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Defects of Lenses
Spherical Aberration
The curved surface of most lenses is a small section of a sphere.
This is not the ideal shape and the resulting problem is called spherical aberration.
After refraction by the lens, the rays do not all pass through the same point.
Rays further from the principal axis are refracted so as to meet nearer to the lens than those which are close to the axis (see diagram below, in which the effect has been exaggerated).
 
All spherical lenses suffer from this problem but it is particularly noticeable if plane-convex lenses are used "the wrong way round".
The problem is most evident when we rely on one surface to do all the refracting of the light.
Imagine this lens is being used as the objective of a telescope.
A telescope is used to look at distant objects (no, really, it's true!) so the light from any point on the object being observed arrives at the lens in a parallel beam.
This means that the first (plane) surface of the lens does not deviate the light and the two image points are quite far apart.
 
If we turn the lens round then both the surfaces do some of the deviation of the light and as a result the spherical aberration is much less of a problem, as shown below.
However, for the same reason (sharing the refraction between the two surfaces of the lens), if the object is placed relatively close to the lens, the spherical aberration is reduced by having the curved face towards the object, as shown here.
 
Of course, image 1 and image 2 are just two extremes, there is actually a whole range of image points in between.
If the object is a point source of light then the image of that point would be a blurred circle of light.
 
An alternative method of reducing the spherical aberration of a lens is simply to reduce the aperture using a diaphragm (also called an iris or a simply a stop).
Although this very effectively reduces the spherical aberration, it also reduces the brightness of the image because less light passes through the lens.
 
Chromatic Aberration
The refractive index of the material of which a lens is made is different for different wavelengths (colours) of light.
Some dispersion of the light occurs, as with a prism.
The resulting false colouring of the image is called chromatic aberration.
This effect can be reduced by having a combination of a convex and a concave lens made of glasses having different refractive indices.
The dispersion caused by the convex lens is just cancelled by the dispersion caused by the concave lens.
 
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