Magnification

Question 1

Case examples on how to predict magnification

Answer 1

• pt with AMD presents with 20/100 BCVA OD, OS after refraction
• use kestenbaums rule (100/20) to start with a +5.00D lens for MNRead
• pt has critical print size of 2M with +5.00 add over refraction at 20cm
• use +10D add at 10cm and retest MNRead
• pt is able to read 1M fluently
• can proceed to use devices with equivalent power of +10D (microscope, magnifiers, etc)

Question 2

The apparent enlargement of an object by an optical instrument that is the ratio of the dimensions of an image formed by the instrument to the corresponding dimensions of the object-also called power

Answer 2

Magnification

Question 3

Types of magnification

Answer 3

Relative distance magnification
Relative size magnification
Angular magnification
Electronic magnification

Question 4

If an object is closer, it appears relatively larger, because it has greater angular subtense even though its size is unchanged

Answer 4

Relative distance magnification

Question 5

Relative distance magnification=

Answer 5

R/d (old distance/new distance)

Question 6

RDM: an object moved into half the original distance will appear ____ as large

Answer 6

Twice

Question 7

RDM: the magnification produced related to the change in viewing distance how?

Answer 7

Inversely proportional

Question 8

A patient reports that they are able to easily read the weather reports on TV since rearranging the living room furniture. The TV was 12 feet from their favorite chair, but is not only 4 feet away. How much RDM does the new arrangement provide

Answer 8

3x (double check)

Question 9

Relative size magnification

Answer 9

Making something bigger so that it can seen

• large print books
• large print check books
• bold face watches

Question 10

Relative size magnification=

Answer 10

New size/reference size

Question 11

Print in large print version of The NY Times measures 4.5mm tall for a lower case “e”. The regular print version measures 1.5mm tall. How much RSM is provided by the large print version

Answer 11

3x (double check)

Question 12

What is the M size of the large print NY time letter (4.5mm tall lower case letters) version?

Answer 12

4.5M (double check)

Question 13

Results from apparent increase in angular subtense

Answer 13

Angular magnification

Question 14

What is not changed in angular magnification

Answer 14

Neither the objects size nor viewing distance is changed

Angular subtense of light rays is changed by an optical system

Question 15

Maximum angular magnification

Answer 15

M=1 + hF

M=magnification relative to the objective viewed without the lens
h=distance from lens to eye (in meters)
F=dioptric strength of lens

For this relationship to hold true, the lens must be one focal length from the object

Question 16

What is the MAM for an object 40cm from the eye using a +10D lens?

Answer 16

1 + .4(+10)

5x

Question 17

Total magnification

Answer 17

M(t)=RDMxMAM
M(t)=(r/d)x(1+hf)

Simplified

M(t)=rf

Mrf Mrf

R=old distance
f= dioptric power of lens

Question 18

What is the total magnification if we measured out reference acuity at 40cm, and now we use a +10D lens at its focal length to view the same object at 20cm?

Answer 18

M=rF
M=.4x10
M=4x

Question 19

Electronic magnification

Answer 19

-image of an object is electronically enlarged (CCTV-consists of a monitor and a camera)

• magnification altered with camera lens
• contrast and brightness can be controlled

Question 20

Magnification from multiple sources

Answer 20

Is multiplicative

M(t)=M1xM2xM3X….

Question 21

You patient replaces their 26” TV with a 52” screen AND moves their seat from 9 feet to 3 feet away. How much magnification did they achieve?

Answer 21

52/26=2X
9/3=3x
2x3=6x

Question 22

Labeled magnification

Answer 22

• aka Rated Magnification
• determines using an arbitrary reference distance chosen by the manufacturer (typically but not always, 25cm)
• labeled magnification=f*0.25=F/4

-because labeled magnification is somewhere arbitrary, we don’t use it. We use dioptric power

Question 23

What is the expected power of a magnifier labeled 4x?

Answer 23

+1D

4/4=1

Question 24

Some additional concerns about magnification

Answer 24

• magnification is not clarification
• magnification does not improve contrast
• magnification does not eliminate scotomas
• determining the predicted magnification only gives you a starting point. Not all patients will meet their goals using the predicted magnification

Question 25

Determining needed magnification

Answer 25

- first, patients refractive error MUST be corrected - magnification needed is relative to their reference acuity Magnification required=reference size/goal size Provides a starting point

Question 26

Choosing goal size for magnification

Answer 26

Desired task - continuous text reading - spot reading - distance viewing Some typical goals are - reading newsprint (1M) - general distance viewing (20/40)

Question 27

Choosing your reference size for goal magnification

Answer 27

- if the goal is reading, the reference size should be determined using a reading task - if goal is fluency, do not choose your patients threshold ability to read, choose their critical print size (CPS) - if goal is reading medicine labels, will probably need a smaller goal than 1M

Question 28

Predicted magnicaition

Answer 28

M=what you got/what you need Mrf

Question 29

Your patient reads 3M and want to read the newspaper (1M). How much magnification do you predict they’ll need?

Answer 29

3x 3/1 (what you’ve got/what you need)

Question 30

Your patient is 20/300 OD, NLP OS. How much magnification would you predict would be needed to see 2/60

Answer 30

300/60=5x What you’ve got/what you need

Question 31

Equivalent power

Answer 31

- were going to learn the basic optics of several low vision devices - in order to compare between near devices well use the concept of equivalent power - the basic premise if that magnification can be boiled down to one simple dioptric power and then we can use our knowledge of devices to determine how to achieve that power

Question 32

Magnification formula for hand held lenses and those in the spectacle plane

Answer 32

M=rF We’ve already shown how lens to eye distance is considered in this formula

Question 33

Stand magnifier equivalent power

Answer 33

-F(ET)=F2E F2=power of add E=enlargement factor of the magnifier

Question 34

Telemicroscope equivalent power

Answer 34

-F(eq)=F(2)M(ts) F2=power of add M(ts)=mag of telescope?

Question 35

In clinic for near device determination we regularly use

Answer 35

-What we got/what we need -a version of M=rF —take the magnication needed and uses it to find out much closer the patient needs to hold reading material -(power of lens)=Add x (mag of device)