5.2 - Blurred image height

Question 1

What is h’

Answer 1

Image size at the optical image plane

Question 2

hb’

Answer 2

Basic blurred image height on retina - less clear

Question 3

What happens in myopic ray diagram including blurred image size and image size at optical plane

Answer 3

• Sharp image focused infront of retina and has height which we can calculate
• By the time that image height reaches retina, not sure what it is because in diagram central ray reaches retina at one speicfc height but rays from top and bottom of pupil reach retina at other heights = point at top of object is imaged as a blur circle on retina

Question 4

What happens at the retina for myopia and hypermetropia

Answer 4

Point at top of object is imaged as a blur circle on retina

Question 5

What is Basic blurred image height

Answer 5

Height of the principal (chief) ray at the retina

Question 6

What is chief ray

Answer 6

The ray that passes through the centre of the entrance pupil ie. The central ray – don’t want extrme rays, this is good ray – average

Question 7

Where is hb’ located

Answer 7

On retina

Question 8

Where is h’ located in myopic eye

Answer 8

Sharp image height ( h’) formed in front of the retina

Question 9

What is equation of basic blurred image height for distance vision

Answer 9

h’b = h’ ( Fe / K + Fe )

Question 10

What is equation for image height h’ for distance object

Answer 10

h’ = -n tan x / Fe

Question 11

What is the correction factor in the equation of basic blurred image height for distance vision

Answer 11

Fe / K + Fe

Question 12

What do you need to do 1st to determine basic blurred image height

Answer 12

Need to work out optical image height h’

Question 13

What does letter recognition depend on

Answer 13

The size of the blur circle

Question 14

What is blur ratio

Answer 14

• Blur-circle diameter / Basic blurred image height on
  retina
• It is a comparison between blur circle and basic blurred image
  Height on retina

Question 15

Why is it good to workout basic blurred image height on retina

Answer 15

As optical image height not available to eye because its not on retina

Question 16

Why is blur ratio useful

Answer 16

Can see how actual visual performance behaves when using blur ratio - what level of visual blur can out system tolerate and still recognise letters

Question 17

What is vision when over 1D of ametropia

Answer 17

Vision falls to 6/24 level

Question 18

What is vision when 2D of ametropia

Answer 18

Up to 6/60 level

Question 19

What letters blurred ratio can we recognise for modest levels of spherical ametropi

Answer 19

1

Question 20

What is blur circle size same as

Answer 20

As letter size ( not limb/gap width ) = some info still there in blurred letter that allows us to recognize it

Question 21

What is blur ratio equation for distant object

Answer 21

BR = -pK / tan x

• p = pupil size
• K = ocular refraction
• x = angular subtend of letter height NOT gap or limb width e.g. 6/6 – 5’,

Question 22

What does blur ratio depend on

Answer 22

Ocular refraction and pupil diameter

these are at back of eye – inaccessible to us

Question 23

What is blur ratio on projected blur circle on 6/60 letter

Answer 23

1

Blur circle diamtert is equal to height of letter

Question 24

What is projected blur

Answer 24

Project the retinal blur circle back on to the object in object space

Question 25

What is the actual blur and where is it located

Answer 25

Blur circle diameter

On back of eye

Question 26

What happens in projection of blur circle

Answer 26

• Refraction happens at vertex of reduced eye ( angle theta become theta’ smaller value as its refracted – both rays refracrted )
• Image forms on retina at distance of k’ but it’s a blur circle because image out of focus on retina
• You project that blur circle diameter back on object at which point it becomes projected blur pb

Question 27

What is equation for projected blur

Answer 27

pb = DpK

Question 28

Why is n theta = n’ theta’

Answer 28

BECAUSE DEALING
WITH PARAXIAL APPROXIMATION SO EVEN THOUGH
BLURRED IMAGERY, ITS STILL IDEAL PARAXIAL IMAGERY FORMATION)

Question 29

Where does refraction occur and what does this mean

Answer 29

• At surface of reduced eye

- So link between blurred circle diameter on retina and projected blur on object space at distance D

Question 30

Why is projected blur interest to us

Answer 30

• Because get people to put markers on that blur to get them to perceive how big the blur is – present blur to them
• Can be used for comparison with an experiment of perceived blur – how big they perceive blur

Question 31

What will object with projected blur circles superimposed be

Answer 31

The object with projected blur circles superimposed will only be a scaled version of the image – BLUR CIRCLES ON TOP OF IMAGE – link between 2

Question 32

Why might actual perceived blur be different from our values (geometrically calculated )

Answer 32

Geometric approaches are simplistic: edges of real blurred objects are unclear so to assess blur on object is difficult as it becomes difficult to know where edge of that object is and the objects often appear to have structure possibly from diffraction in the eye – lens itself – its 1st apporximation

Question 33

What letters with blur ratios can be recognised

Answer 33

Letters can be recognised with blur ratios as high as 1 provided level of ametropia is low to moderate

Question 34

What does Blur ratio and projected blur provide

Answer 34

A first approximation to understanding letter recognition with blur present