Spectacle Lens Aberrations Flashcards
7 cards from real ABO NOCE Basic Opticianry practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Spectacle Lens Aberrations flashcards as text
A patient wearing high-plus spherical lenses complains that straight lines appear curved inwards, especially at the edges of the lens. This is a classic example of which aberration?
Answer: Pincushion Distortion
Pincushion distortion is caused by the magnification increasing from the center to the periphery of a plus lens. This makes straight lines viewed through the lens periphery appear to curve inwards towards the center. Barrel distortion, the opposite effect, is common in high-minus lenses.
Which lens property is measured by the Abbe value, and what aberration does a low Abbe value indicate a higher amount of?
Answer: Abbe Value; Chromatic Aberration
The Abbe value is a measure of a lens material's chromatic dispersion. A lower Abbe value signifies more dispersion, which results in more prominent chromatic aberration, seen as color fringing around objects.
Spherical aberration results in light rays passing through the periphery of a lens focusing at a different point than rays passing through the center. This aberration is most effectively minimized by using what type of lens design?
Answer: An aspheric lens
Aspheric lens designs gradually change in curvature from the center to the edge. This change is calculated to correct for spherical aberration, allowing parallel light rays to focus at a single point, resulting in sharper vision.
When a wearer looks away from the optical center of a lens at an angle, they may experience blur. This is caused by light hitting the lens obliquely, creating two separate focal lines. What is this aberration called?
Answer: Marginal Astigmatism
Marginal astigmatism, also known as oblique astigmatism or radial astigmatism, occurs when viewing through the periphery of a lens. It is a significant off-axis aberration that corrected-curve lens designs aim to minimize.
A comet-shaped blur or flare extending from a point of light, viewed off-axis, is characteristic of which monochromatic aberration?
Answer: Coma
Coma is an off-axis aberration that causes points of light to be imaged as a comet-like shape. The 'tail' of the comet points either away from or towards the optical axis, depending on whether it is positive or negative coma.
The aberration that causes a lens to focus an image onto a curved surface rather than a flat plane is known as:
Answer: Curvature of Field
Curvature of field, or Petzval curvature, describes the tendency of a simple lens to form an image on a curved surface. This can cause the periphery of an image to be out of focus when the center is in focus, or vice-versa.
All of the following are considered Seidel aberrations (monochromatic aberrations) EXCEPT:
Answer: Chromatic Aberration
Chromatic aberration is caused by the dispersion of light as it passes through a lens material, meaning different wavelengths (colors) focus at different points. The five Seidel aberrations (spherical aberration, coma, oblique astigmatism, curvature of field, and distortion) are monochromatic, meaning they would occur even with single-wavelength light.