5.3 - Spectacle magnification

Question 1

What lens is needed to correct myopia

Answer 1

Negative sp lens

Question 2

What happens in Spectacle Lens Correction of Myopia

Answer 2

• Start from infinitrly distant object = parallel rays of light coming in
• Spectacle lens then forms image at 2nd focal point fsp’ – object for eye
• Here, image produce by sp lens subtends angle omega at sp lens
• Ray incident on eye, it subtends a smaller angle at eye – u = image h2’ ( retinal image ) is reduced in comparison to same object h1’ subtended at omega at eye befoe going through sp lens =

Question 3

What does spectacle correction affect

Answer 3

Magnification at final retinal image

Question 4

What does sp lens do in myopic lens

Answer 4

Minification - reduce angle subtended at eye

Question 5

Is retinal image smaller or bigger in spectacle lens correction of myopia than uncorrected eye and how

Answer 5

Smaller

The angle subtended by h1’ at the eye, u, is less than the angle subtended at negative sp lens

Question 6

What happens when no sp lens in place

Answer 6

It is uncorrected eye

Question 7

What does Fe/K act as

Answer 7

Correction factor

Question 8

What is spectacle magnification known as

Answer 8

The ratio h2’ / hb’

Question 9

What is spectacle magnification formula

Answer 9

SM = 1 + dK

Question 10

What happens to retinal image in spectacle corrected myopes

Answer 10

Retinal image is smaller

Question 11

What happens to retinal image in spectacle corrected hypermetropes

Answer 11

Retinal image is larger

Question 12

Why is retinal image in spectacle corrected myopes smaller

Answer 12

• Product d i.e vertex distance which is always positive x K i.e ocular refraction which is negative in myopia
• Sp magnification is less than 1 - Minifcation

Question 13

Why is retinal image in spectacle corrected hypermetropes larger

Answer 13

• Value of k is positive, value of vertex distance d is positive
• Spectacle mag is greater than 1
• Magnifcation

Question 14

What happens in contact lens wearers in terms of mag

Answer 14

No mag effect

Question 15

Why is there no mag effect for contact lens wearers

Answer 15

Vertex distance is 0

Question 16

What is other spectacle magnification formula

Answer 16

SM = K/Fsp

Question 17

Do you need higher of lower power for myopic correction in sp plane than eye

Answer 17

Higher

-Denominator bigger than numerator – sp mag of 1

Question 18

Do you need higher of lower power for hypermetropic correction in sp plane than eye

Answer 18

Less positive power

Numerator larger than denominator = sp mag larger

Question 19

What is consequence of spectacle mag in myopia

Answer 19

Spectacle correction of myopes produces minification of what looking at; objects appear slightly smaller and darker possibly leading to patient preference during subjective refinement

Question 20

What is consequence of spectacle mag in hypermetropia

Answer 20

Spectacle correction of hypermetropes makes objects slightly larger including their eyes – less problem – more aware of blur but read just as much – eyes magnified

Question 21

What is consequence of spectacle mag in contact lens wearers

Answer 21

Nothing - differences in visual performance