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schematic eye & effective lens power

Terms in this set (32)

power of human eye
60D

focal length short, focal length long, strong power
power of cornea
40D

high because air
power of crystalline lens
20D

low because water
ametropia
refractive error present when eye at rest

(ie: myopia, hyperopia, astigmatism, presbyopia)
axial distance
distance from front to back of eye
normal vision

F' and retina
what F' on retina

secondary focal length & length of eye matches
myopia

F' and retina without correction
light in front of retina (secondary focal length) F
ways to correct myopia (ie if -4D)
use -4D lens at corneal plate
move real object where has -4D vergence entering eye
once add lens to myopic patient what happens to image
virtual image for lens is real object for eye
causes of myopia
1) eye LONGER than "normal"
- eye length normal, axial length longer than f'
- normally the cause
2) eye power STRONGER than normal
- axial length normal, still behind F'
- less likely

can be mix of 2, although normally one
PR
far point or punctum remotum

any object farther away from this point will be blurry unless have lens
emmetrope PR
infinity
myope PR
PR inside infinity
hyperope PR
PR is virtual object
hyperopia

what is it
light shorter than normal, behind retina

fix with: plus lens that adds convergence
hyperopia what happens once add lens

in terms of image and object
real image for lens becomes virtual object for eye
hyperopia describe why virtual object when corrected
for instance, if eye needs +4.00D going into it, it implies need virtual object! PR = virtual object. NO position a real object can be placed to form a real image aka converge to form real image
PR for hyperope - ways to understand
1) beyond infinity - by accommodating +4D eye, will bring object to infinity to correct its vision - making artificial emmetrope
2) virtual object point = no real, physical object location can give plus vergence (convergence) going in
how to correct hyperopia (2 ways)
- plus lens of appropriate power
- eye change own power via accommodation
why can you have different prescriptions for specs and CL RX?
different distance from eye.

vergence increase as get closer to image point.
moving plus lens upstream (left) also moves image

what does to power?
more plus
moving plus lens downstream (right) also moves image

what does to power
less plus
moving minus lens upstream (left) also moves image

what does to power?
less minus
moving minus lens downstream (left) also moves image

what does to power?
more minus
upstream what does it do for minus and plus
more plus, less minus
downstream what does it do for minus and plus
less plus, more minus
downstream

hyperopia
add plus from spec to CL
downstream

myopia
add minus from spec to cl
plus lens too strong, converge before screen

what happened to lens
what need to do to refocus light
lens: INCREASED effective power (moved focal length anteriorly) -> stronger power lens than before (when could focus on screen)
to refocus light: need WEAKER power to refocus light
minus lens, too weak (focus behind lens)

what happened to lens
what need to do to refocus light
lens: decreased effective power - acts as WEAKER power lens than before when it was on the screen

to refocus light: need STRONGER lens
lens effectivity formula
F(new) = F(old)/1-dF(old)

d: distance changing
+ : upstream
- : downstream
vertex distance
how far a lens is from the cornea

11-13 mm (think 12 mm for class)
CL : 0 mm distance lens to CL
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