lecture 22 - factors that minimise the effects of aberrations and scattered light

Question 1

What can the pupil diameter decreases in?

Answer 1

Pupil diameter decreases from ~ 8 to 2 mm as the ambient illumination changes from absolute scotopic (threshold for seeing) to high photopic (bright daylight)

• Bright day light causes 2mm
• Complete darkness causes big pupil - 8mm

Question 2

What does the massive change in pupil size detect?

Answer 2

16 fold change in retinal illuminance
-The change in retinal illuminance is not a great advantage when considering the overall massive range of ambient light levels over which VI response and produces useful visual signals

Question 3

What does a large pupil size cause?

Answer 3

more spherical aberration

-very small depth of field

Question 4

Why does a large pupil size cause Spherical aberration ?

Answer 4

• Spherical aberrations increases with the 4th power of ray height and coma in the 3rd power

Question 5

What does spherical aberration do?

Answer 5

remains constant over the field and proportional to ~ h4 (ray height).

Question 6

What does coma do?

Answer 6

it increases linearly with object field angle () (i.e., coma is absent or very small for on-axis object points, but is proportional to ~ h3)

Question 7

What affects the depth of field?

And when does this happen in the eye ?

Answer 7

like in any other lens, the size of the pupil and the resolving power of the retina affects the depth of field.
-In the eye this is particularly evident at lower light levels when the spatial resolution of the retina is reduced

Question 8

What does a big aperture result in?

Answer 8

a small depth of field

Question 9

What does a big aperture result in?

Answer 9

a small depth of field

Question 10

If you want to keep the object of interest in focus only ?

Answer 10

use a large aperture and a very short exposure time

Question 11

However if you wish to bring into focus objects over a large depth of field?

Answer 11

• small aperture

- long exposure time

Question 12

What is not in the human eye?

Answer 12

no field stop

- and in some meridians vision extends up to 90 deg.

Question 13

How useful is this peripheral vision?

Answer 13

directing our saccadic eye movement system

Question 14

What is the effective directional sensitive of cones?

Answer 14

cone photoreceptors respond less well to light from periphery of pupil
-This can be modelled by adding these apodisation functions

Question 15

What does the apodisation function (stiles Crawford effect) do?

Answer 15

is to weight the amount of light entering the pupil depending on paint of entry

• So when light enters from centre of pupil the function is 1
• When you increase the value of r the function becomes less than 1
• now add the amount of light contributed by the pupil with this function to work out the total retinal illuminance. (dont need to work out)

Question 16

What is the percentage reduction in ‘effective’ retinal illuminance due to Stiles-Crawford apodisation over the pupil?

Answer 16

• the retinal illuminance decreases with pupil size
• over 40% for pupil size of 8mm
• only applies to cone photoreceptors

Question 17

If rod photoreceptors mediate vision at very low light levels, how effective is the apodisation when the pupil size is large (i.e., 8 mm)?

Answer 17

if you only have rods

• the apoidisation even fro pupil sizes large as 8mm makes no difference because each photon irrespective of the entry point and the pupil is equally likely to be absorbed by rods
• only get pupil 8mm If light levels very low and only using rod photorecpeotrs

Question 18

How does the wavefront aberration increase with pupil size in the eye?

Answer 18

increasing size of pupil , root mean square aberration (high order aberration) increases rapidly (RMS).

• Dependence of spherical aberration on pupil size.
• Vision at low light levels. Retinal sensitivity to contrast at low light levels.

Question 19

RMS wavefront aberration and pupil size. Does one benefit from correction of higher order aberrations at low light level?

Answer 19

yes - in subjects who dont have large pupil size in high light levels
-However, -if you allow pupil to vary naturally according to ambient light level (low light) there is little benefit in correction high order aberrations

Question 20

What happens in the low mesopic, scotopic range?

Answer 20

• because larger pupil size is when aberrations become important
• require very low light levels -and when ambient light I=level is low you have rod vision
• and rod photoreceptors have very poor visual acuity and spatial resolution
• So the limit of what we see in the low light levels in the Coptic range is determined by the resolving power of retina and the way the rod photoreceptor is arranges opposed to the quality of image in eye

Question 21

What happens to the off axis astigmastim aberration ?

Answer 21

off axis astigmatism which increases with the square of object field angle is not that important as you can not see fine detail in the periphery of vision field

Question 22

What happens in the on axis astigmatism aberration ?

Answer 22

Howver on axis astigmatism ( different curvature in one meridian - steeper curvature in comparison to orthogonal meridian) - this is important because that affects the quality of axial image.

Question 23

What is the important difference between oblique astigmatism (in any system when object is off axis) and astigmatism in eye (caused by changes in curavtriue of corneal surface) ?

Answer 23

the astigmatism in the eye does not depend on object field angle and does not increase with object field angle
-but it causes the same effects as off axis astigmatisms which is inevitable in symmetrical optical systems

Question 24

field curvature (~ 2) in the eye. Does this affect visual performance in the periphery of the visual field?

Answer 24

it is an advantage because the image or retina itself follows surface of eyeball
-thereffore field curvature keeps the image plane onto the retina

Question 25

What is the effect of spectacle lenses on eye aberrations?

Answer 25

• when looking through spectacle lens, the quality of this lens in terms of aberrations is very poor
• varifocal lenses (power varies) - image will be extremely poor and distorted

Question 26

Why is it when we use the lens in conjunction with eye there is no problem and object is in focus?

Answer 26

• because the aperture stop in this arrangement when the spectacle lens is used with the eye is actually the pupil
  -only have high spatial resolution over a very small region of retina, where we see Fien detail.
  -

Question 27

What happens when we want to view the image/detail off axis?

