Aberrations

Question 1

Monochromatic aberrations

Answer 1

Wavelength independent, distorts image quality: spherical, coma, radial astigmatism, deform image plane: curvature of field and distortion

Question 2

Spherical aberrations

Answer 2

Paraxial approximation- incident rays are close to the optical axis
Marginal rays - on periphery, bend more
Longitudinal spherical aberration - point no longer forming a point image = image is blurred
Aberrations get worse further in periphery, iris can help block
Can be on or off axis

Question 3

Coma

Answer 3

Only for off-axis, magnification varied as the height of incident rays above axis is varied, asymmetric comet-shaped patch.
Bigger deal in higher Rx’s

Question 4

Radial astigmatism

Answer 4

Due to off-axis rays hitting lens or interface obliquely from tilted lens. Can be sagittal or tangential meridian, associated with “teacup and saucer” image

Question 5

Curvature of field

Answer 5

When a lens brings light to a focus across a curved image plane (Petzval surface), quality of an image on a flat screen decreases for larger distances. Ex. Projector moving backwards gets worse image quality. Related to radial astig (which is warping on two axes) but still can happen without radial astig (Petzval). Curvature of field will be present in an ophthalmic lens system any time the Petzval surface does not correspond to the far pointsphere of the eye.

Question 6

Distortion

Answer 6

Affects image size and shape (more than blur), magnification and minification effect of lenses increases as you move away from the optical center causing objects or fields of view to appear curved or misshapen especially with high power lenses
Barrel (minus) and pincushion (plus lenses)

Question 7

Chromatic aberrations

Answer 7

Dependent on wavelength, shorter wavelengths bend more

Question 8

Achromatic doublet

Answer 8

0 = F1/V1 + F2/V2
Combining a positive lens with a negative lens of a different material to eliminate CA

Question 9

Transverse/lateral chromatic aberration

Answer 9

Produces different size images depending on wavelength, related to prism effects
TCA = dF/V or prism/V

Question 10

Longitudinal chromatic aberration

Answer 10

series of points images along axis from differing wavelengths
LCA = P/V

Question 11

Aberrations of most concern in ophthalmic lenses

Answer 11

1. Oblique astigmatism
2. Curvature of field
3. Distortion

Question 12

Tscherning ellipse

Answer 12

Collection of points on graph Base Curve vs Back Vertex Power which shows the best value of the base curve for eliminating oblique astigmatism (radial and curvature of field will be reduced). Ostwald is the flatter curve.

Question 13

Aspheric lenses

Answer 13

Optically correct for lens aberrations, high powered lenses >-23 or >+7, flattens the lens reduces mag, reduces weight of the lens and is useful for progressive designs

Question 14

Point focal lens

Answer 14

A lens corrected completely for radial astigmatism (but not curvature of field)

Question 15

Percival form lens

Answer 15

Lens corrected completely for curvature of field (radial astigmatism not corrected)

Question 16

What to consider when using materials with low Abbe value

Answer 16

Use shorter vertex distance, use monocular PDs, include sufficient panto tilt (less than 10)