Week 5

Question 1

Michelson Contrast

Answer 1

How much brighter or darker different parts of an object are compared to its average brightness
Commonly measured using (Lmax-Lmin)/(Lmax+Lmin)
Lmax = Lmin, contrast becomes 0

Question 2

Modulation Transfer Function

Answer 2

-A function to specify the image forming capability of an optical system ( or how well the contrast details of an object can be transferred to the image)
OR
-How well an optical system preserves the contrast and sharpness of an object as it is captured in an image
-It describes the effect of both diffraction and aberration in an optical system and how they affect the contrast and resolution of the image
-Contrast decreases with increasing spatial frequencies
-Resolution increases with increasing spatial frequencies
-Sin wave gratings of increasing spatial frequencies are imaged through the optical system and contrast modulation at each frequency is measured by the MTF
-Each optical system will have its own Diffraction limited MTF ( the best possible image free of aberrations)
-MTF is optimizes by changing different optical parameters – lens surface curvature, thickness etc to push it as close to the diffraction limited MTF curve

Question 3

Fourier Analysis

Answer 3

-A process of decomposing every pixel of an image into its corresponding series of sin waves of different spatial frequencies

Question 4

Spurious Resolution

Answer 4

-In low refractive errors, MTF oscillates between positive and negative values
-Contrast transfer from object to image is reversed

Question 5

Refractive Errors

Answer 5

• “blur” or “defocus” of the optical system
• Person with a refractive error is an ammetrope, and a person without is an emmetrope
• The anterior corneal curvature and axial length are the most significant contributors to refractive error

Question 6

Emmetropization

Answer 6

• The process of the eye growth
• The axial length and optical power of the eye precisely match to bring the eye into perfect focus
• Any disruption to the emmetropization mechanism results in refractive error

Question 7

Myopia

Answer 7

-Longer axial length or steeper cornea
-Image of distant objects appear blurry because they are imaged in front of the retinal plane
-Near objects appear sharp if coincides with the far point
-Far point of a myopic eye is in front of the eye
-Correction with minus lenses (concave) to create a virtual image of the distant object at the far point; light diverges and is then re-imaged onto the retina

Question 8

Hyperopia

Answer 8

-Shorter axial length or flatter cornea
-No object distance will provide a clear image formation in a relaxed eye
-Distance vision may be poor depending on the degree of hyperopia and amplitude of accommodation
-Near objects appear blurry in eyes with high hyperopia or reduced amplitude of accommodation
-Far point of a hyperopic eye is behind the eye
-Correction with plus lenses (convex lenses) to form a real image at the far point

Question 9

effect of low refractive error on vision

Answer 9

-Low refractive errors primary affect low spatial frequencies that carry contrast information
-In low refractive errors ( up to +/- 1.00D) there is a significant loss of contrast and chromatic aberration influences image quality.
-The contrast transfer from object to image are reversed

Question 10

low refractive errors and chromatic aberrations

Answer 10

-Chromatic aberrations benefit low myopes in colour perception more than low hyperopes because red colour is focused closer to the retina than blue
-In uncorrected low myopes, the longer wavelength red light will be in focus at the retina
-For uncorrected hyperopes, the short wavelength blue light will be focused at the retina

Question 11

higher refractive errors

Answer 11

• Higher refractive errors affect both contrast and resolution
• Chromatic aberration as similar effect on both myoped and hyperopes

Question 12

astigmatism

Answer 12

• A condition where a point object incident on the cornea is not focused to form a point image, after refraction
• Due to the toricity of the surface ( unequal radius of curvature along two principle meridians)
• Forms two or more focal lines

Question 13

with the rule astigmatism

Answer 13

• Cornea is steeper ( radius of curvature is shorter) in the vertical meridian
• Axis of minus correcting cylinder between 0-20 and 160-180

Question 14

against the rule astigmatism

Answer 14

• Cornea is steeper ( radius of curvature is shorter) in the horizontal meridian
• Axis of minus correcting cylinder between 70-110