NBEO Contact Lenses & Fitting 1 — Questions and Answers
Question 1: Which parameter describes the radius of curvature of the posterior surface of a contact lens?
- Back vertex power
- Base curve radius (Correct answer)
- Overall diameter
- Dk value
Correct answer: Base curve radius
The base curve radius defines the curvature of the posterior surface of the contact lens that interfaces with the tear film and cornea.
Question 2: Which parameter best describes the oxygen transmissibility of a contact lens material of a given thickness?
- Water content percentage
- Lens modulus of elasticity
- Dk/t value (Correct answer)
- Wetting angle
Correct answer: Dk/t value
Dk/t combines the oxygen permeability (Dk) of the material and lens thickness (t), making it the best indicator of how much oxygen reaches the cornea.
Question 3: According to the FDA classification system, a soft contact lens with water content at or above what percentage is classified as 'high water content'?
- 30%
- 45%
- 50% (Correct answer)
- 70%
Correct answer: 50%
The FDA classifies soft contact lens materials into Groups I–IV; lenses with water content ≥50% are classified as high water content.
Question 4: When evaluating a soft contact lens fit, which finding best indicates an optimal fit?
- Complete limbal clearance with no movement on blink
- Full corneal coverage, good centration, and 0.5–1 mm movement with blink (Correct answer)
- Tight fit with less than 0.1 mm lag on upgaze
- Slight apical clearance with heavy edge lift
Correct answer: Full corneal coverage, good centration, and 0.5–1 mm movement with blink
An ideal soft lens fit shows complete corneal coverage, good centration, and 0.5–1 mm of movement with each blink to allow adequate tear exchange.
Question 5: Which FDA contact lens group is characterized by both high water content and ionic properties, making it most susceptible to protein deposits?
- Group I
- Group II
- Group III
- Group IV (Correct answer)
Correct answer: Group IV
FDA Group IV lenses have high water content (≥50%) and are ionic, which promotes protein binding and deposit formation on the lens surface.
Question 6: Back vertex power of a contact lens is best defined as:
- The power measured at the anterior surface of the lens
- The vergence of light leaving the posterior surface of the lens (Correct answer)
- The base curve converted to dioptric power
- The prismatic effect at the optical center
Correct answer: The vergence of light leaving the posterior surface of the lens
Back vertex power is the vergence of light leaving the posterior surface of the contact lens, which is the standard clinical method for specifying and measuring contact lens power.
Question 7: A patient's spectacle prescription is –10.00 DS at a vertex distance of 12 mm. When converting to a contact lens power, the contact lens should be:
- More minus than the spectacle Rx due to reduced vertex distance
- Less minus than the spectacle Rx because the effective power increases at the corneal plane (Correct answer)
- The same power because vertex distance only matters for plus lenses
- More plus than the spectacle Rx to compensate for tear lens effect
Correct answer: Less minus than the spectacle Rx because the effective power increases at the corneal plane
Moving a high minus lens closer to the eye (shorter vertex distance) increases its effective minus power at the corneal plane, so the contact lens must be prescribed with less minus power.
NBEO Contact Lenses & Fitting 1
Which parameter describes the radius of curvature of the posterior surface of a contact lens?