subjective refraction

Question 1

outline subjective refraction (1)

Answer 1

a clinical procedure that determines the combination of spherical and cylindrical lenses necessary to artificially place the far point of each of the patient’s eyes at infinity

Question 2

describe BVS (best vision sphere) (1)

what happens if spherical power is:

1. less than bvs
2. more than bvs
3. equal to bvs

Answer 2

BVS is most positive / least negative spherical lens for which vision is least blurred

1. less than bvs:
   - vision will still be clear due to accommodation by subject
2. more than bvs:
   - vision will be more blurred as subject cannot relax accomodation
3. equal to bvs:
   - vision is least blurred with 0 accom

Question 3

outline the relationship between BVS and astigmatism (2+1)

Answer 3

• in the absence of astigmatism, BVS becomes the spectacle correction
• for astigmatic subject, the BVS places circle of least confusion on the retina so vision is least blurred

nb: BVS cannot correct astig

Question 4

outline fogging and its procedure (1+2)

Answer 4

• fogging is a procedure of subjective refraction that ensures far point of the eye is placed at infinity when accomodation is fully relaxed

procedure:

• place +ve lens to ensure image is formed in front of retina (making eye artificially myopic)
• if fogging is done correctly, accomodative effort will result in poorer vision

Question 5

outline duochrome test (3)

Answer 5

• based on chromatic aberration of the eye (doesn’t affect colour deficiency patients)
• different wavelengths are bent to a different extent (short green focused in front of longer red wavelength)
• > myopic see red clear, hyperopic see green clear
• used for refinement of within 1.00D end point of sphere refraction

Question 6

define end point

Answer 6

maximum plus or minimum minus power for best visual acuity

Question 7

outline refraction WITHOUT astigmatism (3)

Answer 7

1. place enough plus power to make artificially myopic to fogged vision of 6/12
2. reduce plus power by 0.25 steps
3. stop reducing plus power (or increasing minus power) until best visual acuity is achieved

Question 8

outline refraction WITH astigmatism (1+2+1)

Answer 8

• uncorrected astigmatism can not form point images (2 line images are formed instead)

1. using sphere to move all of interval of sturm so that posterior focal line lies on rentina
2. using cylinder to move anterior focal line back to the posterior focal line

horizontal and vertical focus is then located in same plane
-> aka collapsing the conoid of sturm

Question 9

identify 2 astigmatic charts

Answer 9

1. clock dial

2. rotating T