5.1 - Blur circles and the pinhole Flashcards
When does blur circles on the retina take place
Myopia/Hypermetropia causes points to be imaged as blur circles on the retina
What happens in myopia and hypermetropia
Image formed away from retina ( behind or in front )
= A SINGLE POINT OF LIGHT IS IMAGED AS A BLURRED CIRCLE
What happens with increasing blur
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
What happens in infinitely distant object forming image at 2nd focal point behind retina
- 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
What is an object
Single point of light
A collection of point sources
What is an image
A collection of blur circles if blurred image
What makes up a straight line
Series of points lined up along the straight line
What happens in letter e
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
What does blur circle depend on
- 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
Blur circle equation
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’
What happens in blur circle for myopia
Image formed in front of the retina
What happens to image if distant object
Image come closer to front of eye or move anteriorly
In myopia if blur circle is negative why is this of no relevance
- 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
What is not present in blur circles and what does this mean
- No orientation or asymmetry
- Negative sign of no consequence – it’s the magnitude of blur circle that counts
How is blur circle negative
If k’ is a larger positive dimension than l’ – numberator negative
When do you get a negative blur circle
Where image is formed front of retina
When do you get a positive blur circle
Where image is formed behind retina
What is equation for blur circle distance vision
b = p ( K / K’ )
For distant object, the blur circle diameter is equal to the pupil size x (ocular refraction/dipotric length of eye )
What is the blur analysis based on
Geometrical optics
What happens in an ideal system
- Points are imaged as points
- A single point object will be imaged as single point
What happens in a real system
The wave nature of light leads to diffraction where images of points are diffraction patterns even with no focus error
What is diffraction pattern
- Bright central point with series of rings surrounding that
- Best point
What happens as things get smaller
Wave nature of light becomes more dominant
What does diffraction cause
- A spreading of light outside of this single point image
- Get astigmatic point to point imagery
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:
Within body of letter, blur circles overlap and limb appears dark, egdes loose its sharpness because edge has les blur circles so lower intensity
Can you simulate the effects of blur using a camera and trial case lenses or photoshop software?
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.
What does pinhole disk do
- 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
What happens as the amount of spherical ametropia increases
The blue circle diameter increases
What happens as pupil size increases
Blur circle diameter increases
What is blur circle diameter directionally proportional to and what does this mean
- 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
If vision good already how does the pinhole reduce vision slightly
- Reduces light level by smaller pupil size adversely affecting vision
- Increases diffraction if vision already good possibly decreasing vision
How does smaller pupil size decrease vision from pinhole disc
- Smaller pupil size – light levels at back of eye is less – reduced light level = reduced vision
How does smal pinhole increase diffraction and decrease vision
- 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
How does pinhole improve vision
Add pinhole – reduce pupil size = diatmeter of rays entering eye is less = blur circle on retina is much less
What does it mean if vision improves with pinhole
The indication is that deficit is due to retinal image blur
Why is pinhole a useful diagnostic tool
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