Your FREE ABO Flashcards 2026 – 200+ Cards

The unit of lens power — the reciprocal of the focal length in meters. Power (D) = 1 ÷ f (m).

Power (D) = 1 ÷ focal length (meters). A +2.50 D lens focuses at 1 ÷ 2.50 = 0.40 m = 40 cm.

f = 1 ÷ 5.00 = 0.20 m = 20 cm.

Converges parallel rays to a real focal point; corrects hyperopia and presbyopia; thicker in the center.

Diverges parallel rays to a virtual focal point on the incoming side; corrects myopia; thicker at the edge.

Prism (Δ) = decentration (cm) × lens power (D). Gives the prism induced by looking away from the optical center.

Δ = 0.5 cm × 4.00 D = 2.0 Δ. (Convert 5 mm → 0.5 cm first.)

By decentering the lens (Prentice's rule) or by a prism-grind, moving the optical center relative to the visual axis.

The prism diopter (Δ): 1 Δ deviates light 1 cm at 1 m (1 cm per meter).

By vector (Pythagorean) addition: resultant = √(a² + b²). E.g. 3 Δ up + 4 Δ out = √25 = 5 Δ.

1) New sphere = sphere + cylinder. 2) Reverse the cylinder sign. 3) Rotate the axis 90° (keep 1–180).

→ +3.00 −1.00 × 180 (optically identical, minus-cyl form).

SE = sphere + (cylinder ÷ 2). For −2.00 −1.00 × 090, SE = −2.50 D.

The distance from the back surface of the lens to the front of the cornea.

It acts stronger (more minus). A plus lens moved away acts weaker.

Around ±4.00 D and stronger; weak prescriptions are negligibly affected.

The front (or reference) surface curvature of a lens, read in diopters with a lens clock; sets the lens's form.

A base curve chosen to minimize oblique (marginal) astigmatism for off-axis gaze.

Add the front and back surface powers algebraically (with sign).

Effective power is the lens's power experienced at the eye's plane, which shifts with vertex distance.

The point where light passes with no prismatic deviation; ideally placed in front of the pupil.

Base-up (BU), base-down (BD), base-in (BI, toward the nose), base-out (BO, toward the ears).

Light bends toward the base; the image displaces toward the apex.

Near PD is smaller than distance PD because the eyes converge for near tasks.

Plus lenses magnify (enlarge) the image; minus lenses minify (reduce) it.

Cylinder power in a meridian θ from the axis ≈ cyl × sin²θ added to the sphere.

−3.00 diopters sphere with no cylinder (a purely spherical lens).

Vertical prism imbalance from unequal lens powers when the patient reads below the OCs (Prentice's rule per eye).

f = 1 ÷ 2.00 = 0.50 m = 50 cm (virtual, on the incoming-light side).

Δ = d(cm) × P, so d = 3 ÷ 6.00 = 0.5 cm = 5 mm.

Resultant = √(2² + 1.5²) = √6.25 = 2.5 Δ.

√(36 + 64) = √100 = 10 Δ.

The depth of a curve's bulge; relates surface power, lens diameter, and thickness.

More diverging curvature is needed, removing material from the center and leaving the edge thick.

Recompute with the effective-power/vertex formula; a minus lens worn closer needs slightly less minus.

Yes — they describe the identical lens; transposition converts between them.

Approximate total power ≈ front surface power + back surface power.

Different wavelengths focus at slightly different points; worse with low-Abbe materials.

Toward the apex; the eye rotates toward the apex to refixate.

Convergence problems (it shifts images outward so the eyes converge less).

The cornea — about two-thirds of the eye's focusing power (large air-to-tissue index change).

Fine-tunes focus by accommodation; its stiffening with age causes presbyopia.

The light-sensitive layer at the back of the eye that converts light into neural signals.

The small central retinal area responsible for the sharpest, most detailed central vision.

Controls the amount of light entering the eye by changing the pupil size.

