Refraction Flashcards
What is classified as an ametrope?
myopia, hyperopia, presbyopia and astigmatism
Which is more difficult to correct, myopia or hyperopia?
hyperopia (because of accommodation affects)
ametropia defined?
refractive state of the eye in which no accommodation is being used, object is at optical infinity (20ft) and no correction is being used and all results in an image being formed not on the retina causing blur.
myopia is what kind of sight? image is focused where? hyperopia?
nearsightedness, image is formed in front of the retina.
farsightedness, image is formed in back of retina (without accommodation)
define astigmatism
a point is focused as two lines resulting in a blurred and distorted image (lines may be in front of retina, behind or on in part).
presbyopia is what?
normal decrease in amp of accommodation occurring with increasing age, therefore an ADD is needed over their distance correction to see clearly at near.
define emmetrope
when parallel light enters the eye (object at optical infinity 20ft) the light is focused directly on the retina without any accommodation.
what is the goal of refraction?
to make a perfect eye (emmetrope)
myopes typically have what sized eyes? hyperopes? babies are normally born slightly what?
myopes have longer eyes while hyperopes have shorter eyes.
slightly hyperopic
what causes ametropia?
- failure of emmetropization
- axial length of eye and power of eye are mismatched resulting in RE, discrepancy or ametropia.
- axial length too long or too short for the power of eye
if the power the eye has is greater than what it needs what is the discrepancy? Rx needed?
discrepancy is + (myope), Rx is -
if the power of the eye is less than what it needs what is the discrepancy? Rx needed?
discrepancy is - (hyperope), Rx is +
the Rx is always equal and opposite in power to what?
the discrepancy (or RE or ametropia)
an emmetropic eye has a power of what usually?
around +60D
what does pinhole do?
decreases the size of the blur circle on the retina making the image (or circles) appear smaller and can therefore help the patient determine the image or shape.
pinhole changes the size of each blur circle without changing the defocus or dioptic blur.
if we find increased vision with pinhole what does this suggest? what does the size of the letter suggest?
increased vision with pinhole suggests the vision issues can be corrected with a larger Rx. the size of the letter they are seeing with pinhole estimates the least we expect with new Rx (we could get better).
if after refraction VA is not at least 20/30 what should you do?
pinhole again, refraction not done.
real objects have a power of what? objects at infinity? virtual objects?
RO= negative power
O at infinity= 0 power
VO= positive power
how do we convert cm to diopters? inches to diopters?
100/value in cm= D
40/value in inches= D
accommodation always has what kind of power? what is the max amount we can accommodate? amplitude peaks at what age and then what happens?
positive (0- some amount of plus)
use formula, 15-1/4age
peaks at 14 an then steady decline
positive blur means what? negative?
\+= focus in front of retina - = focus in back of retina
lens or Rx if minus what does this mean? positive? what do we use for adds?
minus used for myopes
positive used for hyperopes
positive used for add
what is the summation principle?
B=L+O+A+D (blur, lens, object, accommodation and discrepancy. all are expressed in diopters).
if an eye is fogged, it sees best with what?
0 accommodation (we want this when using fog because we want light hitting the retina while using no accommodation therefore fogging used to get patient to stop accommodating).
