Ophthalmic Lens Aberrations OPTOM FASLU MUHAMMED

• In actual world, the eyes turn behind the lens to view through off-axis visual points and it is then that the form assumes importance.

Lens Aberrations

•There are six major lens aberrations that work against obtaining a perfect image through the periphery of the lens.

• CHROMATIC ABERRATION• SPHERICAL ABERRATION• MARGINAL ASTIGMATISM• CURVATURE OF FIELD• COMA• DISTORTION

CHROMATIC ABERRATION

•Chromatic aberration is a defect in the lens in which the various colours of the spectrum are not brought to the same focus.

• Blue light is refracted more than the red light when it passes through a lens. The result is out of focus image.

•The wearer complains of peripheral colour fringes around the object which is more pronounced off-axis.

•The higher the power of the lens—the greater is the chromatic aberration.

•Chromatic aberration depends upon the material of the lens. Since, the lens materials have a different refractive index for each wavelength.

•The lens will have a different focal length for each wavelength. The refractive index is larger for blue than the red wavelength, so focal length is less for blue than the red.

Importance

•Since chromatic aberration occurs because the refractive index of the lens material varies with the wavelength of the incident light.

• It gives rise to what is called the Abbe value of the lens material which is denoted by V value.

•Higher Abbe value implies low chromatic aberrations and vice versa.

•So polycarbonate lens with Abbe value of 30 causes more chromatic aberration than CR39 lens with Abbe value of 58.

Correction1. The easiest solution to minimize chromatic

aberration is to change the lens material to higher abbe value.

2. Careful placement of optical centre with monocular pupillary distance and its height in a small frame may reduce the chromatic aberration.

3. Reducing the vertex distance may also result in minimizing the effect of chromatic aberration.

4. Anti-reflection coating with consumer education may also be tried to minimize the effect of chromatic aberration.

5. The best solution is the Achromatic lens system.

Achromatic lens•The achromatic lens uses two different lens materials – one has a regular focal length and the other corrects the dispersion of the first lens.

•For this purpose one lens is made of crown glass, i.e. low dispersion, while the other is made of flint glass, i.e. high dispersion.

Ocular Chromatic Aberration

SPHERICAL ABERRATION•Spherical aberration is an axial and wide beam

aberration.

•The light rays from the peripheral edge of the lens are refracted to a greater degree than the light rays passing through the center of the lens.

•Peripheral rays bend more than the paraxial rays. This creates a slight blurring of the image that is minimized by the size of the lens.

MARGINAL ASTIGMATISM

MARGINAL ASTIGMATISM•Marginal astigmatism or oblique astigmatism

aberration is a small angle aberration.

•When a narrow beam of light enters obliquely to lens axis of a spherical lens, the refracted rays become astigmatic.

•The emerging rays, instead of uniting in a single image point, form two foci at right angles to one – another with a disk of least confusion.

Correction• It is much worse in bi-convex and bi-concave lens

than meniscus lens form.

• It may be reduced by the use of an aspheric surface or by a suitable choice of lens bending.

•Proper use of pantoscopic tilt with optical centre height may help reducing marginal astigmatism.

COMA

•Oblique rays passing through the periphery of the lens are deviated more than the central rays and come to focus near the principal axis.

•The result is unequal magnification of the image formed by the different zones of the lens. The composite image is not circular , but elongated like a comet and coma.

Correction

•The effect of cometic aberration can be minimized by using parabolic curves.

•Aspheric lens design helps reduce coma in high plus power.

CURVATURE OF FIELD

DISTORTION

•Distortion is another aberration of thick lenses. In distortion the object is sharply imaged but does not retain its shape.

Barrel Distortion

Pincushion Distortion

Barrel Distortion

•Barrel distortion is produced in minus power lens where the rays in the centre are more magnified than the rays farther off-axis.

•This is due to minification of corners of a square grid more from minus lens.

Pincushion Distortion

•Pincushion effect is produced in plus lens where the central rays are less magnified. This is due the magnification of corners of a square object more from plus lens

Thank u……