1. Sphere, Cylinders, Spherocylinders, & Lenses / abberations and best form lenses

what is a Convex Lens (Convergent Lens)

A convex lens is thick in the middle and thin at the
edges

What is a Concave Lens (Divergent Lens)

A concave lens is thin in the middle and thick at the
edges.

What is Myopia

short sightedness ( difficulty seeing far)

Hyperopia

long sightedness ( difficulty seeing near)

What causes hyperopia

Hyperopia occurs when light focuses behind the retina of the eye

What causes myopia

Myopia occurs when light focuses in front of the retina of the eye

what type of lens is needed to correct myopia

concave lens / diverging lens

what type of lens is needed to correct hyperopia

convex lens / converging lens

What is an optical centre

The centre point of a lens which lies on the principal axis of the lens is called its optical centre.

Which letter represents the optical centre

C

What is a Principle axis

The principal axis of a lens is defined as a straight line passing through the optical centre and the centre of curvature of two surfaces of a lens

What is a principle focus

The principal focus of a lens is a point on its principal axis to which the light rays parallel to the principal axis converge (convex lens) or appear to diverge (concave lens) after passing through it

Which letter represents the principal focus of a lens

F

What is the focal length

The focal length of a lens is the distance between its optical centre and principal focus.

which letter represents the focal length

f

Meniscus lenses

convex front curves and concave back curve

What is the relationship between the front and back surface curves of a lens is referred to as

lens form / lens profile

Which lens combinations can be used for plus lenses

1. Bi-convex: flat lenses use either convex curves for both the front and back.

2. Plano-convex: one convex curve and one Plano curve

Which lens combinations can be used for minus lenses

1. Bi-concave: concave curves for both the front and back

2. Plano-concave: one concave curve and one Plano curve.

Astigmatism

curvature of the eye , the eye is not spherical

what type of lens is used to treat an astigmatism

cylindrical lens

In cylindrical lenses the scale is always __________

anticlockwise

lens aberration

an imperfection in a lens that prevents light from converging at a single point, resulting in blurred or distorted images

types of lens aberration

1. oblique astigmatism

2. spherical aberration

3. coma

4. distortion

types of distortion

1. barrel

2. pincushion

3. mustache

chromatic aberration

when the speed of light differs due to colour of light

what are the types of chromatic aberration

1. longitudinal

2. lateral

define longitudinal chromatic aberration

when different wavelengths of colour do not converge at the same point after passing through a lens

define lateral chromatic aberration

when different wavelengths of colour coming at an angle focus at different positions along the same focal plane

Define eccentricity

circle eccentricity

e=0

ellipse eccentricity

e= between 0 and 1

parabola eccentricity

e=1

hyperbola eccentricity

e= greater than 1

aspheric lens

a lens which is both good cosmetically and optically makes the eye seem natural and normal

what is a power meridian

where the lens has the maximum or minimum curvature (or refractive power)

what is an axis meridian

The axis meridian is the direction (in degrees) where the cylindrical power of the lens is located

what is a cross cylindrical lens

Two cylindrical lenses are combined with axes 90 degrees apart from one another

what is an oblique astigmatism

an aberration that results when rays of light from an object in the periphery strike the lens obliquely and are refracted differently by the tangential and sagittal meridians of the len

what is a spherical aberration

an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface

what is a barrel distortion

Image magnification decreases with distance from the optical axis

what is a pincushion distortion

image magnification increases with distance from the optical axis. Convex (plus) spherical lenses tend to have barrel distortion

what is a mustache distortion

It starts out as barrel distortion close to the image centre and gradually turns into pincushion distortion towards the image periphery, making horizontal lines in the top half of the frame look like a handlebar

what are the benefits of aspheric lenses

High plus lenses steeper base curve worse lenses look

◦Choosing flatter base curve make the lens less bulbous & decrease magnification ◦

Cosmetically look much better

◦ If flattened lens surface is spherical optics suffers

◦ Aspherics mean both good cosmetic & good optics

◦ Eye look like normal or natural

◦ Decrease magnification or minification

◦ Flatter base curve is easier to glaze to keep lens tight in frame ◦ Reduction of oblique astigmatism

◦ Magnification of the eye and surrounding structure as seen by the observer is reduced

◦ Spectacle magnification for the patient is also reduced

◦ It is flatter, thinner and lighter lens

◦ The lens looks flatter which makes the eyes look more natural due to reduced magnification or minification

◦ Flatter lens fits better ensuring the attractive look of the spectacles, thereby, offering the subjects a wider variety of frames to choose from

◦ If the aspheric lens design is coupled with high index material, it will provide thinner, lighter and flatter lens that looks cosmetically the best

What is a best form lens

Lens with given focal power can be produced by many different lens forms as long as SUM of front and back are constant