Basic Lens Dispensing Part 3

The ODMA board recognises that a crucial part of industry growth is ongoing education and training.

To that end, in this issue we continue our series of articles from the ODMA Optical Retail Guide with the third part of our series on the Basics of Lens Dispensing: Industry Terminology.

From parts 1 and 2 of the Basic Lens Dispensing series, we know that poor vision is caused by the eyes' inability to focus light onto the retina.

An optometric professional will examine a person's eyes to determine the level of correction required, and then a prescription is produced requesting a lens in terms of lens power.

Lens Power

Lens power is created by using two or more curves, resulting in a bending of light.

This lens power is measured in dioptres.

Focal Length

Lenses will form either a real focal point (+ power) or a virtual (imaginary) focal point (- power).

The focal length is the distance from the focal point to the lens. This length is measured in metres.

Example: A magnifying glass forms a point focus on paper and causes it to burn.

Dioptre Power

 

The dioptric power of a lens is inversely related to the distance between the lens and the focal point (focal length).

Lens Power (D) = 1/f (focal length) in metres.

Example:

1. Dioptre = 1/1=1 metre focal length
2. Dioptre = 1/2=1/2 metre focal length

Lens Types

Plus Lens

• Magnifies image
• Focal point on right
• Centre thicker than edges

Minus Lens

• Reduces image size
• Imaginary focal point on left of lens
• Edges thicker than the centre of the lens

Plano Lens

Front and back curves the same, therefore, the light does not bend at all.

Constructing a Plus Lens

Front curve radius is shorter than back curve radius. Therefore, has more curvature.

Lens Form, Spherical Plus

 

Correct Form

• Allows for Rotation of the eye
• Better Peripheral Performance
• Avoids Eyelash Contact

Incorrect Form

• Increases Peripheral Distortion
• Likely to cause Eyelash Contact

Plus Lens Power

Front Curves are usually Convex ( + )

Back Curves are usually Concave ( - )

F Curve + B Curve = Approx Lens Power Target power +6.00

1. +12.00 FC + (-6.00 ) BC - Poor Cosmetics
2. +6.00 FC + ( 0.00 ) BC - Poor Optics
3. +9.00 FC + (-3.00) BC - Best Balance Successfully balances optical performance and cosmetic appearance.

Constructing a Minus Lens

Front curve radius is longer than back curve radius. Therefore, has less curvature.

Lens Form, Spherical Minus

Correct Form

Front Surface curvature, Reduces Reflections

Incorrect Form

• Flat surface increases surface reflections
• Cosmetically unappealing
• Only used for very high minus

Minus Lens Power

Front Curves are usually Convex ( + )

Back Curves are usually Concave ( - )

F Curve + B Curve = Approx Lens Power Target Power -6.00

1. 0.00 FC + (-6.00 ) BC - Poor Cosmetics
2. +5.00 FC + (-11.00 ) BC - Too Difficult
3. +2.00 FC + ( -8.00 ) BC - Best Balance

Successfully balances cosmetics against the difficulty of surfacing.

Lens Form

Lenses can be spherical or cylindrical in nature.

Spherical - lens curves have the same power across the whole surface). e.g. soccer ball shape.

Cylindrical - lens curves are elliptical and the power varies depending on the viewing axis) e.g. rugby ball shape. Cylindrical lenses are used to correct astigmatism.

Spherical Lenses

Two lenses with the same curvature providing the same focal lengths.

 

Cylindrical or Toric lenses

Used to correct vision when the amount of correction varies across the eye's surface.

Lenses can be referred to as cylindrical, toric, or compound.

Cylindrical lenses have two meridians of curvature that are 90 degrees apart and are used to focus light onto the retina.

Cylindrical or Toric lenses

Two different lens curvatures providing two different focal lengths.

Standard Notation - Axis

 

This diagram shows the positions of the principle power meridians.

Prism

 

• What is it? What is its effect?
• Can be described as a wedge.
• Has the effect of moving or displacing an image.
• Can be prescribed or unwanted due to incorrect P.D. or vertical optical centre errors.

Prescription Interpretation

• Ametropia requires a Sphere power.
• Astigmatism requires a Cylinder power and axis.
• Presbyopia requires an Addition which is always plus in power.

Transposition

• Add Sphere and cyl powers together - this becomes your new sphere power.
• Change the sign of the cyl power (magnitude remains the same) - this becomes your new cyl power.
• Change the axis by 90 degrees.