Magnification Flashcards
Case examples on how to predict magnification
- 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)
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
Magnification
Types of magnification
Relative distance magnification
Relative size magnification
Angular magnification
Electronic magnification
If an object is closer, it appears relatively larger, because it has greater angular subtense even though its size is unchanged
Relative distance magnification
Relative distance magnification=
R/d (old distance/new distance)
RDM: an object moved into half the original distance will appear ____ as large
Twice
RDM: the magnification produced related to the change in viewing distance how?
Inversely proportional
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
3x (double check)
Relative size magnification
Making something bigger so that it can seen
- large print books
- large print check books
- bold face watches
Relative size magnification=
New size/reference size
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
3x (double check)
What is the M size of the large print NY time letter (4.5mm tall lower case letters) version?
4.5M (double check)
Results from apparent increase in angular subtense
Angular magnification
What is not changed in angular magnification
Neither the objects size nor viewing distance is changed
Angular subtense of light rays is changed by an optical system
Maximum angular magnification
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
What is the MAM for an object 40cm from the eye using a +10D lens?
1 + .4(+10)
5x
Total magnification
M(t)=RDMxMAM
M(t)=(r/d)x(1+hf)
Simplified
M(t)=rf
Mrf Mrf
R=old distance
f= dioptric power of lens
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?
M=rF
M=.4x10
M=4x
Electronic magnification
-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
Magnification from multiple sources
Is multiplicative
M(t)=M1xM2xM3X….
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?
52/26=2X
9/3=3x
2x3=6x
Labeled magnification
- 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
What is the expected power of a magnifier labeled 4x?
+1D
4/4=1
Some additional concerns about magnification
- 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
Determining needed magnification
- 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
Choosing goal size for magnification
Desired task
- continuous text reading
- spot reading
- distance viewing
Some typical goals are
- reading newsprint (1M)
- general distance viewing (20/40)
Choosing your reference size for goal magnification
- 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
Predicted magnicaition
M=what you got/what you need
Mrf
Your patient reads 3M and want to read the newspaper (1M). How much magnification do you predict they’ll need?
3x
3/1 (what you’ve got/what you need)
Your patient is 20/300 OD, NLP OS. How much magnification would you predict would be needed to see 2/60
300/60=5x
What you’ve got/what you need
Equivalent power
- 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
Magnification formula for hand held lenses and those in the spectacle plane
M=rF
We’ve already shown how lens to eye distance is considered in this formula
Stand magnifier equivalent power
-F(ET)=F2E
F2=power of add
E=enlargement factor of the magnifier
Telemicroscope equivalent power
-F(eq)=F(2)M(ts)
F2=power of add
M(ts)=mag of telescope?
In clinic for near device determination we regularly use
-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)
