Main Reasons for Patient Complaint
1- they didnt get what they thought they were getting
2- image quality/vision is poor
3- frame is not fitting comfortably
Spherical Lenses
- Power is the same all around the lens, in all directions
- Used to correct myopic/hypermetropic patients only
Toric Lenses
Power of cylindrical lens = no power along the axis, has 2 principal meridians at perpendicular meridians to one another
- axis direction of cylinder is always 90 to meridian where the power is positioned - axis always 90 to cyl power
-still has a max and min power but min power is not zero as it is in a spherical lens
toric is a spherical and cyl lens combined to neutralise both meridians
Prisms
- Transparent block of material, has an apex and a base at the bottom which is the thickest part of the lens
- Light deviates towards the base of prism and its because it slows down as it passes through the lens
- Because light is deviated towards the base, vision and so image is deviated towards the apex
Why is a prism used?
- Used for patients with muscle imbalance
- there are some pathologies which cause frequent change in vision, so surgery isnt an option
- some people are born with muscle imabalances - EOM can be too long/short so pull eye in a certain direction
- can still get px to see one image, even if both eyes see diff images
Surfaces of a lens?
Front surface = away from eye
Back Surface = closest to eye
Plano concave/ Plano convex
No power
Bi concave/Bi convex
- same curve on front and back surface. but not necessarily of equal power
- when power is same = equi convex/equi concave
Meniscus
Most common lens type = gives best optics so ensures image quality
- has one convex and one concave curve
What is a Corrective Lens?
- worn in front of the eye to correct vision
- in specs its mounted in a frame to hold the lens in a short distance from the eye
- in cls it fits directly onto the surface of the cornea
Sign Convention
Light always travels from left to right
Emmetropia
When patient has no prescription
When parallel rays of light coming from a distant object converge to a point to form an image on the retina
Second focal point coincides with the retina
- no distortion to the image
Depends on:
- curvature of the cornea
- refractive combined power of the cornea and the lens
- length of the eyeball
Hypermetropia
Point image blurry to px
Image falls after the retina and positive lens needed for image to fall on retina
Second focal point falls behind the retina
Could be due to:
- power of eyeball is too weak, so isnt focusing rays enough
- eye is a bit shorter
Hypermetropes and Accommodation
Your eye can accommodate when you’re younger - lens can alter its shape to give you more/less power. I f this system is working well, they can still see objects and have good vision
Myopia
Image falls before the retina and a negative lens is needed for image to fall on retina
Second focal point falls in front of the retina
- May have longer eyeballs than normal
- have v efficient system near front of the eye, so power is greater than it should be, meaning rays of light converge too much
Astigmatism
Means non point forming - objects are blurred and can appear distorted
Two line foci are produced instead of one dot image
- Condition where the two optical surfaces of the eye, usually cornea , are aspherical. There are two diff curves so one will be a higher power than the other
- creates two principal powers, a max power in one meridian and a min power in the second meridian which falls perpendicular to the first
How to correct astigmatism?
Lens must have two principal powers at perpendicular meridians to one another
- cylinder lens - has a surface with no power (axis meridian) which is 90 degrees away from a surface with max power (power meridian)
Thin Lens
When the two surfaces are in close contact so we can ignore the refractive index of the material bet the surfaces
- power of a thin lens is the sum of the power of both the surfaces
Thin Lens Equation
Fv’ = F1 + F2
Effective Power of the Lens
In distance vision:
The back vertex distance in mm is the distance bet the back vertex of the lens and the cornea and it affects the power of the lens (BVD)
If you increase the distance from the lens to the eye then the lens becomes more positive - so you have to make the prescribed lens more negative
