ABO Optician — All Questions

What is the focal length of a +2.00 D lens?

• a.10 cm
• b.50 cm✓
• c.25 cm
• d.200 cm

Focal length in meters equals 1 divided by the lens power in diopters. 1 / 2.00 = 0.50 m, or 50 cm. A stronger plus lens would have a shorter focal length.

A lens has a focal length of 25 cm. What is its dioptric power?

• a.+2.00 D
• b.+25.00 D
• c.+0.25 D
• d.+4.00 D✓

Power equals 1 divided by focal length in meters. 25 cm = 0.25 m, so 1 / 0.25 = +4.00 D. Focal length and power are reciprocals of one another.

Light traveling from a less dense medium into a more dense medium will do what?

• a.Stop completely
• b.Speed up and bend away from the normal
• c.Slow down and bend toward the normal✓
• d.Continue at the same speed

When light enters a denser medium (higher index of refraction) it slows down and refracts toward the normal. This bending of light is the basis of how lenses focus rays.

Which lens material has the highest Abbe value, meaning the least chromatic aberration?

• a.1.67 high-index
• b.CR-39 (1.498)✓
• c.Polycarbonate
• d.1.74 high-index

CR-39 plastic has an Abbe value near 58, the highest among these options, so it produces the least chromatic aberration. Higher-index materials such as 1.67 and 1.74 have lower Abbe values and more color dispersion.

A minus (concave) lens forms what type of image and has what type of focal point?

• a.No image at all
• b.Real image, real focal point
• c.Virtual image, virtual focal point✓
• d.Real image, virtual focal point

A concave minus lens diverges light, so its rays only appear to come from a point behind the lens, creating a virtual focal point and a virtual, upright image.

The power of a lens surface increases as its radius of curvature does what?

• a.Stays the same
• b.Decreases (steeper surface)✓
• c.Increases (flatter surface)
• d.Doubles the index

Surface power is inversely related to radius of curvature. A shorter radius means a steeper, more curved surface and greater power; a longer radius means a flatter surface and less power.

What is the total power of a lens with a front surface of +6.00 D and a back surface of -2.00 D (thin lens approximation)?

• a.+3.00 D
• b.+4.00 D✓
• c.+8.00 D
• d.-4.00 D

For a thin lens, total power is the algebraic sum of the front and back surface powers. +6.00 + (-2.00) = +4.00 D.

Vergence describes what property of light?

• a.Its polarization angle
• b.Its intensity
• c.Its color wavelength
• d.The spreading or converging of light rays✓

Vergence is the degree to which light rays diverge or converge, measured in diopters. Plus lenses add convergence (plus vergence); minus lenses add divergence (minus vergence).

The base curve of a lens refers to which surface?

• a.The bevel edge
• b.The segment line
• c.The temple curve
• d.The reference curve from which the lens is made, usually the front✓

Base curve is the fixed reference curve used as the starting point for making a lens, conventionally the front surface. The back surface is then ground to achieve the required prescription power.

Add power on a multifocal lens represents what?

• a.The prism amount
• b.The distance sphere power
• c.The additional plus power for near vision✓
• d.The cylinder axis

The add is the extra plus power in the reading segment that supplements the distance prescription to help a presbyopic patient see clearly at near. It is always a plus value.

If the distance Rx is -3.00 D and the add is +2.00 D, what is the total power in the reading portion?

• a.-5.00 D
• b.+2.00 D
• c.-1.00 D✓
• d.-3.00 D

The near power equals the distance sphere plus the add. -3.00 + 2.00 = -1.00 D in the reading zone.

Vertex distance is the measured space between what two points?

• a.The frame front and the temple
• b.The two pupils
• c.The back of the lens and the front of the cornea✓
• d.The optical center and the segment

Vertex distance is the distance from the back surface of the lens to the front of the eye (cornea). It matters most for higher-powered prescriptions where a change alters effective power.

Chromatic aberration is caused by what?

• a.Different wavelengths of light focusing at different points✓
• b.Uneven frame pressure
• c.Scratches on the lens
• d.Anti-reflective coating

Because a lens refracts short (blue) wavelengths more than long (red) wavelengths, colors focus at slightly different points, producing color fringing. Materials with a low Abbe value show it most.

Which of the following describes a plano lens?

• a.+2.00 D of plus power
• b.A lens with no refractive power✓
• c.A lens with cylinder only
• d.A high minus lens

Plano means flat or zero power. A plano lens has no sphere or cylinder power and does not change vergence, though it may still carry a tint or coating.

Spherical aberration in a lens is best described as what?

• a.Color separation of white light
• b.Distortion from prism
• c.Peripheral rays focusing differently than central rays✓
• d.Reflection off both surfaces

Spherical aberration occurs when light passing through the outer (peripheral) zones of a lens focuses at a different point than light through the center, blurring the image. Aspheric designs reduce this effect.

