Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

VO and MT > 7.3 the astigmatic image > Flashcards

7.3 the astigmatic image Flashcards

1
Q

What happens when observing an object point at presence of astigmatism?

A

line foci are formed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are one of the paradox of astigmatism?

A

the lines are perpendicular to the meridian aghast is at focus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What does astigmatism mean?

A

not stigmatic-

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does stigmatic mean?

A

means point to point imagery that means a single point object is imaged as single point image which is not the case in astigmatic images

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What do the line foci represent?

A

represents extremes of shapes that can be formed in a astigmatic image

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

How do you and what happens when viewing an astigmatic image?

A

putting magnifier glass over

-get progression of blurring between the 2 extremes the horizontal line focus and vertical line focus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What happens as you move away from the horizontal line focus ?

A

get horizontal blur elipse
line limiting form of elipse
-the more ways you get a circular patch -disc of least confusion
-even more away the ellipse becomes vertical before it becomes the vertical line of focus
-progression of shapes of blur as we move through focus in a astigmatic image
-circles and lines are special form of elipse.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the disc of least confusion (DLC)?

A

-circular blur patch located dioptrically midway between the astigmatic foci

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the best approximation to point object?

A

circular blur- all diagrams show single point of light

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the best approximation to point object called ?

A

-circle of least confusion- point in between 2 line foci

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is Best vision sphere (BVS)?

A

-It is the spherical correction in the spectacle plane that places the disc of least confusion on the retina for distance vision.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the definitions for analysis?

A

Take θ to be the least powerful meridian with power Fe θ ; this will be the sphere component when the prescription is written in negative cyl form
-always at right angles to each other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the most powerful meridian ?

A

θ + 90 degrees is the more powerful Meridan with power Fe theta + 90

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the least powerful meridian ?

A

Take θ meridian as least powerful meridian with power Fe θ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the expression of the length of the astigmatic line for the least powerful meridian?

A

From similar triangles: a=p(l’θ-l’θ+90/l’θ)=p(L’θ+90-L’θ/L’θ+90)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

How does the pupil size and astigmatic line correlate?

A

the bigger the pupil size the longer the astigmatic line

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the expression of the length of the astigmatic line for the most powerful meridian?

A

b=p(l’θ+90-l’θ/l’θ+90)=p(L’θ-L’θ+90/L’θ)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the length of focal line expression for a distance object?

A

L’θ=Feθ

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is astigmatism defined as ?

A

the difference in power between the principal meridians

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is the Ast equation?

A

Ast= Fe θ+90 - Feθ

21
Q

What are the expression when distant object- in the θ+90 meridian?

A

a= p(-Ast/Feθ+90)

22
Q

What are the expression when distant object- in the θ meridian?

A

b= p(-Ast/Feθ)

23
Q

What are the dimensions of the elliptical blur?

A
  • working at general distance (retina can be at any point) - l’z
  • The method used to find the length of the astigmatic lines can be used to find the major and minor axes of the elliptical blurs
24
Q

What is l’z?

A

the general distance

25
Q

What is l’θ?

A

focus distance of the least powerful meridian - which can be used to determine size of blur elipse

26
Q

What are the dimension of the elliptical blur equation for major ?

A

x= p(L’z-L’θ/L’z)

27
Q

What can the major and minor do?

A

Can flip around depending on relative magnitudes

28
Q

What are the dimension of the elliptical blur equation for minor?

A

y=p(L’θ+90-L’z/L’z)

29
Q

What is the dioptric location of DLC (L’z)

A

The DLC occurs when major and minor axes of the elliptical blur are equal

30
Q

What is the dioptric location of DLC (L’z) equation?

A

p=(L’Z-L’θ/L’z)=p(L’θ+90-L’z/L’z)

31
Q

What is the DLC for a distant object equation?

A

L’θ= Feθ
L’θ+90=Feθ+90
THEREFORE

L’z= (Fe θ+90 + Fe θ / 2)- shows dioptrically midway

32
Q

What do we do to find location of DLC?

A

L’z equation

33
Q

How do we find size of DLC?

A

knowing the location, we can substitute into either expression for the major or minor axis of the ellipse to find the diameter of the DLC
-IF PUPIL size is higher,, blur is larger , then the DLC will be larger and astigmatism is higher

34
Q

What is the equation for the size of the DLC?

A

z= (p x Ast / Fe θ+90 + Feθ)

35
Q

Take an eye with compound myope astigmatism
-0.50DS/-2.00DC X135
Astigmatic blur

A
  • This could be produced by an eye with
    Fe135 = 60.5 D
    Fe, 45= 62.5D
    K’ = 60D

Means main blur is along the 45 meridian
and the along 135 is the less blur
-Blurr elipse from examiners view

36
Q

How to calculate the major and minor axes of the blur elipse ?

A

E.g (p=4mm)

Draw table - have in notes
-Blurr elipse from examiners view

37
Q

How to predict the appearance of astigmatic image?

A

superimpose blurs on lines to predict blurred image

  • can treat any object as a collection of points and each of those points become a blur ellipse
  • Have series of lines (horizontal, diagonal, vertical etc for lines of letters)
  • Blur elipse is from the subjects point of view
  • subject behind- so flips

go over???

38
Q

What does a point object appear as in general?

A

blur elipse in presence of astigmatism.

39
Q

When does point object appear as a blur circle?

A

when the DLC falls on the retina or a line focus if were at one of the foci principle meridians

40
Q

What can astigmatism be considered as?

A

Astigmatism can be considered as two different degrees of spherical ametropia along the principle meridians

41
Q

What is astigmatic blur eclipses (major axes of elipse )?

A

are perpendicular to the meridian that is closest to focus- reforged to paradox to astigmatism

42
Q

What is a simple myopic astigmatism ?

A

One meridian focus lies on the retina and the other in front of the retina

43
Q

An eye with simple myopic astigmatism views a point source. The image on the retina is:

A

a line

44
Q

What type of astigmatism is represented by the prescription -1.00/-1.75 x35?

A

compound myopic astigmatism

45
Q

What type of astigmatism is represented by the prescription +0.75/-1.25 x170?

A

Mixed astigmatism

46
Q

What does The length of the astigmatic line depends on?

A

(p x Ast)/Fe,theta

47
Q

An astigmatic reduced eye has principal meridian powers of +59D and +63.5D. What is the Dioptric location of the disc of least confusion?

A

+61.25D

48
Q

Calculate the size of the disc of least confusion if an eye has principal meridian powers of +60D and +63.5D and the pupil size is 4mm:

A

0.113mm

49
Q

An astigmatic reduced eye has an ocular refraction of -0.50/-1.50 x10. Calculate the major and minor axes of the blur ellipse if K’=+60D and the pupil size is 5mm:

A

minor: -0.042mm; major: -0.167mm

VO and MT (38 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions