Visual Impairment 3- Near LVAs

Question 1

Where should an object be placed for parallel lights to emerge?

Answer 1

Anterior focal length of lens

Question 2

What is the definition of a magnifier?

Answer 2

A plus lns allowing px to obtain increased retinal image size by holding objects closer to eye without extra demands on accommodation

Question 3

How would a hand held mag be calculated?

Answer 3

Mag= Feq/4

Question 4

What happens if the is not at anterior focal point?

Answer 4

Distance closer= not parallel, need reading add

Dist further= rays converging, image blurred

Question 5

Why does dist between object and mag matter?

Answer 5

If there’s no dist= magnification is minimum

Dist increases= magnification increases

Question 6

Why does dist between mag and pxs eye matter?

Answer 6

Effects the FOV

Relative distance mag needs to decrease as mag and eye dist increases , otherwise no effect on mag

Question 7

How can we maximise the FOV and factors would reduce the FOV?

Answer 7

Increase FOV= Large diameter

Decrease FOV= aberrations and high powers

Question 8

What type of lens may give a increase d field of view with minimal peripheral aberrations?

Answer 8

Aspheric lenses

Question 9

Whar is the linear field calculation?

Answer 9

FOV= D/ d (dist b/w mag and cornea) x Feq

Question 10

How is FOV increased in a hand held mag?

Answer 10

Move closer to lens

Question 11

What image is created at the focal point?

Answer 11

Virtual imagev

Question 12

What is required to help magnify an image if the object is closer than focal length?

Answer 12

Near add

Question 13

When is illumination added to a mag?

Answer 13

If the power increases= the lens diameter decreases= the focal length decreases—– illumination added

Question 14

How is a clear image produced in a stand magnifier news why?

Answer 14

The focal length is less than the object dist to len

Better quality image and less aberrations

Question 15

What are the 2 components to a stand magnifier?

Answer 15

Magnifier

Reading add lens

Question 16

What is the calculation to the 2 component system?

Answer 16

Feq= FM + FA - zFM . FA

Question 17

What are the advantages of using a stand magnifier?

Answer 17

Accurate WD
Hands free
Internal illumination
Prolog reading

Question 18

What are the disadvantages to a stand magnifier?

Answer 18

Special reading specs required
Prevents access
Flat surface
More bulky

Question 19

When will the FOV be effected?

Answer 19

FOV will reduce as px moves AWAY from the lens

Question 20

What mag calculation is used in a stand magnifier?

Answer 20

Mag= Fequiv/4

Question 21

What ways can the mag be increased in a stand mag?

Answer 21

Increase reading add

Decrease separation between Reading add and magn

Question 22

What is the starting point reading add for a spectacle microscope to become considered an LVA and why?

Answer 22

+4.00

Why?… standard add as 1/4= 25cm

Question 23

What are the different types of spectacle magnifier?

Answer 23

SV readers
Hyperoculars (12x)
Prismatic half eyes (+12.00D)
Bifocal units 
Clip to lenses (4x)
Head band magnifier (3x)

Question 24

What is the mag calculation used for a spectacle mag?

Answer 24

Mag= Fnew/4

Question 25

What are the advantages of a spectacle microscope?

Answer 25

Hands free FOV max as lens close to eye Similar to conventional specs

Question 26

What are the disadvantages of spectacle microscope?

Answer 26

``` Reducing WD- writing/sewing Postural discomfort Illumination difficult Convergence demands More head movement ```

Question 27

What 3 categories are spectacle mags subdivided into?

Answer 27

High read additions Simple spec mag Compound spec mag

Question 28

What powers are typically found in high reading adds specs?

Answer 28

>+3.00-

Question 29

What mags are typically seen in a simple spec mag?

Answer 29

2x- 10x

Question 30

When would a compound spectacle magnifier be useful?

Answer 30

When >10x mag required

Question 31

What does a compound spec look like?

Answer 31

2 lenses cemented together | Can be mistaken for telescope

Question 32

What is the problem with binocular spec magnifiers?

Answer 32

Close WD demand too much convergence

Question 33

If the convergence demand is too much, what rule of thumb is used?

Answer 33

Every 1D in reading add= 1 prismD added to each eye

Question 34

At what power and mag does it become difficult to achieve good BV?

Answer 34

>+8.00 (2x)

Question 35

What happens to one of the lenses in spec mag if only one lens out of the 2 is being used?

Answer 35

A balance, frosted, chavasse or semi opaque tape is used over the lens

Question 36

Which way does a bar magnifier magnify an image?

Answer 36

Vz not hz

Question 37

Which way does a flat field magnifier magnify an image?

Answer 37

All meridians

Question 38

What are the advantages of a bar/flat field magnifier

Answer 38

Self support Inconspicuous Light gathering properties Binocular viewing and normal WD

Question 39

What are the disadvantages of a bar/flat field magnifier

Answer 39

No access underneath | Flat surface

Question 40

What do manufacturers give magnification based on?

Answer 40

F/4+1

Question 41

What are the advantages of a hand magnifier?

Answer 41

``` Discrete Light Easy to use Inexpensive Convenience Portable and compact Used c dist rx Internal illumination ```

Question 42

What are the disadvantages of a hand magnifier?

Answer 42

``` Handling- occupy a hand and steady hand required Restrictive field of view Can case shadows Become tired Shorter/spotting tasks ```