The index of refraction of a material is a ratio comparing what?

• a.Speed of light in a vacuum to speed of light in the material✓
• b.Weight to volume
• c.Diameter to thickness
• d.Front curve to back curve

Index of refraction (n) equals the speed of light in a vacuum divided by the speed of light in the material. A higher index means light slows more, allowing thinner lenses for a given power.

An aspheric lens design primarily reduces what?

• a.Peripheral distortion and lens bulge✓
• b.Tint density
• c.Segment height
• d.Weight of the frame

Aspheric lenses flatten the front curve and change curvature toward the edges, which reduces peripheral aberrations and the magnified, bulging appearance of higher-plus lenses.

Compared with crown glass, polycarbonate is valued mostly for what property?

• a.Lowest cost
• b.Scratch resistance without coating
• c.Higher Abbe value
• d.High impact resistance✓

Polycarbonate is highly impact resistant, making it the preferred material for safety, children's, and sports eyewear. Its drawbacks include a low Abbe value and a soft surface that needs a scratch coating.

Snell's law governs which optical phenomenon?

• a.Polarization
• b.Reflection intensity
• c.Diffraction
• d.Refraction of light at a surface✓

Snell's law relates the angle of incidence and angle of refraction to the indices of the two media, describing how much light bends when passing between materials of different density.

As the index of refraction of a lens material increases, for a given power the lens can be made which way?

• a.Softer
• b.Thinner✓
• c.More curved
• d.Thicker

Higher-index materials bend light more efficiently, so less material and shallower curves are needed to achieve a given power, producing a thinner and often lighter lens.

A prism deviates light in which direction?

• a.Along the axis
• b.Toward the base
• c.Toward the apex✓
• d.There is no deviation

Light passing through a prism bends toward the thick base, while the image seen through it displaces toward the thin apex. Prism power is measured in prism diopters.

One prism diopter of power displaces an image by how much and at what distance?

• a.1 m at 1 meter
• b.1 mm at 1 meter
• c.1 cm at 1 meter✓
• d.1 inch at 1 foot

One prism diopter deviates a ray of light 1 centimeter at a distance of 1 meter. Prism diopters are the standard unit for specifying and measuring prismatic effect.

Which lens curve produces plus power?

• a.A prism surface
• b.A convex surface✓
• c.A flat surface
• d.A concave surface

A convex surface is thicker in the middle and converges light, producing plus power. A concave surface is thinner in the middle, diverges light, and produces minus power.

Effective power change from vertex distance is most significant for which prescriptions?

• a.High-powered lenses✓
• b.Cylinder-only lenses
• c.Plano lenses
• d.Low-powered lenses

The further a lens is moved from the eye, the more its effective power changes, and this effect grows with lens power. For high plus or minus Rx, vertex compensation is important; weak Rx changes negligibly.

The optical center of a lens is the point where what happens?

• a.The bevel is thickest
• b.The cylinder axis crosses
• c.Light passes with no prismatic deviation✓
• d.Maximum prism occurs

At the optical center, light passes straight through with zero prismatic effect. Moving the line of sight away from the optical center introduces prism, per Prentice's rule.

A photochromic lens changes tint in response to what?

• a.Blue light from screens
• b.Ultraviolet (UV) radiation✓
• c.Polarized reflections
• d.Temperature only

Photochromic lenses darken when exposed to ultraviolet light, which is why they may not darken fully inside a car whose windshield blocks UV. They lighten again when UV exposure decreases.

Anti-reflective (AR) coating improves vision primarily by doing what?

• a.Blocking UV only
• b.Increasing prism
• c.Reducing surface reflections and increasing light transmission✓
• d.Adding tint

AR coating reduces reflections off the lens surfaces, increasing the amount of light transmitted to the eye and reducing glare and ghost images, which improves visual clarity and cosmetic appearance.

A polarized lens reduces glare best from which source?

• a.Horizontal reflective surfaces like water and roads✓
• b.Digital device blue light
• c.Indoor fluorescent light
• d.Direct overhead sunlight only

Polarized filters block horizontally polarized light, which is the glare reflected off flat horizontal surfaces such as water, snow, and road pavement, greatly improving comfort and contrast outdoors.

What does the term 'minimum blank size' refer to?

• a.The smallest segment
• b.The smallest frame available
• c.The smallest lens blank that will cut out to fill the frame✓
• d.The smallest bevel angle

Minimum blank size (MBS) is the smallest diameter uncut lens needed to fully cut the shape into a given frame after accounting for decentration, ensuring the lens covers the entire eyewire.

Cylinder power in a lens corrects for what refractive condition?

• a.Presbyopia
• b.Color blindness
• c.Astigmatism✓
• d.Simple myopia

Cylinder power corrects astigmatism, in which the eye focuses light at different meridians unequally. The cylinder is oriented at a specified axis to counter the meridian needing correction.