6: Ocular Image Formation Flashcards
if the real object moves towards the eye, then the image moves _____ in the eye
posteriorly
if the real object moves away from the eye, then the image moves _____ in the eye
anteriorly
a diverging wavefront is imaged ______ to the retina in a relaxed emmetropic eye
posterior
a converging wavefront is imaged ______ to the retina in a relaced emmetropic eye
anterior
if the eye’s power increases, then the image moves _______ in the eye
anteriorly
if the eye’s power decreases, then the image moves ______ in the eye
posteriorly
in an emmetropic eye with relaxed accommodation, a distant object is imaged ___ the retina
at the retina
in a myopic eye with relaxed accommodation, a distant object is imaged ______ to the retina
anterior
in a hyperopic eye with relaxed accommodation, a distant object is imaged ______ to the retina
posterior
there are many causes of retinal image blurring, give some examples:
spherical and astigmatic defocus, chromatic and monochromatic aberrations, diffraction, and intraocular scatter
what is motion blur, and is it perceptual or optical?
if the image moves too quickly across the retina, perceptual smearing results
-this is perceptual rather than optical
examples of spherical defocus are
myopia and hyperopia
if the eye’s power is equal in all meridians, and there is a mismatch between eye power and axial length, then _____ _____ is found
spherical defocus
in spherical defocus, a point object is imaged as
a blur circle
if the eye’s power is not equal in all meridians, but has one max power in one meridian and min power in a second meridian, and these meridians are separated by 90 degrees, then ____ ____ occurs
astigmatic defocus
astigmatic optical systems produce astigmatic intervals, and point object can be imaged as:
a blur circle, ellipse, or a line
longitudinal chromatic aberration (LCA) causes
short wavelengths to image more anteriorly in the eye than long wavelengths
what can cause transverse chromatic aberration
certain displacements of the natural pupil (or external pupil)
transverse chromatic abberation (TCA) results in a
transverse (sideways) mis-location of the light rays of different colors
chromatic difference of magnification (CDM) can affect:
off-axis objects
chromatic difference of magnification (CDM) appear as
the large object would be perceived to have a rainbow edge, with blue towards the middle and red towards the edge
describe monochromatic aberrations
if the eye’s surfaces differ from the ideal forms, then the rays from a monochromatic point object may not form a single focus
describe diffraction
due to the wave nature of light, light waves passing through a small pupil form patterns of constructive and destructive interference at the retinal plane
the most common and important source of scatter is the
crystalline lens and cataracts of the lens
what is the point spread function (PSF)
is the two dimensional intensity distribution of a point source at the retinal plane
is the intensity of the PSF disc proportional or inversely proportional to the area of the disc
inversely proportional
what is the Smith’s approach used for
used a paraxial approach to derive an approximate expression for the perceived angular size of the blue disc in object space
why is Smith’s approach not as accurate for levels of defocus less than 3D?
at smaller levels of defocus, diffraction and aberrations become greater sources of blur than defocus
blur is ______ with larger defocus
greater
blur is ______ with larger pupil diameters
greater
_______ and _____ _______ may be traded against each other to maintain an equivalent amount of perceptual blue
defocus and pupil diameter
what are the two steps for imaging of an extended object
- trace a ray from each object point to its intersection with the retina
- place at each intersection point its respective point spread function (PSF)
of the construction methods, which one is most versatile/ preferred?
the entrance-exit pupil method
3 construction methods for imaging of extended object
- classical lateral magnification
- nodal points method
- entrance and exit pupil method
describe the classical lateral magnification method for imaging of extended objects
simple paraxial method for estimating image height using principle points, equivalent power, and lens formula (M=L/L’)
-this method is not often useful in visual optics
describe the nodal points method for imaging of extended objects
trace a ray from an object point to the first nodal point (N) and then to the second nodal point (N’) to its intersection with the retina (called the nodal ray)
-this method is not valid if the retinal image is defocused
describe the entrance and exit pupil method for imaging of extended objects
- the chief ray is traced from an object point to the center of the entrance pupil and then from the center of the exit pupil (E’) to its intersection with the retina, using (m=u’/u) where m= the paraxial pupil ray angle ratio
- this method is valid for both focused or defocused retinal images
the retinal image for extended image is imaged
inverted horizontally and vertically
convolution is a mathematical procedure that may be used to
estimate the appearance of the image of an extended object based on the known point spread function of the optical system
what is optic array
it is a set of solid angles surrounding a point in air in the real world
