5.1 - Blur circles and the pinhole

Question 1

When does blur circles on the retina take place

Answer 1

Myopia/Hypermetropia causes points to be imaged as blur circles on the retina

Question 2

What happens in myopia and hypermetropia

Answer 2

Image formed away from retina ( behind or in front )

= A SINGLE POINT OF LIGHT IS IMAGED AS A BLURRED CIRCLE

Question 3

What happens with increasing blur

Answer 3

The blur size increases and amount of light present is spread out over a larger blur circle diameter ( small one – dark grey, big one – light grey

Question 4

What happens in infinitely distant object forming image at 2nd focal point behind retina

Answer 4

• Image that falls on retina is a blurred image/circle of that point because retina doesn’t have access to sharp image
• Distant object object i.e. single point of light, rays come in parallel to OA

Question 5

What is an object

Answer 5

Single point of light

A collection of point sources

Question 6

What is an image

Answer 6

A collection of blur circles if blurred image

Question 7

What makes up a straight line

Answer 7

Series of points lined up along the straight line

Question 8

What happens in letter e

Answer 8

Take every point on letter e as an individual point and so its image becomes the combination of all of the blur circles formed from each one of those points individually

Question 9

What does blur circle depend on

Answer 9

• The blur on the retina i.e. blur circle = diameter is b
• Pupil diatmeter – p
• k’ – axial length
• l’ – image position
• Numerator - l’-k’ = difference between 2 nearly equal numbers ( fractions of mm – dipotres of change in refraction ) = prone to error

Question 10

Blur circle equation

Answer 10

b = p ( K’ - L’ / K’ )

OR

b = p ( K - L / K’ )

BETTER TO USE VERGENCES SO USE CAPITAL L AND K AND K’ INSTEAD OF k and k’ and l’

Question 11

What happens in blur circle for myopia

Answer 11

Image formed in front of the retina

Question 12

What happens to image if distant object

Answer 12

Image come closer to front of eye or move anteriorly

Question 13

In myopia if blur circle is negative why is this of no relevance

Answer 13

• No difference between 2 blur circles, if there was orientation or asymmetry e.g. A is differet in normal direction and negative direction i.e. upside down because its asymmetric so clear indication of orientation – dont have in blurred circles

Question 14

What is not present in blur circles and what does this mean

Answer 14

• No orientation or asymmetry

- Negative sign of no consequence – it’s the magnitude of blur circle that counts

Question 15

How is blur circle negative

Answer 15

If k’ is a larger positive dimension than l’ – numberator negative

Question 16

When do you get a negative blur circle

Answer 16

Where image is formed front of retina

Question 17

When do you get a positive blur circle

Answer 17

Where image is formed behind retina

Question 18

What is equation for blur circle distance vision

Answer 18

b = p ( K / K’ )

For distant object, the blur circle diameter is equal to the pupil size x (ocular refraction/dipotric length of eye )

Question 19

What is the blur analysis based on

Answer 19

Geometrical optics

Question 20

What happens in an ideal system

Answer 20

• Points are imaged as points

- A single point object will be imaged as single point

Question 21

What happens in a real system

Answer 21

The wave nature of light leads to diffraction where images of points are diffraction patterns even with no focus error

Question 22

What is diffraction pattern

Answer 22

• Bright central point with series of rings surrounding that
• Best point

Question 23

What happens as things get smaller

Answer 23

Wave nature of light becomes more dominant

Question 24

What does diffraction cause

Answer 24

• A spreading of light outside of this single point image

- Get astigmatic point to point imagery

Question 25

By considering the limb of a letter as a collection of points that each produce blur circles, try and argue what the edge of a blurred letter would look like:

Answer 25

Within body of letter, blur circles overlap and limb appears dark, egdes loose its sharpness because edge has les blur circles so lower intensity

Question 26

Can you simulate the effects of blur using a camera and trial case lenses or photoshop software?

Answer 26

Whatever we do ignores the neural processing that occurs. We can reasonably simulate the optical image but that is all. Sometimes it is very instructive to do so since no amount of neural processing can make a poor optical image good.

Question 27

What does pinhole disk do

Answer 27

• Artificially reduces the pupil size significantly decreasing the blur circle ( p ).
• Can improve vision
• If there is significant blurring, the pinhole disk is going to reduce it

Question 28

What happens as the amount of spherical ametropia increases

Answer 28

The blue circle diameter increases

Question 29

What happens as pupil size increases

Answer 29

Blur circle diameter increases

Question 30

What is blur circle diameter directionally proportional to and what does this mean

Answer 30

• The pupil size and ocular refraction k
• So as the amount of spherical ametropia increases so does blur circle diameter but also as pupil size increases, so does the blur circle diameter

Question 31

If vision good already how does the pinhole reduce vision slightly

Answer 31

• Reduces light level by smaller pupil size adversely affecting vision
• Increases diffraction if vision already good possibly decreasing vision

Question 32

How does smaller pupil size decrease vision from pinhole disc

Answer 32

• Smaller pupil size – light levels at back of eye is less – reduced light level = reduced vision

Question 33

How does smal pinhole increase diffraction and decrease vision

Answer 33

• Small pinhole increases amount of diffraction ( spreading of light due to wave nature happening at back of eyes )
  – pinhole smaller than pupil size so increases it more than normally, so if vision already good, increased diffraction can decrease vision

Question 34

How does pinhole improve vision

Answer 34

Add pinhole – reduce pupil size = diatmeter of rays entering eye is less = blur circle on retina is much less

Question 35

What does it mean if vision improves with pinhole

Answer 35

The indication is that deficit is due to retinal image blur

Question 36

Why is pinhole a useful diagnostic tool

Answer 36

BECAUSE if vision is averaged to poor and improves significantly with a pinhole, it means theres significant optical blur present. If vision reduced and doesn’t improve with pinhole, something else like pathology. If vision already good, pinole can reduce vision