Where the optic nerve exits the retina — it has no photoreceptors, creating a natural blind spot.

In front of the retina (eye too long / cornea too steep); corrected with minus lenses.

Behind the retina (eye too short / cornea too flat); corrected with plus lenses.

Uneven corneal/lens curvature focusing light at two meridians; corrected with cylinder.

Age-related loss of accommodation (mid-40s onward); corrected with a near add.

The condition of no refractive error — light focuses exactly on the retina.

The eye's ability to increase its focusing power for near objects by changing lens shape.

Extra plus power for the near (reading) zone, added to the distance prescription for presbyopia.

OD = right eye (oculus dexter), OS = left eye (oculus sinister), OU = both eyes (oculus uterque).

Sphere, Cylinder, Axis, then Add and any Prism/Base.

The meridian (1–180°) along which the cylinder's correcting power is oriented.

The tough white outer coat of the eye that protects and maintains its shape.

Its avascular, regular collagen arrangement and controlled hydration.

Fluid in the anterior chamber that nourishes the cornea/lens and maintains intraocular pressure.

Convergence, accommodation, and pupil constriction when shifting to a near object.

The vascular layer between the retina and sclera that nourishes the outer retina.

The clear gel filling the posterior cavity that helps maintain the eye's shape.

Retinal photoreceptors: rods for low-light/peripheral vision, cones for color and detail.

The cone-rich center of the macula giving the sharpest vision.

The border where the cornea meets the sclera.

The thin membrane covering the white of the eye and the inner eyelids.

A significant difference in refractive power between the two eyes.

They must accommodate even for distance, leaving less reserve for sustained near work.

Reduced vision in an eye that did not develop normally ('lazy eye').

The closest distance at which the eye can keep an object in clear focus.

1.498 — the standard plastic; excellent optics (Abbe ≈ 58) but thick in strong Rx.

1.523 — excellent optics (Abbe ≈ 59); heavy and breakable, rarely dispensed now.

1.586 — most impact-resistant; low Abbe (≈ 30); inherently blocks UV.

≈ 1.53 — light, impact-resistant, drillable; higher Abbe (≈ 43–45) than polycarbonate.

1.60, 1.67, and 1.74 — progressively thinner/flatter for strong Rx, with lower Abbe.

A measure of chromatic dispersion. High Abbe = crisp optics; low Abbe = color fringing/edge blur.

The material bends light more, so the lens can be made thinner and flatter for the same power.

Polycarbonate — the standard for children's, safety, and sports eyewear.

Trivex — light and drillable; polycarbonate can crack at drill holes.

It reacts to UV light, not visible light, becoming dark outdoors and clearing indoors.

The windshield blocks most UV, the trigger they need, so they stay relatively clear.

Block reflected horizontal glare from roads/water using a polarizing filter.

Reduces surface reflections, improving light transmission, night vision, and cosmetic clarity.

Hardens the soft surface of plastic/high-index lenses to resist scratching.

The width of the flat-top reading segment in millimeters (28 mm wide).

A bifocal whose reading segment spans the entire width of the lens.

A multifocal with distance, intermediate, and near zones (e.g. a 7×28 trifocal).

A line-free multifocal with a gradual power change from distance through intermediate to near.

The extra plus power in the near zone of a multifocal, supplied for presbyopia.

Flat-top (FT/D-seg), round seg, curved-top, and executive/full-width.

Cosmetic color, light reduction, or contrast enhancement; gradient or solid.

By chemical or thermal (air) tempering, since glass is not inherently impact-resistant.

Reflects a portion of incident light off the front surface to cut brightness; cosmetic too.

It drives lens weight; lower specific gravity (e.g. Trivex) gives lighter lenses.

One power across the whole lens — corrects a single refractive distance need.

Flatter, thinner, lighter design that reduces magnification and distortion in plus lenses.

Inherently blocks essentially all UV up to about 380–400 nm.

