lecture 22 - factors that minimise the effects of aberrations and scattered light Flashcards
What can the pupil diameter decreases in?
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
What does the massive change in pupil size detect?
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
What does a large pupil size cause?
more spherical aberration
-very small depth of field
Why does a large pupil size cause Spherical aberration ?
- Spherical aberrations increases with the 4th power of ray height and coma in the 3rd power
What does spherical aberration do?
remains constant over the field and proportional to ~ h4 (ray height).
What does coma do?
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)
What affects the depth of field?
And when does this happen in the eye ?
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
What does a big aperture result in?
a small depth of field
What does a big aperture result in?
a small depth of field
If you want to keep the object of interest in focus only ?
use a large aperture and a very short exposure time
However if you wish to bring into focus objects over a large depth of field?
- small aperture
- long exposure time
What is not in the human eye?
no field stop
- and in some meridians vision extends up to 90 deg.
How useful is this peripheral vision?
directing our saccadic eye movement system
What is the effective directional sensitive of cones?
cone photoreceptors respond less well to light from periphery of pupil
-This can be modelled by adding these apodisation functions
What does the apodisation function (stiles Crawford effect) do?
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)
What is the percentage reduction in ‘effective’ retinal illuminance due to Stiles-Crawford apodisation over the pupil?
- the retinal illuminance decreases with pupil size
- over 40% for pupil size of 8mm
- only applies to cone photoreceptors
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)?
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
How does the wavefront aberration increase with pupil size in the eye?
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.
RMS wavefront aberration and pupil size. Does one benefit from correction of higher order aberrations at low light level?
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
What happens in the low mesopic, scotopic range?
- 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
What happens to the off axis astigmastim aberration ?
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
What happens in the on axis astigmatism aberration ?
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.
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) ?
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
field curvature (~ 2) in the eye. Does this affect visual performance in the periphery of the visual field?
it is an advantage because the image or retina itself follows surface of eyeball
-thereffore field curvature keeps the image plane onto the retina
