1.  Myopia

    2. Hypermetropia

    3. Astigmatism 

    4. Presbyopia

  > They are usually corrected with the help of spectacles , contact lenses and refractive surgeries. 

 >Problems with the high refractive errors are:

     • Greater thickness of lens

     • Weight of lens

     • Subject to aberrations

     • Magnification properties

     • Visual field limitation

 >Problems with the aphakia spectacle wearer:

     • Increased retinal image size

     • Decreased field of view

     • Increased ocular rotation

     • Increased lens aberration

     • Motion of objects in the field of view

     • Appearance of wearer

     • Demand of convergence

>  INCREASED RETINAL IMAGE SIZE

          

      The normal phakic eye values

             Refractive power of eye = +58.64D

             Anterior focal length     = 17.05mm

      The aphakic schematic eye values

             Refractive power of eye = +43.05D

             Anterior focal length     = 23.23mm

The correcting lens is placed at anterior focal point of the eye

Retinal image size is directly proportional to anterior focal length and inversely proportional to refractive power of eye

In short: increase in retinal image size by 36%

      Retinal image size in aphakia = 23.23 = 1.36

      Retinal image size in emmetropia = 17.05

>DECREASED FIELD OF VIEW 

        The prismatic effect of a high plus lens reduces the size of field of view through lens

> PRESENCE OF RING SCOTOMA

      The strong plus lens causes an angular gap in object space completely around the lens

      The patient sees nothing either through the lens or beyond the lens unless a head movement is made

      Beyond the scotomatous area wearer has uncorrected vision and centrally to scotoma the wearer has corrected vision

      Jack in the box phenomenon

      Lens power : stronger the lens larger the ring scotoma

      Vertex distance : farther the lens from eye smaller the  scotoma and moving the lens farther out makes  scotoma move centrally

      Lens size : the lens size increases the ring scotoma increases in size but it also moves towards periphery

      Pupil size : the smaller the pupil larger the ring scotoma

      Lens thickness : the lens thickness increases so does the magnification and the size of ring scotoma

      Base curve : with increasing steepness of base curve both the magnification and the size of the ring scotoma increases

> INCREASED OCULAR ROTATION 

      When the spectacle is worn the angle of eye must turn in changing fixation from one object pt to another increases or decreases

      The excursion of eye is increased for plus lenses and decreases for minus lenses

      The increase or decrease of ocular rotation is caused by the presence of prismatic effect

> INCREASED LENS ABERRATION 

     Spherical aberration

     Coma

     Oblique astigmatism

     Curvature of the image

     Distortion

     Chromatic aberration

> MOTION OF OBJECTS IN FIELD OF VIEW

     When a high ‘+ve’ lens wearer holds the eyes steady and moves the head to look an object indirectly, a reversed motion (against motion) of field of view is experienced. This motion is known as “Swim”. It occurs when the field from which visual axes are moving away is seen in indirect vision

     It can be avoided by moving the eyes from one fixation point to another while the head is held stationary

> APPEARANCE OF WEARER 

     It may convey 2 impressions

     1st impression: apparent enlargement of the eyes & their unusual appearance behind bulbous, thick lenses

     2nd impression: General awkwardness of the patient’s bearing and movements

     Both these are come from the magnification & distortion effects of lenses

> DEMAND OF CONVERGENCE

     If the distance center of aphakic lenses are placed in front of the center of the wearer’s pupil, convergence of the visual axes towards a near fixation point creates a base out prismatic effect

     Because of this the eyes when reading, must converge considerably more than for a wearer of lenses of low power

> CORRECTION FOR HIGH MYOPIA

 PROBLEMS CAUSED BY HIGH MINUS LENSES

 >  LENS WEIGHT AND THICKNESS

    Lens weight is not a problem for myope as a plastic lens is used

    Lens thickness is more of a problem for myope

> MINIFICATION

    High minus lenses causes minification

    Both the apparent size of the wearer’s eyes and the retinal image size increases

> MINIMIZING THE EDGE THICKNESS

           Small lens size

           High index material

           Aspheric lens

  

> SMALL LENS SIZE

     Lens edge gets thicker farther away from the center

     Frames with rounded corners should be used

> HIGH INDEX MATERIAL

    1.64 to 1.74 are high index

    Advantages of this are lens will be lighter, thinner, good cosmesis and reduced magnification.