Low Abbe value (chromatic aberration), most noticeable peripherally in strong/large lenses.

A tint that is darker at the top and fades lighter toward the bottom.

The filter is oriented vertically to block horizontally polarized reflected glare.

UV exposure and temperature — they darken more in cold, less in heat and in cars.

Segment width: 28 mm vs 35 mm reading area.

A small round reading segment; less image jump location but a curved dividing line.

The sudden prismatic shift as the eye crosses the seg line; least with the OC at the seg top (executive/no-jump).

Lighter weight (lower specific gravity) and better tensile strength for drilled mounts.

Flatter/thinner but requires correct vertex/pantoscopic fitting to perform well.

Commonly ~70–85% absorption (about 15–30% light transmission).

The front (convex) surface of the lens.

Thinner/lighter but typically lower Abbe (more aberration) and more surface reflection (AR recommended).

About +0.75 D to +3.00 D depending on the patient's near demand and age.

Sphere, cylinder, axis, add, and prism of a finished lens, and locates the optical center.

To eliminate the operator's accommodation so your eye doesn't add/remove power and bias readings.

A spherical lens (no cylinder correction).

Rotate until the single (sphere) lines and the triple (cylinder) lines are each sharp; note the axis.

A 3-pin gauge that reads a surface's curvature (base curve) in diopters.

Calibrated for crown glass, index 1.523; readings need correction on other materials.

Lower than the true surface power (it under-reads), requiring correction.

The vertex distance between the back of the lens and the cornea.

A corneal-reflection instrument that measures monocular and binocular PD accurately.

A distant target so the eyes are in parallel (non-converged) alignment.

It removes the influence of differing back-vertex distances between distance and near zones.

Subtract the distance sphere reading from the segment (near) sphere reading.

Center and edge thickness of a lens, in millimeters.

By the displacement of the target center from the lensmeter's reticle center (in Δ).

Both read power/axis/add/prism; the automated unit displays values digitally, reducing operator error.

Marking the optical center and the 180° (horizontal) reference for layout and prism checks.

Measure from the nasal-bridge center to each pupil/corneal reflection separately.

A template of the frame's eyewire shape used by the edger to cut the lens to size.

A higher diopter value (more curvature).

The lensmeter — used to check power, axis, add, prism, and OC against ANSI Z80.1 tolerances.

Use the front-vertex (neutralizing) method: read distance, then seg; subtract.

Vector-combine the vertical and horizontal target displacements (Pythagorean).

Marks the optical center (middle dot) and the horizontal 180° reference.

Singles read one principal meridian (sphere), triples the other (cylinder); axis when both are sharp.

The lens material's actual index vs the 1.523 calibration index.

Millimeters of vertex distance.

Faster, reduces operator accommodation/parallax error, and reads progressive power maps.

Uses the light reflex on the cornea to mark each pupil center for monocular PD.

Binocular PD is pupil-to-pupil total; monocular PD is each pupil's distance from the nose center, measured separately.

Faces are rarely symmetric — it places each OC in front of its pupil to avoid unwanted prism.

Near PD is smaller because the eyes converge for near tasks.

The angle where the bottom of the frame front sits closer to the face than the top.

Lower the optical center ~1 mm for every 2° of pantoscopic tilt.

Unwanted vertical prism and oblique astigmatism.

The horizontal wrap of the frame front so the lenses curve back toward the temples.

At or just below the lower eyelid margin / lower limbus.

At the patient's pupil center in primary gaze, per the manufacturer's layout.

From the lowest point of the lens shape up to the desired seg top (or fitting cross).

From the back surface of the lens to the front of the cornea.

Rectangle (box) — used to standardize frame and decentration measurements.

Horizontal width of the lens box — the eye size.

Vertical height of the lens box.

Distance between lenses — the bridge size (gap between the two lens boxes).

The longest diagonal across the lens box from its geometric center; drives minimum blank size.

