1 Why do all round-seg bifocals give rise to jump? (A) They exert base-up prism at the dividing line; (B) They exert base-down prism at the dividing line; (C) The segments are inset from the distance OC; (D) It depends on the power of the distance Rx; (E) They do not give rise to jump.
The correct answer is B. Because light is deviated towards the base of a prism, to the observer it appears to have been displaced in the opposite direction; in this case upwards.
2 How far would an object be displaced at 3m when viewed through a 3Æ prism? (A) 27cm; (B) 9cm; (C) 1cm; (D) 3cm; (E) 0.3cm.
The correct answer is B. For every prism dioptre, an object is displaced 1cm per metre; therefore a 3Æ prism will displace an object 3cm at 1m and therefore 9cm at 3m.
3 Which of the following unwanted, equal right and left prism base directions is likely to be accepted by the wearer? (A) R: in L: in; (B) R: out L: out; (C) R: up L: up; (D) R: up L: down; (E) R: down L: up.
The correct answer is C. When prism base directions are the same vertically they cancel.
4 If a hypermetrope's PD is smaller than the frame 'box centre distance' and the required diameter lens blank is not large enough to produce the correct centration, what prism base direction will the subject encounter if this lens is glazed to 'max decentration'? (A) No prism; (B) Base-down only; (C) Base in; (D) Base out; (E) Base-up only.
The correct answer is D. The OCs and in this case the thickest part of the lenses will be wider than the subject's PD.
5 A +5.50D hypermetrope has mono PDs of R: 33 L: 29. What is the subject likely to complain of if spectacles were made with the OCs coincident with the pupil centres? (A) Chromatic aberration when looking straight ahead; (B) Blurring in one eye; (C) Double vision; (D) Lens thickness differences; (E) Suppression.
The correct answer is D. Greater decentration for the left lens requires the need for a larger blank size and therefore a thicker lens.
6 What decentration is required to create 3Æ base in with an Rx of: -5.00/-1.00x90? (A) 5mm out; (B) 5mm in; (C) 6mm out; (D) 6mm in; (E) 3mm in.
The correct answer is A. The horizontal power of the lens is -6.00D. By rearranging P=cF, c = 0.5cms. or 5mm. Minus lenses are required to be decentred in the opposite direction to the required base direction.
7 R: 3Æ base-up may be split: (A) R: 1.5 Æup L: 1.5 Æ up; (B) R: 2Æ up L: 1Æ up; (C) R: 1.5 Æ up L: 1.5 Æ down; (D) R: 1.5 Æ down L: 1.5Æ down; (E) Cannot be split.
The correct answer is C. Both (A) and (D) cancel and (B) results in 1Æ up in the right.
8 A -2.00D myope with 4Æ base-out each eye and mono PDs of R: 30mm L: 30mm requires varifocal lenses. What fitting cross centration distances should be ordered? (A) R: 34 L: 34; (B) R: 31 L: 31; (C) R: 30 L: 30; (D) R: 29 L: 29; (E) R: 28 L: 28.
The correct answer is D. The centration points are adjusted by 1mm (1/4mm/every 1Æ prism) in the direction of the prism apex (in).
9 Which centration is least likely to be tolerated by a subject with an uncompensated exophoria for near and a reading Rx of +5.75DS each eye? (A) OCs glazed to the subjects near PD; (B) OCs glazed wider than the subject's near PD; (C) OCs glazed narrower than the subject's near PD; (D) OCs glazed 3mm below the subject's pupil centres; (E) OCs glazed 1mm below the subject's pupil centres.
The correct answer is B. Wide OCs with a plus Rx creates base-out prism. Base-in prism is usually associated with exophoria.
10 A 30-year-old +6.00D hypermetrope has a distance PD of 30mm each eye and a near PD, when measured in the plane of an empty spectacle frame, of 28mm each eye. When reading, how much prismatic effect will each eye encounter, assuming the subject uses spectacles centred for distance only? (A) 1.2Æ base-out; (B) 1.2Æ base-in; (C) More than 1.2Æ base-out; (D) More than 1.2Æ base-in; (E) No prismatic effect.
The correct answer is C. The base-out prism causes even greater convergence of the eyes towards the apex of the prism, resulting in the near visual point being more inset than that determined from the near PD measurement.
11 When an esophoric myopic subject is given the full spectacle distance correction what is the likely outcome when viewing a near target? (A) The esophoria increases; (B) The esophoria decreases; (C) The esophoria remains the same; (D) The subject becomes exophoric; (E) They will be unable to accommodate.
The correct answer is A. Giving the full minus Rx normalises the accommodation increasing the likelihood of greater convergence.
12 Which of the following lenses could not be used to correct the Rx: R & L +2.25 / -0.50 x 180 Add +2.25 2Æ base in? (A) Franklin split bifocal; (B) Prism segment solid bifocals; (C) Cemented segment bifocals; (D) Bi-prism bifocals (slab off); (E) Single-vision distance and near lenses.
