spectacle magnification 5.3

Question 1

What happens to retinal image size with correction?- of a myope

Answer 1

• Starting from object produced by spectacle lens- start from infinitely distant oibject
• Spectacle lens forms an image at 2nd focal point - which acts a s an object for the eye
• Image produced by spectacle lens subtends at angle omega at sp lens
• If you look at a ray at incident of the eye which subtends at a smaller angle
• which tells us the retinal image is reduced / smaller than in the uncorrected eye.
• Spectacle correction has an effect on the magnification of the final retinal image.

Question 2

What is the retinal image size in corrected ametropia compared to basic blurred image size?

Answer 2

• the ratio between height of retinal image in corrected eye (h2’)to height of the blurred retinal image (hb’)
• h’2/h’b= 1+dK - this tells us spectacle magnification

Question 3

What is the spectacle magnification equation?

Answer 3

SM= 1+dK

-The ratio h2’/hb’ is known as the spectacle magnification.

Question 4

What is the SM for a myope?

Answer 4

• Vertex distance (d) is positive
• Ocular refraction (K) is negative
• therefore it will be (NEGATIVE) - less than 1 which is minification
• Retinal image is smalller for spectacle corrected myopes

Question 5

What is the SM for a hypermetrope?

Answer 5

• Retinal image is larger for spectacle corrected hypermetropes.
• K is positive
• d is positve
• SM is positive- value greater than 1 - therefore magnified

Question 6

What is the SM for contact lens wearers?

Answer 6

• d is =0
  -no magnification effects for contact lens wearers
  SM doesn’t change

Question 7

What is another version of the SM equation?

Answer 7

-SM= K/Fsp

explained- (for a myopic correction we need a higher power in spectacle plane than we do at the eye
-therefore denominator bigger than numerator therefore get a negative SM)

Question 8

What are the consequences of spectacle correction ?

Answer 8

• Spectacle correction of myopes produces minification ; objects appear slightly smaller and darker possibly leading to patient preference during subjective refinement
• Spectacle correction of hypermetropes makes objects slightly larger including their eyes.
• not for cL wearers

Question 9

What does SM demonstrate?

Answer 9

-minification for spectacle corrected myopes and magnification for spectacle corrected hypermetropes