Lesson 7 - Aberrations

• Explain the meaning of spherical aberration and of chromatic aberration as produced by a single lens.

• Describe how spherical aberration in a lens may be reduced.

• Describe how chromatic aberration in a lens may be reduced.

Spherical Aberration

This problem is due to the spherical shape of lens surfaces.

It occurs because rays that are incident upon the lens far from the principle axis have a different focal length to those incident close to the principle axis:

An idealised convex lens

Convex lens showing spherical aberration.

The effect can be reduced by reducing the aperture of the lens, i.e. reducing the diameter of the hole in front of the lens. This is called ‘stopping down’.

In effect this cuts out the shorter focal length rays coming from points furthest from the principle axis.

Spherical aberration results in uneven focus from the middle to the outside of an image...

Stopping down can cause barrel distortion at the image corners:Out of focus

In focus

Chromatic Aberration

Each wavelength of light refracts different amounts. This results in each part of the visible spectrum having a slightly different focal length for a particular lens:

As a result the edges of an image may appear coloured:

Chromatic aberrations can be eliminated for two colours (and reduced for all) by an achromatic doublet. This consists of a diverging lens stuck to the converging lens:

The two lenses create equal but opposite amounts of dispersion so the two colours recombine at the focal point.

Note: other colours are still focused at different points but the difference has been decreased (see the green ray below):