hand and stand magnifiers

Question 1

equivalent viewing power:

Answer 1

another term for magnification
magnification depends on how the lens is used
calculated using diff component of optical system

Question 2

equivalent viewing distance:

Answer 2

reciprocal of equivalent viewing power

Question 3

hand magnifiers

Answer 3

low cost
easy to use/ maintain
portable
can have some internal illumination 
large variety in cost and quality, doesn't need professional consultation, can be used in combination with rx, good for variable head psotions

disad- use hand, wot if parkinsons? text has to be on flat surfaces. hand gets tired, lack of patience, has to be at correct position

Question 4

how do u work put enlargement for a bar/flat field magnifier-

Answer 4

nr/ (t(1-n)+nr)

Question 5

bar and flat field magnifiers

Answer 5

lens acts as a collector of light- lens that lives bar field is diverging
provide real enlargement

Question 6

whats the aim of low vision

Answer 6

MAGNIFICATION

aka image enlargement

Question 7

what are the four main types of image enlargement

Answer 7

1. relative size enlargement
2. relative distance enlargement
3. electronic-transverse- enlargement
4. angular enlargement

Question 8

whats relative distnace enlargement

Answer 8

half wd
then u double retinal image size
if hey cannot accommodate- then u may need a add

when starting any low vision assessment- get a REFERENCE point-

Question 9

whats the reference point in a low vision assessment

Answer 9

what can the px read at 25cm with a +4.00 add

Question 10

how do u work out relative distance enlargement

Answer 10

u figure out what they can read at 25cm using a +4.00D add
then u divide this by what they want to read
this gives u the ENLARGEMENT RATIO
the new WD is now the old WD/ENLARGEMENT RATIO
new MAG power- 1/new WD * 100

Question 11

what’s acuity reserve

Answer 11

when in consulting room u have optimal conditions, with easy to read text, and they read for a short period of time
therefore u assume an acuity reserve of 2:1/ 3:1

Question 12

Whats relative size enlargement

Answer 12

increase the retinal image size
by keeping wd same
u enlarge size of print
u work out ENLARGEMENT= new print size/ old print size

Question 13

you may need to combine relative size and relative distance enlargement :

Answer 13

as a lower powered lens- better image quality an digger field of view

Question 14

electronic enlargement

Answer 14

use camera? to introduce colour, improve contrast, or increase size
electronic enlargement- image on screen/ original image

adv- can get high levels of mag- more comfortable WD- can underline-maintains binocular view- looks good- larger fov
but well expensive,lack of portability? disadvantaged by quality of camera and screen

Question 15

what’s angular magnification

Answer 15

use of telescopes
d and N
for N u need a lens cap
ENLARGEMENT- angle subtended through optical system/ angle subtended when viewed directly

Question 16

whats the max and min wd

Answer 16

max is focal length of lens

min depends on accom

Question 17

each lens has 2 functions; i.e.

Answer 17

focusing power

magnification power

Question 18

whaats nominal mag

Answer 18

F/4

Question 19

if px is wearing glasses/ using accom with magnifier. u need to use equivalent power formula

Answer 19

Fe= Fm + Fa - (dFmFa)

d- distance between eye and magnifier

Question 20

what mag is labelled on a magnifier

Answer 20

trade

Question 21

what do u assume for trade mag

Answer 21

the px is using +4.00 add or +4.00 accomm
and also magnifier is held up to the eye

easy way to work out is fe=fm/4+1

Question 22

The enlargement of a simple magnifier will remain the same, regardless of reading add/accommodation, if

Answer 22

the lens is held at its focal length away from the eye

Question 23

Holding the magnifier closer than its focal length

Answer 23

INCREASES the equivalent power

Question 24

Holding the magnifier further away than its focal length

Answer 24

DECREASES the equivalent power

Question 25

take into account FOV

Answer 25

w=y / (Fm x z)

where y- lens diamtwe
z- eye to magnifier
fm- power of mag

Question 26

stand magnifiers

Answer 26

illuminated/
placed on object- so constant distance
can have a tilted magnifier, better for those who don’t want to hold, high range of mags
however heavy

not as portable
need to be flat surface
need correct specs
not the best looking

Question 27

stand magnifiers

Answer 27

Fixed focus: object- lens distance is fixed
The object- lens distance is always less than the focal length of the lens
This means that light leaving the lens (towards the eye) is DIVERGENT
Reading glasses or accommodation are needed to focus the light


Question 28

the equivalent power of a stand mag is

Answer 28

less than th potential mag of the lens alone

compared with nominal mag

Question 29

If the image is improved when the patient brings the magnifier closer

Answer 29

the reading add needs to be increased

If the image is improved when the patient takes the magnifier further away, the reading add needs to be decreased

Question 30

spectacle magnifiers

Answer 30

can either be binocular/ monocular
specs with ugh +
if binocular- increased convergence needed to avoid diplopia- one base in for each dioptre add
improved fov, don;t need hands, can use a pen at the same time, cheap

however after +12.00 it’s diff to maintain binocularity
if more enlargement u need a telescope. need to be trained how to use them tho, close wd, hard to adapt, less depth of focus