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THE UNIVERSITY OF NEW SOUTH WALES

SCHOOL OF OPTOMETRY AND VISION SCIENCE

SEMESTER 1 2009

VISN2131 - OPTICS AND THE EYE

(Paper-1: Visual Optics)

TIME ALLOWED: 2 hours

THIS EXAMINATION HAS 4 PAGES

TOTAL NUMBER OF QUESTIONS: 5

ANSWER ALL QUESTIONS

TOTAL MARKS AVAILABLE – 50

ALL QUESTIONS ARE OF EQUAL VALUE

THIS QUESTION PAPER MAY BE RETAINED BY THE CANDIDATE

CANDIDATES MAY BRING Drawing Instruments or Rules and approved

calculators TO THE EXAMINATION.

ALL ANSWERS MUST BE WRITTEN IN INK. EXCEPT WHERE THEY ARE

EXPRESSLY REQUIRED, PENCILS MAY BE USED ONLY FOR DRAWING,

SKETCHING OR GRAPHICAL WORK.

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1. i) For a static astigmatic eye of pupil size g viewing an object at distance

l draw the rays to show the formation of the line foci and hence derive

an expression for the length a of the horizontal line focus in terms of

the pupil size, astigmatism, and the corresponding image vergence.

Assume that the eye is more myopic along the vertical. Label the key

features and the distances in your diagram. 4 marks

ii) While using a simple optometer a practitioner makes a systematic error

of +1.0 mm in deciding the target location from the primary focal

plane. The power of the optometer lens is +8 D. The subject‟s eye is

close behind the lens. The target location from the focal plane was

measured to be +50 mm for a subject. What is the actual value of his

refractive error? 3 marks

iii) For a refractive myope of refractive error K = −3.00 D, and pupil size g

of 5 mm, viewing a “36-metre” letter at a distance of 6 metres, find the

blur ratio. Will the subject be able to recognize this letter? Justify.

3 marks

2. i) An observer E sees with one eye a man at a position A distant 100

metres. The observer continues to fixate on the point A, as the man

walks away from A in a direction perpendicular to EA. If the man

suddenly disappears from view, how far did he walk from A before he

disappeared from view? The blind spot is 5 degrees in horizontal

diameter and its centre is 15 degrees from the fovea.

2 marks

ii) At what distance would you expect an observer with a visual acuity of

6/12 to be able to read a notice with letters 50 mm high? 2 marks

iii) A subject has a contrast sensitivity of 100 at a spatial frequency of

10 cycles per deg. He is viewing a „30-metre‟ letter from a distance of

12 m. If the letter has a contrast of 1%, will he be able to recognize

it? What is the threshold distance at which he can recognize this letter?

3 marks

iv) Show that if a brightly lit pinhole is held at the anterior focal plane of

the eye, the projected shadow of a vitreous opacity seen by the subject

is magnified by d/fe, where fe is the anterior focal length of the eye and

d the distance from the pinhole at which the projected shadow is

measured. 3 marks

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3. i) A 0.50 jcc is used in the power mode to find the power of the negative

correcting cyl. for a subject. The subject requires a spectacle correction

of Fsp = +4.00/ 1.00x180. A trial cyl of power 1.50x180 is

given to the subject after his axis has been found. Determine

(a) The total power of the trial sphere required to retain the CLC

on the retina.

(b) The residual error in each of the two presentations of the 0.50

jcc in the power mode with the given trial cyl and correct

sphere in place.

(c) Based on the residual error in the two presentations determine

whether the trial cyl power should be increased or decreased in

the next step.

6 marks

ii) The radius of curvature r1 of the cornea of a subject is 7.80 mm and its

index is 1.376. The refractive error K of the subject is 8.00 DS.

Calculate the axial thickness of the cornea to be removed by surgery,

to establish emmetropia, if the diameter of the altered zone is 5 mm.

4 marks

4. i) What is spherical aberration? Describe how the human eye copes with

it. 4 marks

ii) The refracting power, Fc, of a cornea is +48.00D. naque = nvitre = 1.336.

The actual length of the eye = 28.00 mm. nimplant = 1.490. The radius of

curvature r2 of the front surface of the implant is +50 mm. The location

d1 of the front surface of the implant from the cornea is 3.70 mm. The

implant thickness d2 = 1.0 mm. Calculate the required FE of the implant

in air. 4 marks

iii) Calculate the least distance and the gap or overlap in the range of clear

vision through bifocals when Add = +1.50D, Amps = 2.00D

2 marks

5. i) For K = 4.00 / 3.00 x 180, calculate the near correction for an object

distance of 300 mm from the spectacle plane S when the Add power is

1 D. Assume a vertex distance of 16 mm. 3 marks

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ii) In comparison phakometry, the separation of the Purkinje I images is

8 mm and the separation of the Purkinje III images is 13 mm, when

the eye is looking at a distant object. The radius of curvature of the

cornea is 7.8 mm and the apparent and real depth of the anterior

chamber are 3.05 mm and 3.6 mm respectively. Assume the index of

the aqueous to be 1.336 and the index of the lens to be 1.416 and find

the radius of curvature of the anterior surface of the lens and hence its

power. 3 marks

iii) Typically, what is the maximum retinal eccentricity for which V is

essentially diffraction limited when the pupil size is 4 mm?

1 mark

iv) Define the visual axis and illustrate with a sketch. 3 marks

END OF PAPER