1. 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.
2.
3. 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.
4. 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.
5. 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
6.
7. 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:
8. As a result the edges of an image may appear coloured:
9. 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:
10. 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):
Editor's Notes
Conceptual ray diagrams of ideal and spherically aberrated lenses. A perfect lens (top) focuses all incoming rays to a single point on the optic axis, but a real lens with spherical surfaces (bottom) focuses different rays to different points along the optic axis, depending on the radial position of each incoming ray.
Conceptual ray diagrams of ideal and spherically aberrated lenses. A perfect lens (top) focuses all incoming rays to a single point on the optic axis, but a real lens with spherical surfaces (bottom) focuses different rays to different points along the optic axis, depending on the radial position of each incoming ray.
Link to info on distortion: http://toothwalker.org/optics/distortion.html
Chromatic aberration. Top image shows a photo taken with a built-in lens of digital camera (Sony V3). Bottom photo taken with the same camera, but with additional wide angle lens. The effect of aberration is visible around the dark edges (especially on the right). The images show only a part of the photo from the corner of the original ones (to emphasize the effect of aberration).
