Aberrations

Coma

- occurs when oblique rays are refracted by a large-aperture optical system. affects the sharpness of image points - rarely a problem with spectacle lenses (b/c the limiting effect of the pupil)

Vogel's Approximate Base Curve formula: minus lenses

SE = spherical equivalent unit: diopters

Vogel's Approximate Base Curve formula: plus lenses

SE = spherical equivalent unit: diopters

Zernicke polynomials

a more complete representation (than Seidel) of aberrations that could be present in an eye or lens - a popular system to describe and measure ocular monochromatic aberrations - does not assume spherical surfaces

Corrections of spherical aberration

aspheric surfaces can be used - these also reduce oblique astigmatism and distortion

Chromatic aberration

normally represented as dispersion (dispersive power) or abbe value - two types: longitudinal (axial) and transverse (lateral)

Spherical aberration

occurs when a pencil of light is refracted by a large aperture optical system, which occurs because different zones of the aperture have different focal lengths - more important for lenses of high power (+10D or more) - may be controlled by using aspheric surfaces

What do you need to know the calculate the approximate axial chromatic aberration of the human eye?

power of the eye and Abbe number of the eye

spherical aberration vs coma

spherical aberration occurs for beams of light parallel to the optic axis; coma occurs for oblique beams

defocus

spherical refractive error (simple myopia or simple hyperopia)

Contributors to wave front error

the eyes' entire optical system may contribute to higher order wave front error - cornea - lens - vitreous - retina

Wavefront

the outer/"leading" border formed by light rays diverging from their point of origin

3rd order Zernicke Terms

- Coma: often described by patients as a "comet" when looking at a light - Trefoil: less problematic in terms of visual contrast concerns

Tscherning Ellipse

- Shows the best values for BCs to eliminate oblique astigmatism, distortion, and curvature of field - The ellipse varies based on viewing distance, lens material **Made of two curves - Wollaston and Ostwalt (the flatter one we use)

Oblique astigmatism

- aka "radial astigmatism" or "marginal astigmatism" - occurs when oblique rays are refracted by a small-aperture system and affects both sharpness of image points and image position - inability of a lens to form a point image of an oblique point object - interval of sturm, 2 line foci and the circle of least confusion **same idea as lens tilt!

Longitudinal aberration

- aka Axial chromatic aberration

Transverse aberration

- aka lateral aberration

Curvature of Field

- aka power error - occurs when light entering the peripheral areas of a lens does not properly focus on the far point sphere (FPS) -> instead focuses on Petzval Surface (PS)

Bichrome test

- aka red-green imbalance - sphere refinement testing that utilizes the ACA to create an interval of sturm's

Reduction of chromatic aberration

- chromatic aberration cannot be eliminated in an optical element made of a single material - achromatic systems (lenses) --> two elements (doublet) of different materials that produce equal but opposite dispersions can be used to reduce chromatic aberrations

Chromatic vs monochromatic aberrations

- chromatic aberration is caused by the material from which the lens is made - monochromatic aberrations occur when incident light is not confined to paraxial rays

Higher order aberrations

- defined as any refractive error that cannot be corrected by spherocylindrical lens combinations - ex) coma, spherical aberration, chromatic aberration - higher order aberrations make up ~20% of total refractive error

Seidel aberrations

- depends on lens diameter, object size, and lens position - independent of wavelength - full correction of one aberration requires correction of all previous aberrations (they are all inter-related) - assumes spherical surfaces

Signs of non-optimized vision

- double images - low contrast, lack of crispness - lack of color perception - glare sensitivity - night driving problems - "Halos", "starburst patterns", "comet's tails" around lights at night - compromised far and near vision

Why aren't most higher order aberrations an issue?

- in spite of the fact that we can study many orders of aberrations, the human eye is able to perceive distortion effects only up to 4th to 5th order - clinically relevant aberrations (mainly 3rd and 4th) can degrade quality of vision regardless of defocus and astigmatic correction

Why do we need the Zernicke polynomial system? (rather than just Seidel)

- its has been found that refractive surgery may alter/increase higher order aberrations; this realization has created a need for controlling and managing them in an effort to maximize post-surgical visual performance - if higher order aberrations can be netter measured (even in non-surgical patients); a logical follow-up would be to find a way to correct them

When is distortion a problem?

- mainly a problem for lenses of high power - aphakes - can be reduced with aspheric lenses

Monochromatic aberrations: geometrical optics assumptions

- monochromatic assumptions - rays of light involved in image formation are confined to a small cylindrical region immediately surrounding the optical axis (paraxial region) - "first order optics"

Distortion

- occurs when the magnification of an extended object varies with its distance from the optical axis - distortion affects image shape and lateral position, but not image clarity - inability of a lens to form an image of the same shape as the object - when the ratio of the image size to the object size has a constant value for all object sizes, no distortion exists and the conditions of orthoscopy exists

Tscherning Ellipse: Ostawalt Form

- shallower (flatter) form - becasue most near vision tasks are relatively coarse and best acuity is normally demanded at distance, lens manufacturers have restricted their lens design to lens form requires for distance vision - used as basis for lens design for modern (but "traditional") ophthalmic lenses

4th order Zernicke terms

- spherical aberration: often described as a halo by patients - quadrafoil: less debilitating since the central aspect has limited change - secondary astigmatism: adversely affects vision quality and was once referred to as a "central island"

Tscherning Ellipse: Wollaston Form

- steeper form - eliminates oblique astigmatism and distortion - more expensive, difficult to manufacture and cosmetically poor - fell into disuse by the end of the 19th century

2nd order Zernicke Terms

- the second order terms are defocus and astigmatism - in a normal ametropic eye, they are the largest contributor to wave front error - normally corrected by the ophthalmic spectacle Rx

What is the base curve?

- the surface curve of a lens that becomes the basis for which the other remaining curves are calculated - usually the front surface spherical curve of the lens - chosen to reduce/eliminate monochromatic aberrations

Factors that control Coma

1. aperture size 2. lens form 3. angle of obliquity

Types of Seidel aberrations (monochromatic aberrations)

1. spherical aberration 2. coma 3. oblique astigmatism 4. curvature of image 5. distortion

what is the abbe value of the eye

45

what is the power of the eye

60 diopters

what is the approximate ACA of the eye?

60/45 = 1.33 Diopters

Distortion: pincushion vs barrel

As long as the pupil of the eye acts as a stop behind a spectacle lens... - pincushion = plus lenses - barrel = minus lenses **note: the natural distortion of the lens in the eye is barrel, even though the lens is plus b/c pupils is in front of it.