52 mm eye size · 18 mm bridge (DBL) · 140 mm temple length.

A + DBL — the frame's mechanical center distance; compare to the patient PD to find decentration.

Decenter each lens inward by half the difference so each OC sits over the pupil.

A properly adjusted frame rests evenly: both temples and both pads/sides contact without rocking.

Lengthen the temple bend / adjust pads so the frame lowers to the proper vertex and pantoscopic angle.

Acetate becomes pliable with heat; adjusting it cold risks cracking the plastic.

Confirm the seg top sits at the marked height relative to the patient's pupil/lower lid.

Skull (most common), library (straight), comfort-cable, and riding-bow temples.

Saddle, keyhole, and adjustable nose-pad bridges.

The patient hits the reading zone too soon and complains the distance field is small.

The patient must depress their gaze excessively or tilt the head to read.

About 8°–12° for everyday wear.

A few degrees for dress eyewear; high-wrap sport frames need power compensation.

Roughly 12–14 mm for standard fittings.

Total in-decentration 8 mm → 4 mm inward per lens.

ED + 2 × (decentration) + a few mm tolerance.

Frame front straight, even temple spread, level fronts, proper pad/temple contact (four-point touch).

Widen/spread the pads so the frame settles lower on the nose.

Add or increase the temple bend behind the ear (earpiece) for grip.

It must reflect the actual fitted pantoscopic tilt and how the frame sits.

Essential — it prevents induced prism by centering each OC over its own pupil.

Off-center OCs induce unwanted (usually base-out or base-in) prism.

Tolerances for prescription (dress) ophthalmic lenses — power, cylinder, axis, add, and prism.

±0.13 D; above 6.50 D it is about ±2%.

Tightens as cylinder rises: ±14° (very low cyl), ±7°, ±5°, ±3°, down to ±2° (cyl >1.50 D).

±0.12 D for adds up to +4.00 D; ±0.18 D above.

Occupational and educational personal-eye-protection (safety eyewear) impact requirements.

The lenses meet the high-velocity (high-impact) requirement of ANSI Z87.1.

21 CFR 801.410 — dress (street-wear) eyeglass lenses must be impact-resistant.

A 5/8-inch (~16 g) steel ball is dropped 50 inches onto the lens, which must not fracture.

Usually no — they are certified impact-resistant by the manufacturer and exempt.

FDA 801.410 = minimum impact for dress eyewear; ANSI Z87.1 = higher-impact occupational safety eyewear.

Prescribers must give the patient a copy of the eyeglass Rx at the end of the exam, free, without being asked.

No — under the FTC Eyeglass Rule the Rx must be released regardless of whether the patient buys from you.

Provides similar Rx-release protections for contact lenses.

Interpret prescriptions and fit/measure/dispense eyewear — not diagnose or treat eye disease.

It requires safeguarding patients' protected health information (PHI) in the optical office.

Yes — some states license/certify opticians and others do not; ABO certification supports licensure where required.

The American Board of Opticianry & National Contact Lens Examiners (ABO-NCLE).

Remaking the lens — it must meet ANSI Z80.1 before it is dispensed.

OSHA requires employers to provide ANSI Z87-compliant eye protection in hazardous workplaces.

Advise UV-blocking lenses/coatings; polycarbonate and Trivex inherently block UV.

About ±0.13 D.

Between the eyes: vertical imbalance ±0.33 Δ, horizontal ±0.67 Δ.

Prescription safety eyewear meeting ANSI Z87.1 (the '-2' denotes Rx).

5/8 inch diameter, about 16 grams (0.56 oz).

50 inches.

The patient, at the completion of the eye exam, automatically and free (FTC Eyeglass Rule).

No — but the prescription itself must be released without conditioning it on a purchase.

Optician dispenses eyewear; optometrist examines/prescribes; ophthalmologist is a medical/surgical eye physician.

Meeting that state's licensure standard for opticians.

HIPAA — protect patients' protected health information.