4.1 - Magnification

Question 1

Why consider image size?

Answer 1

• Leads to important results about blur = understand refractive error
• Gives an insight into the magnification produced by different refractive corrections (spectacles, contact lenses, refractive surgery) – don’t all produce same level of magnification
• Has direct clinical relevance to aniseikonia

Question 2

What is transverse magnification

Answer 2

• Ratio of image height to object height ( h )

- Gives a relative size of how big image ( h’ ) is compared to object

Question 3

What is equation for transverse magnification - for finite object

Answer 3

M = L/L' 
M = h'/h

Question 4

What can the equation for transverse magnification be applied to

Answer 4

Any single surface or thin lens ( one surface where refraction takes place )

Question 5

Why is transverse magnification called transverse

Answer 5

As its measuring image and object heights across the optical axis i.e. vertical heights

Question 6

What values do we need to find out mag and image height

Answer 6

L and L’

Question 7

How to work out retinal image height

Answer 7

1. Object location l: L = n/l SO l = n/L
2. Refraction at eye ( image vergence ): L’ = L + Fe
3. Magnification: M = L/L’
4. Retinal image height: h’ = M X h

Question 8

What is the power of the eye

Answer 8

+60D

Question 9

Is eye low or high power

Answer 9

High

Question 10

What is magnification for real objects forming real images on or near the retina

Answer 10

• Negative therefore image on back of eye from real object is inverted
• Less than 1 cause power is high and positive = retinal image is much smaller than the object looking at

Question 11

What does image on retina have to be for it to produce a response

Answer 11

Real

Question 12

What happens in multiple surface/thin lenses

Answer 12

Image formed by one surface acts as object for the next ….
-If we can work out what the 1st lens does in terms of image it produces we take that info/image and use it as the object for the next surface and keep going through the surfaces all the way through the system

Question 13

What happens when sp lens in front of reduced eye

Answer 13

Have 2 surfaces to take into account

Question 14

How do you work out magnification for finite object for multiple surfaces i.e sp lens before eye

Answer 14

1. Object location l1: L = n/l SO l1 = n/L
2. Refraction at lens ( image vergence ): L’1 = L1 + Fsp
3. Step along i.e. vergence at eye: L2 = L1’/(1 − dL1’)
4. Refraction at the eye: L2’ = L2 + Fe
5. Magnification: M = L1/L1’ X L2/L2’

Question 15

What is finite object transverse magnification equation for multiple surfaces

Answer 15

M = L1/L1’ X L2/L2’