Magnification

If a low vision Px cannot resolve the retinal image despiteit being optimally focussed, then it is necessary for it to bemade larger.

There are a number of different means via which this can be achieved:

1. Relative Size Magnification

2. Relative distance Magnification

3. Angular Magnification

4. Real Image Magnification

Magnification is relative

It is the ratio comparing the situation before and after some change in the viewing environment.

Magnification is NOT about making objects clearer it is simplyabout making them bigger.

Magnification (M) = new retinal image sizeold retinal image size

d1

h1 NE

h1’

Unmagnified Object

d1

h2

NE

’ h2’

Increasing the Object Size

M = ’

Which for small angles tan ’

M = tan = h2 x d1 = h2

’ d1 x h1h1

M = new object size old object size

Examples include large print books, large TV screens etc.

The magnification is calculated in comparison to a standard size,e.g. if large print books are N24 and standard books are N10, then the relative size magnification provided = 2.4X

Relative Size Magnification

d3

h1 NE

h3’’

Decreasing the Viewing Distance

d1

h1 NE

h1’

Unmagnified Object

Relative Distance Magnification

M = tan = h1 x d1 = d1

’ d3 x h1d3

M = old object distancenew object distance

The simplest way to increase magnification is to decrease the viewing distance (Approach Magnification).

If a Px watched TV at 3m and they move closer to 1m, what is The magnification?

Moving a reading task from 30cm to 5 cm will produce what value of magnification? Any problems with this approach?

Angular Magnification.

d1

h1 NE

h5’’

Optical Devicee.g. Telescope

Angular Magnification

M = angle subtended at the eye by instrument image angle subtended at the eye by object

This is generally refers to the magnification provided by optical instruments (e.g. telescopes).

Versatile means of magnification as it does not involve changingobject or viewing distance.

d1

h4

NE

’ h4’

Real Image or Transverse Magnification

h1

Real Image or Transverse Magnification

M = tan = h4 x d1 = h4

’ d1 x h1h1

M = size of real image size of object

This is typical of the situation used with a CCTV device wherea magnified image of the object is created on a TV screen.

This real image is created in approximately the same location as the object.

Different types of magnification can be used in conjunction with one another.

The total magnification is the product of the two values.

A Px can barely see N12 at a reading distance of 40cms.They now use large print books (N24) at 30 cm.

Relative size magnification = 24/ 12 = 2x

Relative distance magnification = 40/30 = 1.33x

Total Magnification = 2 x 1.33 = 2.66

Combining Magnification

Prescribing Magnification

Even when low vision Pxs are optimally refracted this is notsufficient to improve their vision to required levels for a particulartask.

Therefore some value of magnification is required.

How in practice can the optometrist obtain an appropriate valuefor Magnification?

1. Identify specific tasks and predict the amount of magnification needed.

2. Is binocular or monocular correction preferable?

3. Select an appropriate LVA, if required.

4. Perform a trial for suitability

5. Determine any spectacle correction that might be necessary

6. Loan out device after training.

7. Follow-up visits.

Predicting the magnification required

Magnification required = required VA present VA

In Snellen notation to improve from 6/60 to 6/6

Magnification required = 6 x 60 6 x 6 = 10 x

Watching TV 6/18Reading Bus Numbers 6/6

Magnification used = achieved VA present VA

So achieved VA = magnification x present VA

For example a Px with a VA = 6/36, using a telescope with aMagnification = 4x would achieve a VA of:

4 x 6/36 = 6/9

If VA is measured in a LogMAR notation:

Magnification = (1.25)n

Where n = number of steps

If the present acuity = 0.5 and the required acuity = 0.1

Then Magnification = (1.25)4 = 2.44x

Task N-notation (40 cm)

Medicine bottle labels Telephone Directories Small column newsprint Typewritten books (9-12 yrs) Computer display Typewritten books (7-8 yrs) Large print books Newspaper headlines

3

6

8

13

16

20

24

65

N - notation

N print uses New Times Roman font and is the standard UK test.