> ASPHERIC LENSES

    A lens in which one or both surfaces are not spherical

    Doesn’t have same radius of curvature over the entire surface

    Optically correct oblique astigmatism produced when looking through lens.

    In the middle, starts out as spherical surface

> EDGE REFLECTION AND CONCENTRIC RINGS

    Edges of the high minus lenses produce multiple rings seen - true images of the edges of lens.

    Caused by total internal reflection.

    Total internal reflection repeated and multiple ring reflections seen by observer

    Rings more pronounced on oblique viewing than straight ahead position.

> WAYS TO REDUCE EDGE REFLECTION 

   Antireflection coatings

   Edge coating

   Edge painting

   Semitransparent edge

   Buffing the edge

   Tinting the lens

   Hide-a-Bevel technique

> LENTICULAR LENSES

   Used in case of very high myopes

   The central area of the lens contains the prescribed refractive power of the lens. The peripheral area is the carrier of lens.

1. MYODISC

    Front surface is almost flat contains the cylinder component of prescription.

    A high minus “bowl” in the middle of back surface. Plano back carrier area

2. MINUS LENTICULAR

    Carrier is not plano. Instead back surface of the carrier is made positive so the outer edge will thin down considerably.

3. BLENDED LENTICULAR

    Optical aperture is blended into the flange eliminating the obvious line and improving the appearance

> HIGH INDEX LENSES

    Normal index – 1.48 to 1.53

    Mid index – 1.54 to 1.63

    High index – 1.64 to 1.73

    Very high index – 1.74 to above

> High index available in

    Multiple progressive

    Single vision designs.

    Aspheric and atoric designs.

    High index, especially the index of 1.67 and 1.70, is also an excellent choice for rimless frames

    Polycarbonate or Trivex for children

> ADVANTAGE OF HIGH INDEX LENS

   1.  Power rings round the edges of the lens is 

   2. Reduce thickness 

   3. Look flatter

   4. Thinner

   5. Lighter 

   6. The benefit is greater peripheral vision as well as less magnification or minification of the eye

> DISADVANTAGE OF HIGH INDEX LENS

  1.  Reduced Low Abbe number

  2. Poor light transmission and increased backside and inner surface reflection increasing 

  3. More expensive than normal lenses

  4. Increase chromatic aberration

> WAYS TO REDUCE ABERRATIONS ARE:

> WAYS TO REDUCE CHROMATIC ABERRATION

   Using an achromatic lens or achromat

   The most common type is an achromatic doublet

   Using apochromatic lens or apochromat.

> SPHERICAL ABERRATION

 

   Use an aperture in front of the lens

   Aspheric lenses

> COMA

   Like spherical aberration, coma is a wide angle aberration

   But it applies only to off axial rays.

   The comatic blur figure is asymmetric and "comet shaped", hence the name coma.

   Correction : Using bestform or aplanatic lenses

                           Choosing the correct curvature of lenses

>OBLIQUE ASTIGMATISM 

    Narrow pencil of ray from a extra axial object point to pass through, after refraction give two focal lines perpendicular to each other, that is the pencil has become astigmatic. This aberration is oblique astigmatism.

    Occurs due to oblique incidence of light rays on spherical surface.

 Correction :

      Use meniscus lenses, instead of biconcave or biconvex.

      Orientation of lens so that astigmatism is minimized.

> CURVATURE OF FIELD

      Curvature of field is another oblique aberration

      In curvature of field a plane object is sharply imaged, but on a curved surface.

      It may be handled by bending the lens or curving the receiving the screen.

Correction: insert a field stop (iris) in order to remove edge light rays

> DISTORTION

     Another oblique small angle aberration is distortion.

     In distortion the object is sharply imaged but doesn't retain its shape.

     2 kinds of distortion are barrel distortion and pincushion distortion.

 

Distortion can be dealt with through placement of the aperture or by using lens systems, e.g. a so called orthoscopic doublet.

> ENHANCING COSMESIS

 REDUCING LENS THICKNESS

     Selecting a smaller lens

     A thinner edge reduces light entering into the edge, reducing an additional source of internal reflections.

     Extremely thick lenses for myopia can be beveled to reduce flaring out of the very thick edge

Ways to reduce the weight and thickness of corrective lenses :

    Choose glasses frames with small lenses

    Choose a frame that allows the pupil to occupy the exact middle point of the lens.

    Choose a lens as near round as possible.

    Choose as high a refractive index.