraction, FBC
SM=
(shape factor)(power factor)
shape factor=
1/[1-(t/n)f)]
power factor=
1/[1-(dP)]
relative spectacle magnification compares what?
standard emmetropic eye with magnification of an ametropic eye
RSM=
image size for CORRECTED ametropic eye/standard image size for standard emmetropic eye
What is Knapp’s Law? His untrue assumption?
When a refractive error is caused by excessively long or short eyeball length (axial), vertex distance of 15mm will create retinal images of same size, no matter what
–assuming spectacle lenses are flat and thin
percent gain in magnification =
(Mspec-1)*100%
The aperture of the eye is the entrance pupil is the _, located _mm behind cornea
pupil, 3
in power factor equation, d =
vertex distance + 3mm to entrance pupil (in METERS)
when considering CLs, d=
3mm
Even ~6% is a noticeable difference and is and advantage of contact lenses for myopes
–
only using power factor with - lenses is very similar to shape factor and power factor because:
minus lenses CT doesn’t usually vary much
flatter FBC
Difference with aniseikonic symptoms vs uncorrected refractive error symptoms is:
symptoms are not helped by rx
problems appear after other problems are corrected
indications for clinicially significant aniseikonia:
high anisometropia, high astigmatism
IOL, optic atrophy
spatial distortion
better comfort when only one eye is used
physiologic (natural) aniseikonia is useful in small amounts created how?
gives clues to?
different locations of two eyes
depth, object location
symmetrical aniseikonia is
equally smaller/larger in every meridian
meridional aneiseikonia is
is it symmetrical?
caused by?
meridional size difference in a meridian of one eye compared with that of the other eye
- yes, symmetrical
- significant astigmatism
asymmetrical aniseikonia:
progressive increase/decrease across the visual field
with accommodation relaxed, parallel rays of light do not focus on the retina when _ is present
ametropia
2 types of ametropia?
axial - unequal eye lengths
refractive - unequal curvature of refractive components of the eye
if K readings are equal, rxs are significantly different; _ ametropia is present
axial
K readings are different proportionally to rxs; _ ametropia is present
refractive
is spectacle lenses are used for high astigmatism, each meridian will cause different amounts of magnification
-correction of choice would be _
contact lenses
When using eyewear to correct aniseikonia:
use frame with _ vertex distance
short
When using eyewear to correct aniseikonia:
use frame with _ eyesize
small, reduces vertex distance
When using eyewear to correct aniseikonia:
use _ lens design, which _ base curve
aspheric, flattens
When using eyewear to correct aniseikonia:
use _ lens material
high index, thins plus lens CT
when correcting aniseikonia for presbyopes, it may be necessary to correct _ at the same time
vertical imbalance
_ is a lens to assist in the matching of retinal sizes
size lens or iseikonic lens
when correcting aniseikonia with specs, keep decentration _
to a minimum (frame pd=pt pd as much as possible)
base curves range from _ to _
.50 to 8.00
if you choose to use aspheric lenses, you must _
use it in both lenses!
for poly or trivex:
finished= _ mm min CT
semi-finished= _mm min CT
- 0
1. 5
normal vertex distance is _ mm
_ mm is too close, lash crash
13
11
thickness:
_ mm is industry standard min
poly or trivex, surfaced to _mm or thick stock lens as _ mm
2.0
1.5 mm
1mm
if you bring retinal image size between eyes to be just less than _%, patients can fuse 95% of the time
4%
4 lens components:
FBC
index of refraction
vertex distance
center thickness
we cannot add _
minification
to add magnification: steepening FBC = _ CT increasing CT = _ magnification flattening FBC = _ CT decreasing CT = _ magnification
adds
increases
reduces
decreases
increasing vertex distance for any lens, makes it more _ power, and _ magnification
plus
increases
