(4) Best Vision Sphere

Question 1

What are the aims of subjective refraction

Answer 1

• verify objective refraction results from retinoscopy and auto-refractometer
• get best combo of sph and cyl for best possible VA for subject

Question 2

What is BVS for spherical ametropia

Answer 2

The lens power which brings the focal point to the retina

Question 3

What is BVS for astigmatic ametropia

Answer 3

• sph equivalent shifts circle of least confusion onto retina
• circle of least confusion: midpoint between 2 images/ halfway between 2 lines found in astig
• sph equivalent: compromised clear vision since both meridians cannot be clear at once
• formula: sph + 1/2 cyl

Question 4

Procedure for BVS +/- 0.25 DS challenge

Answer 4

-done after retinoscopy, keep sph, cyl lens from retinoscopy in trial frame. ensure accommodation relaxed

1. Start with plus. Ask if (i) clearer (ii) same (iii) worse.
   (i) add until response is “worse”.
   (ii) add until response is “worse”
   (iii) (valid only upon adding first plus lens) check if VA (from retinoscopy) worsened then don’t add +. Try -
2. Add -0.25. Ask if (i) clearer (ii) smaller and darker or same or worse
   (i) check if VA (from retinscopy) improves by at least one line then add until px responds “smaller and darker”

Question 5

When do you stop +/-0.25 respectively

Answer 5

+0.25: responds “worse” after a first plus at least has been added

-0.25: responds “same/ smaller and darker/ worse”

Question 5

Room condition and target for duo chrome test

Answer 5

• dim illumination
• flash eye for 2 seconds with red/green chart
• at least 2-4 lines above VA achieved from +/-0.25 challenge

Question 6

Instructions for duo chrome test

Answer 6

Looking at the black letters, are they DARKER - SHARPER - DEEPER on the red or green bg

Or

Read the letters out and ask eg. Is P T or T P darker sharper deeper

Question 7

Procedures for duochrome test

Answer 7

1. Red - can stop
2. Both on each side are equally distinct
3. Green - add +0.25 until they respond 1./2.

Question 8

Duochrome test disadvantages

Answer 8

• psychological preference for one colour
• young who accommodates: green
• elderly with yellowing of crystalline lens: red bias

Question 9

explain +1.00 test

Answer 9

• add +1.00, check that 4 lines (including va line from bvs) above va is blurred. 5th should be clear
• if blur=remove +0.25, clear=add +0.25