Answer 27

we rotate the eye all so the same region (of retina where can see fine detail) will be looking off axis

Question 28

What happens when you look at an object on axis in eye?

Answer 28

• the actual area of the lens which you are using to form the image within this centre region fo retina (where resolving power is very high), is extremely small- only use a small region due to the size of pupil
• when region is very small, over this small region the height of the ray is very small -which means spherical aberration and coma is very small and this is the reason why the lens itself, despite its poor optical quality, its not really affecting our visual performance

Question 29

What is another parameter which affects strongly how well we can see and form?

Answer 29

retinal illminance level- how much light we have on retina which depends on the brightness.luminance of the object that we are looking at

Question 30

What do we do to investigate the retinal illuminance ?

Answer 30

• measure the contrast threshold - which is the smallest contrast the subject needs to resolve the gap in a Londolt c i.e
• We do this as a function of light level, is a function of retinal illuminance, the amount of light on the retina- using computer based techniques for assessing functional contrast sensitivity - The acutity plus test

Question 31

How do we alter the light on the retina for this test ?

Answer 31

using the neutral density filters as well as the luminance of screen

Question 32

What does the graph show?

Answer 32

contrast threshold for a number of light levels - form bright light to low mesonic range of light

• as light level drops, the actual contrast threshold increases very rapidly.
• At very low light levels, you need very large contrast - which can only be achieved by Positive contrast
• At higher light level, need same contrast, there is much less change.

Question 33

What else does this experiment results show?

Answer 33

Contrast thresholds for letter recognition (Snellen chart) or gap orientation discrimination (Landolt ring) in the photopic range are in general well below 10%.

• At lower retinal illuminances (as one enters the mesopic rage) the contrast thresholds start to increase rapidly
• The rapid reduction in retinal sensitivity to contrast is attributed to the lower retinal illuminance since correction of higher order aberrations does not improve vision (when the light level is low)
• Small aberrations, hence a small pupil size, is therefore only of advantage at higher light levels in the photopic range when retina responds well to contrast and nota n advantage at low light levels
• Small range which the pupil changes size quite rapidly
• Data suggests that the function of pupil is not to improve the image quality by reducing aberration but is to give us another o1.2 log units range of light levels of which the snensitvity of the retina remains relaivitley good.

Question 34

How DOES THE EYE MINIMISE THE EFFECTS OF CHROMATIC ABERRATION

Answer 34

• luminance contrast channel - which is the most important channel 0 reply on signals only on L and M cones and not on signals from S cones
• S cones respond well to short wavelength;ength of light but do not contribute to luminance econtrast channel
• fewer photoreceptor which reposed to short wavelength of light

Question 35

How is spatial vision , luminance contrast, ability to see fine detail, VA determined by?

Answer 35

determined largely by the luminance contrast channel

• -this relies on Land M cones - not S cones or on rods
• Absence of S cones on the centre region of visual field and of rods- only small density around.
• Have photoreceptor interaction- S cone signal becomes effective when the L and M cone signal is low and the S cone signal, even. if present, is in inhibited by presence of strong L and M cone signals.
• the amount of short wavelength of light we end up on retina depends on the spectral transmittance of the lens and all the amount fo macular pigment in a particular eye
• the lens absorbs more blue light as the age increases- less absorbption of shot wavelength of light when older.
• if we combine the absorption of lens with the absorption of macular pigment- the macula pigment is absorbing light in a young px but then as px age increases , you end up with the absorption of blue light in the eye being dominated by the lens

Question 36

What happens as a result of relying on the 2 L and M cones?

Answer 36

• as result of relying only these 2 cones, which are operated by less than 30nm, the amount of chromatic aberration over the small spectral range where the sensitivity is highest is less than 0.3D and this explains why chromatic aberration is minimised by not involving short wavelength cones
• there is no S cones of rods and in fact the density is low all over the retina in. comparison to L and M

Question 37

What dont we have in the centre region of visual field?

Answer 37

• we dont have the photoreceptors which respond to short wavelength lght
• also have very small amount of short wavelength light due to absorptiton of blue light in 3 receptor filters.
• filters that are inferno of eye- like lens and macular pigment
• S cone density is small and right at the very centre there is no S cones or Rods in central. vision

Question 38

What does the absorption of blue light affect ?

Answer 38

the whole of the retina over a very small part of retina we have another filter which consists of carotenisnds which impregante the henley fibres which are the photoreceptor axons, only in this small region, around fixation, the macula pigment optical density, which indicates how much light is absorbed , decreases very rapidly as you move away from the point of regard.

Question 39

Why does the macula pigment not affect strongly the response of L cones ?

Answer 39

the spectral transmittance of this pigment is smallest int he blue part of the spectrum but then it transmits all the light above 550nm
-it hence affects the responses of S cones and rods

Question 40

How much short wavelength light do we end up with on the retina?

Answer 40

• the amount of short wavelength of light we end up on retina depends on the spectral transmittance of the lens and all the amount fo macular pigment in a particular eye
• the lens absorbs more blue light as the age increases- less absorbption of shot wavelength of light when older.

Question 41

The directional sensitivity of cone photoreceptors reduces the effective size of the pupil and hence the effects of spherical aberration and coma

Answer 41

a

Question 42

The lack of directional sensitivity in rod photoreceptors results in higher sensitivity to light

Answer 42

a

Question 43

The rapid increase in pupil size with decreasing light level in the mesopic range helps reduce the loss of retinal sensitivity to contrast

Answer 43

a