Refractive Error Flashcards
How much of the USA population is myopic?
42%
How much of the population has atigmatism?
40%
By 2050 how much of the world’s population will be myopic?
50%
What is the #1/ #2 leading causes of blindness worldwide?
- Cataract
- Uncorrected refractice error
What happens if an eye has the same axial length but the light is refracted too much?
What if it is refracted too little?
Refractive Myope
Refractive Hyperope
Is most myopia axial or refractive?
Axial
What is the focal point?
When rays from a distant object strike a lens, the image is formed at the focal point of the lens (F). The distance from the lens to the focal point is called the focal length of the lens (f).
Where is the focal point of a plus lens?
Behind the lens
Where is the focal point of a minus lens?
In front of the lens
What is the lens power?
Ie: Focal length 1m=?
f= 0.5m=?
The power of a lens is stated as the reciprocal of the focal length in meters. Lens power is measured in DIOPTERS (D)
1m= 1D
0.5m= 2D
Where does the image lie for a plus lens?
Behind the lens
What sign do distances on the incident side have?
What sign do distance on the emergent side have?
What happens if you reverse where the object is?
Incident= Neg
Emergent= Pos
Does not matter! Incident side is always negative while the emergent side is always positive
What is l?
Location of object
What is l’ ?
Location of image
What is the far point of the eye?
The far point of the eye is the point conjugate with the retina of the unaccommodated eye
What happens if the object is at the far point?
What if the object is on the retina?
If the object is at the far point of the eye then the image is on the retina
If the object is on the retina then the image lies at the far point
Where is the far point (Mr) of a myopic eye?
How do you calculate the far point?
In front of the eye
1/refractive error in diopters
Where is the far point (Mr) of a hyperopic eye?
How do you calculate the far point?
Behind the eye
1/ refractive error in diopters
How does a correcting lens work?
Any refractive error can be corrected by a lens whose second focal point coincides with the patient’s far point
Where is the image in relation to the lens when the object is at optical infinity?
When the object is at optical infinity the image lies at the second focal point of the lens
Where is the object in relation to the lens when the image is at optical infinity?
When the image is at infinity, the object lies at the first focal point of the lens
Where is the second focal point of a plus lens?
Where is the second focal point of a minus lens?
Plus= Behind the lens
Minus= In front of the lens
What occurs in myopia?
Parallel rays of light from a distant object are focused in front of the retina = blurred retinal image
What must the lens power be equal to?
The lens power must be equal to the distance from the lens to the far point
How does accommodation occur?
Increasing the refractive power of the crystalline lens resulting from contraction of the ciliary muscle
How is TOTAL HYPEROPIA measured?
By getting rid of all accommodation with a cycloplegic drug
How is MANIFEST HYPEROPIA measured?
Finding the maximum plus lens that gives the best VA
What is latent hyperopia?
Latent= Total Hyperopia- Manifest Hyperopia
How is facultative hyperopia found?
Hyperopia under fog- hyperopia before fogging
What is absolute hyperopia?
What is the absolute hyperopia for a +8.00 hyperope with a maximum accommodation of 5D?
The amount of hyperopia that exceeds the maximum accommodation
3D of absolute hyperopia
What is the name of the direction of the zero power?
Cylinder Axis
How do you transpose between minus cyl and plus cyl?
+1.00-4.00x180 convert to plus cyl.
+1.00-4.00x180 ->
-3.00+4.00x090
What is the required focal length?
The distance from the lens to the far point of the eye
When writing an Rx, how is the axis of the Rx determined?
The axis is determined by the most plus or least minus axis
Where is the first focal length of the eye?
The distance from the eye to the far point or from the eye to to secondary focal point of the lens (calculate in bvd)
REMEMBER: The correcting lens’ secondary focal point coincides with the patient’s far point.
What is the secondary focal length of the eye
The axial length when viewing an object at infinity
Where is the second focal point of the lens?
When the object is at infinity
Where is the 1st focal point of the lens?
When the image is at infinity
Calculate the second focal length of the eye when the eye has an AL= 21.75mm and can be fully corrected for distance by a -3.25sph (bvd=14mm)
- Calculate power of the eye
1/3.25= -(307.69+14)= -321.69mm
F= 1.333/0.02175 - - 1/0.32169
= 64.40D
- Calculate the axial length of the eye when focused at an object at infinity
1.333/l’ - 1/infinity= 64.40D
l’= 20.70mm
When calculating the power of the eyes from a glasses Rx what do you need to consider?
BVD!!!
