retinal image analysis

Question 1

outline retinal image size in uncorrected ametropia (axial vs refractive) (1+2)

Answer 1

retinal size in uncorrected ametropia depends on the nature of ametropia

axial:
- in axial ametropia, retinal image increases as the eye’s axial length increases
myope > emmetrope > hyperope

refractive:
- image size remains constant (still blurred)

Question 2

outline retinal image formation in corrected ametropia (1+2)

Answer 2

• ametropia corrected by ensuring that the secondary focal point of the correcting lens is placed at the eye’s far point

1. spec lens depends on refractive error
2. this image then becomes object for the eye (which is conjugate with the macula)

Question 3

what affects the size of the blur circle (3)

Answer 3

1. degree of focusing error
2. pupil size
3. axial length

Question 4

when is the blur = zero

Answer 4

for a relaxed eye, blur is zero when L = K

ie. when the object is at far point MR

Question 5

how does accomodation affect focusing error

Answer 5

when the eye is accommodating, the focusing error is reduced by accommodation

Question 6

outline the basic of retinal image rays (sharp v blur) (2+1)

Answer 6

1. if retinal image sharp
   - all rays from a given object point will pass through the retinal image point
   - > image point can be located on retina using the undeviated ray from the object point
2. if retinal image blurred
   - to locate centre of blurred image, trace the central (chief) ray of the pencil entering the eye

Question 7

outline concept of pinhole (1)

Answer 7

• pinhole reduces the effective pupil size -> reducing the blur circle + circle of confusion diameter -> increasing depth of field

Question 8

pinhole advantage vs disadvantage (1 vs 2)

Answer 8

advantage:
1. if retinal image is not in sharp focus, can significantly improve vision

disadvantage:

1. if retinal image is in sharp focus (ie. onlu diffraction limited), pinhole will impair vision by increasing the size of the Airy disc
2. pinhole reduces illumination (can impair vision)

Question 9

describe relationship between blur and:

1. pupil size (g)
2. refractive error (K)

Answer 9

1. blur is proportional to pupil size (g)

2. for distant objects, blur is proportional to the refractive error (K)