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).
is it harder to fog a myope or hyperope? why?
hyperope because they are constantly accommodating all the time therefore it is hard to get them to stop. they do it unconciously.
hyperopia also means?
hypermetropia and farsightedness.
hyeropes get perfect vision without prescription because of?
accommodation (discrepancy is negative therefore they can use accommodation which is positive)
what is myopes revenge? what problems will older hyperopes have?
asthenopia at near (discomfort)
earlier presbyopia
complete dependence on a correction later in life.
what is near point defined as?
closest point the eye can see clearly using maximum accommodation (amplitude of accommodation 15-1/4age)
far point?
point in space conjugate to the retina where accommodation is relaxed (at 0).
when finding near points and far points using BLOAD what is blur equal to?
zero (accommodation for near is amp and accommodation for far is 0).
when at a young age, the far point for both a +4D myope and -4D hyperope are what compared to each other?
the same. until we increase in age and get a decrease in accommodation than hyperopes wont be able to accommodate as well and will require a larger Rx.
what does beyond infinity mean?
the object, no matter how far away is still too close for that eye to see it clearly under these conditions therefore must accommodate even at infinity.
when the hyperopes amp is equal to the hyperopoia power, his near point is where without correction?
at infinity.
when the hyperopes amp is equal to his hyperopia, his far point is where without correction?
beyond infinity
when a hyperopes amp is less than his hyperopia, his near point is where? (he is totally dependent on glasses)
beyond infinity
front surface of an optical lens is always what power? back surface?
positive (convex)
negative (concave)
cylinder lenses are used to correct what?
astigmatism
in plus cyl which is the toric surface (varying between two powers)? minus cyl?
front surface is toric
back surface is toric
if given a plus cyl and told base curve what does this mean?
plus cyl means front surface is toric. base curve means front surface therefore value given for base curve is the least positive power and goes in least positive meridian.
sphero cyl lens is made up of what two things?
simple sph and a plano cyl.
the amount of astigmatism is what value in the Rx?
the cyl power (difference between sph and difference).
if the vertical line foci are focused on retina what is happening with the horizontal?
they are not in focus because not in retina therefore horizontal line foci appear blurry (opposite for when horizontal foci are focused on retina, vertical line foci will be blurry)
if the circle of least confusion is located on the retina what happens to line foci H and V?
they both appear clear.
what do autorefractors read?
Rx NOT discrepancies
