D: Magnification Effects - Week 4 (just read notes instead)

Question 1

Define Spectacle Magnification. How are myopes and hyperopes affected when corrected?

Answer 1

Change in magnification of what you see due to spectacles.

Corrected myopes: see smaller image
Corrected hyperopes: see larger image

Question 2

Define Eye Magnification. How are Myopes and hyperopes affected when corrected?

Answer 2

Is the magnification or minification of the spectacle wearer’s eyes, when someone looks at them.

Corrected myopes: eyes look smaller
Corrected hyperopes: eyes look larger

Question 3

How can we define specular magnification in regards to retinal image size? How does this correspond to angular image size?

Answer 3

SM = retinal image size seen through lens/retinal image size for uncorrected eye

This is proportional to angular size, therefore:

SM = theta’/theta

**theta’ – angular size corrected, theta — original angular size

Question 4

What are the formulas for angular size before and angular size after?

Answer 4

theta =eta’F (b/c eta’ = theta/F)

theta’ = eta’/(f-de)

**note: de is a negative value so f - de ends up being a sum for total distance to eye

Question 5

How do we overcome the fact we can’t physically measure the distance all the way to the entrance pupil?

Answer 5

We use vertex distance (distance to front surface of cornea) + 3mm

(because we assume that the entrance pupil is 3mm behind the cornea)

Question 6

Knowing the relationship between the theta and eta of corrected and ammetropic uncorrected vision, what is the new formula for Spectacle Magnification (for a thin lens)?

Answer 6

SM = theta’/theta
= [eta’/(f-de)]/eta’F

note: f = 1/F, so:

SM = 1/(1-deF)

Question 7

In the spectacle magnification formula, what do the following symbols represent?

• de
• F

Answer 7

de = distance b/w 2nd principle plane of the lens and the entrance pupil fo the eye = vertex dist. + 3mm

F = equivalent power = BVP for a thin lens

Question 8

When calculating spectacle magnification for a thick lens, what do we have to now consider?

Answer 8

Shape factor.

SM = shape factor x power factor

Shape factor = 1/1-(t/n)F1

Question 9

When do the following things cause spectacle magnification to increase?

• lens thickness
• refractive index
• front surface power
• vertex distance
• back vertex power

Answer 9

SM increases when,

• lens thickness increases
• refractive index decreases
• front surface power increases
• vertex distance increases (for +ve lenses) or decreases (for -ve lenses)
• back vertex power increases (becomes more positive)

Question 10

Reminder: This lecture is not the most suitable for flash cards because of all the maths and stuff. So don’t worry too much about these flash cards. Just read the notes for this one.

Answer 10

Yeah. That’s enough.