1.03 flat and meniscus forms

Question 1

what does flat form mean?

Answer 1

• 2 centres of curvature are on opposite sides of the lens
• does not mean lens must have plano surface

Question 2

what are the 3 different flat forms we can have of a positive lens?

Answer 2

1. equiconvex: f1 + f2 are same, power is equally split between 2 surfaces, e.g +2 on both, so lens is +4 DS
2. biconvex: both surfaces are convex but not of equal power, e.g +3 and +1 or +1.75 and +2.25
3. plano convex: all power on F1 , F2 has 0 power

Question 3

another form which is not a flat form?

Answer 3

a curved or meniscus form

Question 4

curved or meniscus forms

Answer 4

• convex and concave surface
• or 2 centres of curvature are on same side of lens
• e.g F1 is +6.50 and F2 is -2.50

Question 5

if one surface of a lens is -4.00 and so is the other, what kind of flat form is this?

Answer 5

• equiconcave
• -8.00 DS lens

Question 6

if one surface is -6.25 and the other is -1.75, what kind of flat form is this?

Answer 6

• biconcave
• -8.00 DS lens

Question 7

if the front surface is plano and the back surface is -8.00 what kind of flat form is this?

Answer 7

plano concave

Question 8

base curve

Answer 8

• different meniscus form differ in their base curves
• which is the LOWEST numerical surface power
• used as the reference surface

Question 9

some forms with particular base curves have what?

Answer 9

special names: periscopic form is + or - 1.25

Question 10

what kinda lenses are chosen for modern prescription spectacles?

Answer 10

meniscus

Question 11

why do we choose meniscus forms?

Answer 11

• all lenses suffer aberrations, meniscus forms minimise these
• different manufacturers make their own calculations depending on what aberrations they consider most significant
• best form = any of the possible alternatives

Question 12

lens positioning: if lens is not equiconcave or convex which side is placed towards the eye?

Answer 12

• for both plus and minus the rule is:
• MOST CONVEX / LEAST CONCAVE AT FRONT
• most concave at back towards the eye
• optimised image quality and reduces aberrations