It has a linear scale:

N10 is 2x the size of N5

Magnification required = present VA required VA

N48 N6

M = 8x

A measurement of near VA should always be accompaniedby the working distance at which it is taken.

Near Magnification.

In theory it should be possible to calculate what letter size aPx will be able to read at near from a knowledge of their distance VA.

However, reading is not equivalent to single letter identification.

The values used to determine magnification for near tasks are based on threshold values.

Therefore we have to incorporate a READING RESERVE intoour calculations so that comfortable reading is possible for longer periods.

This is done by halving the size of the required N value beforedetermining the magnification.

Similarly one can double the size of the magnification is you work with threshold values.

The problem with this approach is that it doubles the amount ofmagnification which has consequences for the field of view.

In practice what often happens is that a magnification 1.5 times that based on threshold values is used.

Example:

65-year-old patient with distance spectacles of -1.00DS both eyes and reading spectacles of +4.00 DS both eyes.

With their reading spectacles, they see N24, and they want to read the newspaper.

N8 for newspaper at threshold

N4 for reading reserve,

Mag. required = present VA/ VA wanted = N24/N4 = 6X

Practical use 6/1.5 = 4X

(N6 is adequate for comfortable reading of newsprint).

Binocular v Monocular Magnification

If the acuities of the two eyes are similar then binocular viewingis beneficial to the Px.

However, the design of the magnifier or the limitations that it places on viewing conditions may only allow monocular correction.

Selection of an LVA

Once a value of the magnification required has been predicted thetask for the practitioner is to find the best way for the Px to obtainthis value.

Remember that we have four options at our disposal:

1. Make the object bigger

2. Decrease the working distance.

3. Increase the Angular magnification with an optical device.

4. Real image magnification.

Type of Magnification

Field of

View

Working

Space

Distance of

Task Increase size Decrease distance (by +ve lens magnifiers) Real image Angular (Telescope)

No change

No change Decreased

Decreased

Decreased

No change

Decreased Decreased

No change

No change

Usually N only

D, I or N Near only

Usually N only

D, I or N

Distance Magnification

The only practically useful aid is the telescope.

In some cases it may be possible to move an object closer(e.g. moving closer to the TV).

Real image magnification is rare beyond arm’s length.

Near Magnification

The most common magnifier for near tasks is the the +ve lenswhich can be used in one of its various guises:

spectacle microscopeshand held magnifiers stand magnifiers

The object can be made larger (relative size magnification) but thistypically restricts magnification to a maximum of 2x.

Real image magnification is possible with CCTV.

Telescopes for near (telemicroscopes) can be very useful particularly if the Px wishes to perform manipulative tasks.

Trial and Modification of Predicted Magnification

Record acuity

Manner of reading:

e.g. ‘fast’ ‘too slow’ ‘one letter at a time’ etc.

Spectacle Correction

For near vision magnifiers the general rule is that for:

hand held magnifiers distance Rx

stand magnifiers reading Rx

In the case of spectacle microscope the Px distance Rx will need to be taken into consideration.

e.g. A –4.00DS myope requires 3x magnification which is suppliedby a +12.00DS lens.

The final lens = +8.00DS

Spectacle Correction (cont’d)

Distance telescopes can be used in conjunction with the distance Rx.

However, the largest field of view is obtained when the exit pupilof the telescope is as close as possible to the entrance pupil of the eye.

Training the Px How to Use the LVA

The Px should receive good task related training on the use of the LVA.

Both written and verbal instruction is advisable,

Be clear about which spectacles will be needed in conjunction with The LVA.

Be clear about the working distance that should be employed.

Should RE, LE or both eyes be used.

Emphasize the importance of good illumination.

Follow Up Visits.

Typically you would see a Low vision Px about 2-3 weeks after the initial supply of the LVA.

Most common reasons for problems:

VA has deteriorated (pathology still active)

LVA being used sub-optimally.

LVA is being used for a task other than that for whichIt was initially intended.