blur circles and the pinhole 5.1 Flashcards
where is image produced in spherical ametropia ?
myopia - image formed in front of retina
hypermetropia - image formed behind retina
so image that falls on retina is a blurred image of point
what happens with increasing blur ?
blur circle size increases and also the amount of light is spread out over a larger blur circle diameter
how are points imaged in myopia/hypermetropia?
myopia/hypermetropia causes points to be imaged as blur circles on the retina
are we able to consider an object as a collection of point sources ?
. it is possible to consider any object as a collection of point sources
. the image is therefore a collection of blur circles formed from each point individually
what does blur circle diameter depend on ?
axial length of eye
pupil diameter
image position
how to calculate blur circle for similar triangles ?
b/p = (l’ - k’ ) / l’
however numerator difference of two nearly equal numbers is prone to error so it is written in terms of vergence
b=p(K’ - L’ / K’) or =p(K-L/K’)
what happens to blur circle in myopia ?
. answer is negative because k’ is larger than l’
. there is no consequence if blur circle is negative , because it’s a blur circle not a asymmetric shape that has any orientation dependence
. only magnitude of blur circle that counts
how to calculate blur circle for distance vision ?
L=0
so b=p(K/K’)
dependent on ocular refraction and dioptric length of the eye
what is the blur analysis dependent on ?
this blur analysis is dependent on geometrical optics where in an ideal system , points are imaged as points this is what we call stigmatic imagery
. in reality the wave nature of light leads to diffraction ( bright central point with rings surrounding it ) where images of points are diffraction patterns even with no focus error
what is diffraction ?
spreading of light outside point image
what is relationship between blur circle diameter and pupil size and spherical ametropia ?
. blur circle diameter is directly proportional to pupil size and to ocular refraction (k)
. as the amount of spherical ametropia increases so does blur circle diameter .
. as pupil size increases so does blur circle diameter
what is the function of a pinhole disc ?
. artificially reduces the pupil size significantly decreasing the blur circle , this increases the depth of focus , doesn’t correct ametropia
since b=p(k/k’)
. will only improve mage if blurring is significant
.
what is the effect of pinhole ?
. pinhole reduces pupil size
. diameter of rays entering eye is much less
. blur circle on retina is much less
what does it mean if vision improves with a pinhole ?
. it indicates that deficit is due to retinal image blur
what is the effect of pinhole if vision is already good ?
. will decrease pupil size therefore reduces light at the back of eye adversely affecting vision
. increases diffraction if vision already good possibly decreasing vision
