correcting ametropia Flashcards
Bring object from infinity to the fat point
myopia correction
Minus lens
Virtual image to the left of the lens
Positive lens
Converges beam
Image on the right
Real image
Bring object from infinity to the far point
Hyperopia correction
To function as correcting lens….
The lens secondary focal point F’ coincides with the far point of the eye
To correct myopia
-decrease the optical power and thus we use a diverging minus lens
To correct hyperopia
-increase optical power, and this we use a converging, positive lens
Because the spectacles are not in contact with the eye, what is involved
Vergence
The unit of vergence is
Diopters
What is the refractive index of the eye?
1.336
Are F and F’ the same?
No they are different because of different RI
How do you find vergence in a medium other than air
n/l
The vergence L of a ray bundle is
- the wavefront curvature
- the reciprocal of the distance L required to converge to a point
- for a collimated ray bundle, vergence is 0
- for propagation to the right, if the bundle converges, vergence is positive, while if the beam diverges, negative
The required power of a lens to produce the SAME optical effect at a different location
Effective (downstream) optical power
Which needs to have more power, spectacles far away from the eye, or one that is closer?
One that is closer
What is the average vergence distance?
10-15mm
What is the vergence distance for CL?
0
This hyperopia is referred to as
+10.00 vertex distance 15mm
I know this is weird, because you cant see the pic. Just know that you use the power of the spectacles and then state the vergence distance (distance between the specs and the eye itself)
When is vertex distance important?
When converting between contact lenses and spec Rx
When does vertex distance become significant?
If the spectacle Rx is +/-4.00D or more or vertex greater than 12
What vertex distances give a bigger jump in power?
12 and 15
When we measure front and back focal lengths what are we supposed to measure and what do we actually measure?
We are supposed to measure from the principal point to the focal point, but we actually measure from the vertex point to the focal point because it is more practical
