Physical Characteristics of Lenses (2.1)

How does a small radius affect curvature?

The smaller the radius the larger the curvature

a completely flat lens has what kind of prescription?

plano

More curvature a lens has means what for its power?

the more power it has

What does a lens clock measure? (direct and indirect)

directly measures sag depth

indirectly measures lens power

What is sagittal depth?

distance between a point on a circle and the midpoint of a chord on a circle

Lens clock measures sag depth to measure what of the lens?

curvature of the Lens

When can you use the approximate sag formula?

for spec lenses

What is a limitation of lens clock?

Can only be calibrated to one index of refraction (usually 1.53)

If the index of refraction of the lens clock>index of refraction of true lens......?

Power of lens clock>True power of Lens

(lens clock will overestimate the power of the lens)

If the index of refraction of the lens clock

Power of the lens clock < True power of the lens

(lens clock will underestimate the power of the lens)

What are convex/plano, Plano/concave, and biconvex/biconcave lenses not really used anymore?

bad optics/distortions and aberrations in periphery

What is an equiconvex lens?

Lens whos front and back surfaces are both convex of equal power

What is a periscopic lens?

One surface power is +/-1.25D

What is a meniscus lens?

lens with a convex front surface and concave back surface

What is a modern sph lens form?

meniscus lenses

What is a lens form?

relationship between the front and back surface curvatures

Examples of cylindrical lens forms

plano-cyl

toric lens

bi-toric lens

What is Plano-cyl lens form?

one surface is flat/plano, other is cyl

What is a toric lens?

one surface is toric, other spherical

What is a bitoric lens?

both surfaces are toric

When are toric lenses used?

almost for all SV spec lenses

When are bitoric lenses used?

RPG's (not really sued in spec lenses)

Which type of lens form are minus/plus-cyl lens form options for?

toric lens form

What is a minus-cyl lens form?

toric lens where the front surface is (+) sphere and the back surface is (-) toric

Majority of toric lenses have what lens form?

minus cyl

What is plus-cal lens form?

toric lens where the front surface is (+) toric and the back surface is (-) sphere

How do manufactures decide which combo of front and back surface powers for a meniscus lens?

try to maximize:

-optical quality

-cosmetic appearance

What is a rough blank?

neither side of lens is ground or polished

What is a semi-finished blank?

one lens surface e is ground + polished

What is an uncut lens?

both surfaces ground + polished, but lens has not been edged

What is base curve (for manufactures)?

standard curvature grind onto one lens surface by manufacturer for a group of lens powers

What is base curve for semi-finished sphere blanks?

Front surface power

What is base curve for semi-finished minus-cyl lens form blanks?

steeper meridian on front surface (sphere curve)

What is the base curve for semi-unfinished plus-cal lens form blanks?

flatter front surface meridian

What is the steeper meridian called on plus-cal lens form lenses?

cross curve

What is the advantage and disadvantage of choosing a smaller base curve?

Advantage: lens is flatter/thinner

Disadvantage: poorer image quality/more aberrations

Choosing the correct base curve is especially important for what kind of lenses/aberrations?

oblique astigmatism

curvature of image

Changing the base curve of a patient's lenses will cause what?

change in magnification (esp in hyperopes)