6: Ocular Image Formation

Question 1

if the real object moves towards the eye, then the image moves _____ in the eye

Answer 1

posteriorly

Question 2

if the real object moves away from the eye, then the image moves _____ in the eye

Answer 2

anteriorly

Question 3

a diverging wavefront is imaged ______ to the retina in a relaxed emmetropic eye

Answer 3

posterior

Question 4

a converging wavefront is imaged ______ to the retina in a relaced emmetropic eye

Answer 4

anterior

Question 5

if the eye’s power increases, then the image moves _______ in the eye

Answer 5

anteriorly

Question 6

if the eye’s power decreases, then the image moves ______ in the eye

Answer 6

posteriorly

Question 7

in an emmetropic eye with relaxed accommodation, a distant object is imaged ___ the retina

Answer 7

at the retina

Question 8

in a myopic eye with relaxed accommodation, a distant object is imaged ______ to the retina

Answer 8

anterior

Question 9

in a hyperopic eye with relaxed accommodation, a distant object is imaged ______ to the retina

Answer 9

posterior

Question 10

there are many causes of retinal image blurring, give some examples:

Answer 10

spherical and astigmatic defocus, chromatic and monochromatic aberrations, diffraction, and intraocular scatter

Question 11

what is motion blur, and is it perceptual or optical?

Answer 11

if the image moves too quickly across the retina, perceptual smearing results
-this is perceptual rather than optical

Question 12

examples of spherical defocus are

Answer 12

myopia and hyperopia

Question 13

if the eye’s power is equal in all meridians, and there is a mismatch between eye power and axial length, then _____ _____ is found

Answer 13

spherical defocus

Question 14

in spherical defocus, a point object is imaged as

Answer 14

a blur circle

Question 15

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

Answer 15

astigmatic defocus

Question 16

astigmatic optical systems produce astigmatic intervals, and point object can be imaged as:

Answer 16

a blur circle, ellipse, or a line

Question 17

longitudinal chromatic aberration (LCA) causes

Answer 17

short wavelengths to image more anteriorly in the eye than long wavelengths

Question 18

what can cause transverse chromatic aberration

Answer 18

certain displacements of the natural pupil (or external pupil)

Question 19

transverse chromatic abberation (TCA) results in a

Answer 19

transverse (sideways) mis-location of the light rays of different colors

Question 20

chromatic difference of magnification (CDM) can affect:

Answer 20

off-axis objects

Question 21

chromatic difference of magnification (CDM) appear as

Answer 21

the large object would be perceived to have a rainbow edge, with blue towards the middle and red towards the edge

Question 22

describe monochromatic aberrations

Answer 22

if the eye’s surfaces differ from the ideal forms, then the rays from a monochromatic point object may not form a single focus

Question 23

describe diffraction

Answer 23

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

Question 24

the most common and important source of scatter is the

Answer 24

crystalline lens and cataracts of the lens

Question 25

what is the point spread function (PSF)

Answer 25

is the two dimensional intensity distribution of a point source at the retinal plane

Question 26

is the intensity of the PSF disc proportional or inversely proportional to the area of the disc

Answer 26

inversely proportional

Question 27

what is the Smith’s approach used for

Answer 27

used a paraxial approach to derive an approximate expression for the perceived angular size of the blue disc in object space

Question 28

why is Smith’s approach not as accurate for levels of defocus less than 3D?

Answer 28

at smaller levels of defocus, diffraction and aberrations become greater sources of blur than defocus

Question 29

blur is ______ with larger defocus

Answer 29

greater

Question 30

blur is ______ with larger pupil diameters

Answer 30

greater

Question 31

_______ and _____ _______ may be traded against each other to maintain an equivalent amount of perceptual blue

Answer 31

defocus and pupil diameter

Question 32

what are the two steps for imaging of an extended object

Answer 32

1. trace a ray from each object point to its intersection with the retina
2. place at each intersection point its respective point spread function (PSF)

Question 33

of the construction methods, which one is most versatile/ preferred?

Answer 33

the entrance-exit pupil method

Question 34

3 construction methods for imaging of extended object

Answer 34

1. classical lateral magnification
2. nodal points method
3. entrance and exit pupil method

Question 35

describe the classical lateral magnification method for imaging of extended objects

Answer 35

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

Question 36

describe the nodal points method for imaging of extended objects

Answer 36

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

Question 37

describe the entrance and exit pupil method for imaging of extended objects

Answer 37

• 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

Question 38

the retinal image for extended image is imaged

Answer 38

inverted horizontally and vertically

Question 39

convolution is a mathematical procedure that may be used to

Answer 39

estimate the appearance of the image of an extended object based on the known point spread function of the optical system

Question 40

what is optic array

Answer 40

it is a set of solid angles surrounding a point in air in the real world