Emmetropia and ametropia

• What is the approximate power of the cornea in diopters?

  ~43 D
• What is the approximate thickness of the lens?

  ~3.5-4 mm
• What is the approximate anterior chamber depth (ACD)?

  ~3.5 mm

  
• What is the approximate vitreous chamber depth (VCD)?

  ~16.5 mm

  
• What is the approximate axial length (AL) of the eye?

  ~23.5 mm

  
• What is emmetropia?

  It is when distant objects are focused sharply on the retina with relaxed accommodation.

  
• What happens to the image of a near object in emmetropia?

  The image is initially focused behind the photoreceptors but can be brought into focus by accommodation.

  
• What is the typical range of spherical equivalent for emmetropia?

  Generally -0.25 to +0.75 DS
• What characterizes myopia?

  Distant objects are focused in front of the retina.

  
• Why does myopia occur?

  It generally occurs because the eye has grown too long for the remaining refractive components.
• What is the visual characteristic of myopia?

  Distance vision is blurred while near vision is clearer.

  
• What type of lenses are used for optical correction of myopia?

  Concave (diverging) lenses.

  
• What is hypermetropia?

  It is when distant objects are focused behind the retina when accommodation is relaxed.

  
• What optical correction is used for hypermetropia?

  Positive/convex lenses.
• What is the typical spherical equivalent for hypermetropia?

  Generally ≥ +0.75 DS
• What is astigmatism caused by?

  It is caused by one or more toroidal refracting surfaces of the eye.
• What are the types of astigmatism?

  • Simple myopic astigmatism
  • Compound myopic astigmatism
  • Simple hyperopic astigmatism
  • Compound hyperopic astigmatism
  • Mixed astigmatism
• What does "with the rule" astigmatism indicate?

  Horizontal lines are blurry with an axis of ~180º.

  
• What does "against the rule" astigmatism indicate?

  Vertical lines are blurry with an axis of ~90º.

  
• What does "oblique" astigmatism indicate?

  Oblique lines are blurry with an axis of ~45º or 135º.

  
• How is a prescription with astigmatism written?

  • Format: OD or OS: ± Sphere power DS/ − cyl power DC x axis
  • Example: OD: +1.00/ -2.00 x 180
• How do you calculate the script for two spheres with different powers?

  Pick the most positive as the sphere power, calculate the difference as the cylinder power, and pick the axis of the most negative as the cylinder axis.
• What is presbyopia?

  It is difficulty with near work due to lens inflexibility with age.

  
• At what age does presbyopia typically onset?

  ~40-45 years old

  
• What are common symptoms of presbyopia onset?

  Eye strain/tiredness and increased working distance for emmetropes/hyperopes.
• What type of lenses are used for optical correction of presbyopia?

  Positive/convex lenses for near work.

  
• What is the typical additional prescription for presbyopia?

  Usually up to +2.50 DS
• How are prescriptions written for refractive errors?

  • Sphere
  • Cylinder
  • Axis
  • Addition
• What is the average pupillary distance (PD)?

  53-72 mm
• What is the significance of aligning optical centers of lenses with pupillary distances?

  It is essential for effective refractive correction.
• What are the key points regarding vision and refractive errors?

  • Vision at distance and near varies for refractive error types.
  • Prescribing refractive correction requires alignment of optical centers with pupillary distances.