The correct answer is D. Slab off can only be used to remove unwanted differential vertical prism.1 Why do all round-seg bifocals give rise to jump? (A) They exert base-up prism at the dividing line; (B) They exert base-down prism at the dividing line; (C) The segments are inset from the distance OC; (D) It depends on the power of the distance Rx; (E) They do not give rise to jump.
The correct answer is B. Because light is deviated towards the base of a prism, to the observer it appears to have been displaced in the opposite direction; in this case upwards.
2 How far would an object be displaced at 3m when viewed through a 3Æ prism? (A) 27cm; (B) 9cm; (C) 1cm; (D) 3cm; (E) 0.3cm.
The correct answer is B. For every prism dioptre, an object is displaced 1cm per metre; therefore a 3Æ prism will displace an object 3cm at 1m and therefore 9cm at 3m.
3 Which of the following unwanted, equal right and left prism base directions is likely to be accepted by the wearer? (A) R: in L: in; (B) R: out L: out; (C) R: up L: up; (D) R: up L: down; (E) R: down L: up.
The correct answer is C. When prism base directions are the same vertically they cancel.
4 If a hypermetrope's PD is smaller than the frame 'box centre distance' and the required diameter lens blank is not large enough to produce the correct centration, what prism base direction will the subject encounter if this lens is glazed to 'max decentration'? (A) No prism; (B) Base-down only; (C) Base in; (D) Base out; (E) Base-up only.
The correct answer is D. The OCs and in this case the thickest part of the lenses will be wider than the subject's PD.
5 A +5.50D hypermetrope has mono PDs of R: 33 L: 29. What is the subject likely to complain of if spectacles were made with the OCs coincident with the pupil centres? (A) Chromatic aberration when looking straight ahead; (B) Blurring in one eye; (C) Double vision; (D) Lens thickness differences; (E) Suppression.
The correct answer is D. Greater decentration for the left lens requires the need for a larger blank size and therefore a thicker lens.
6 What decentration is required to create 3Æ base in with an Rx of: -5.00/-1.00x90? (A) 5mm out; (B) 5mm in; (C) 6mm out; (D) 6mm in; (E) 3mm in.
The correct answer is A. The horizontal power of the lens is -6.00D. By rearranging P=cF, c = 0.5cms. or 5mm. Minus lenses are required to be decentred in the opposite direction to the required base direction.
7 R: 3Æ base-up may be split: (A) R: 1.5 Æup L: 1.5 Æ up; (B) R: 2Æ up L: 1Æ up; (C) R: 1.5 Æ up L: 1.5 Æ down; (D) R: 1.5 Æ down L: 1.5Æ down; (E) Cannot be split.
The correct answer is C. Both (A) and (D) cancel and (B) results in 1Æ up in the right.
8 A -2.00D myope with 4Æ base-out each eye and mono PDs of R: 30mm L: 30mm requires varifocal lenses. What fitting cross centration distances should be ordered? (A) R: 34 L: 34; (B) R: 31 L: 31; (C) R: 30 L: 30; (D) R: 29 L: 29; (E) R: 28 L: 28.
The correct answer is D. The centration points are adjusted by 1mm (1/4mm/every 1Æ prism) in the direction of the prism apex (in).
9 Which centration is least likely to be tolerated by a subject with an uncompensated exophoria for near and a reading Rx of +5.75DS each eye? (A) OCs glazed to the subjects near PD; (B) OCs glazed wider than the subject's near PD; (C) OCs glazed narrower than the subject's near PD; (D) OCs glazed 3mm below the subject's pupil centres; (E) OCs glazed 1mm below the subject's pupil centres.
The correct answer is B. Wide OCs with a plus Rx creates base-out prism. Base-in prism is usually associated with exophoria.
10 A 30-year-old +6.00D hypermetrope has a distance PD of 30mm each eye and a near PD, when measured in the plane of an empty spectacle frame, of 28mm each eye. When reading, how much prismatic effect will each eye encounter, assuming the subject uses spectacles centred for distance only? (A) 1.2Æ base-out; (B) 1.2Æ base-in; (C) More than 1.2Æ base-out; (D) More than 1.2Æ base-in; (E) No prismatic effect.
The correct answer is C. The base-out prism causes even greater convergence of the eyes towards the apex of the prism, resulting in the near visual point being more inset than that determined from the near PD measurement.
11 When an esophoric myopic subject is given the full spectacle distance correction what is the likely outcome when viewing a near target? (A) The esophoria increases; (B) The esophoria decreases; (C) The esophoria remains the same; (D) The subject becomes exophoric; (E) They will be unable to accommodate.
The correct answer is A. Giving the full minus Rx normalises the accommodation increasing the likelihood of greater convergence.
12 Which of the following lenses could not be used to correct the Rx: R & L +2.25 / -0.50 x 180 Add +2.25 2Æ base in? (A) Franklin split bifocal; (B) Prism segment solid bifocals; (C) Cemented segment bifocals; (D) Bi-prism bifocals (slab off); (E) Single-vision distance and near lenses.
The correct answer is D. Slab off can only be used to remove unwanted differential vertical prism.
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