optics of the eye

Question 1

what is far point?

Answer 1

the point farthest form the eye at which an object is accurately focused in the retina when the accommodation is completely relaxed.

Question 2

what is near point?

Answer 2

the point nearest the eye at which an object is accurately focused on the retina at full accommodation.

Question 3

what is emmetropia? (perfect eyesight)

Answer 3

• parallel pencils of rays from a distant object to a sharp focus on the retina for an unaccommodated eye
• Far point at infinity
• Secondary focal point coincides with fovea
• Near point is determined based on the age and ability to accommodate weakens with age
• Estimated accommodation = 15 – (age/4)

Question 4

what is axial ametropia?

Answer 4

• assumed to have its “standard” eye power(+60D)

- refractive error in axial length(e.g. 20mm)

Question 5

what is refractive ametropia?

Answer 5

• axial length of reduced eye is assumed its standard value(22.22mm)
• defect attributed to an “error” in power(e.g. +58D)

Question 6

what is presbyopia?

Answer 6

• is a condition associated with the aging of theeyethat results in progressively worsening ability to focus clearly on close objects
• caused by crystalline lenses losing its flexibility as we get older
• Corrected by: Plus (Convex) Lenses

Question 7

what is astigmatism?

Answer 7

• Two focal line images formed at different distances generally perpendicular to each other for a point object
• Eyeball is not round thus the horizontal and vertical axis have different focal points
• Corrected by: Sphero-cylindrical lenses
• Circle of least confusion is the dioptric midway point between the 2 focal planes
• Interval of Sturm is the dioptric or physical distance between the 2 focal lines

Question 8

what is reduced schematic eye?

Answer 8

• 2 principal points (P) coincide with each other and with the vertex of the surface
• 2 nodal points (N) coincide with each other and with the centre of curvature of the surface
• Optical axis becomes the visual axis
• Hypothetical pupil lies at the refracting surface
• Entrance & exit pupils coincide with the hypothetical pupil. Therefore P is also the centre of the pupil
• Parameters of the standard reduced emmetropic eye
  > Reduced power Fe= +60D
  > Axial length of the eye k’ = +22.22mm
  > Refractive index of the eye ne= 4/3 (1.333)

Question 9

what is myopia?

Answer 9

• rays from a distant object point focus in front of the macula in unaccommodated eye
• Eye too powerful or eyeball too long
• Far point is in front of the eye, therefore k is negative
• Corrected by: Minus (Concave) Lenses

Question 10

what is hyperopia?

Answer 10

• rays from a distant object point focus behind the macula in unaccommodated eye
• Eye not powerful enough or eyeball too short
• Far point is behind the eye, therefore k is positive
• Corrected by: Plus (Convex) Lenses