A spherical aberration is a problem that occurs with a spherical lens. When parallel light rays pass through the central region of the lens, they focus farther away than rays that pass through the edges of the lens. This causes many focal points, creating a blurry image.
With a single lens, spherical aberration can be minimized by bending the lens into its best form. This shape is obtained using the Coddington Shape Factor to calculate the appropriate shape of the lens. In a single lens, however, spherical aberration cannot be eliminated. For multiple lenses, spherical aberrations can be canceled out by over correcting certain aspects of the lens or can be greatly reduced by using a symmetric doublet, such as the orthoscopic doublet.
The concept of principle focal length presumes that all parallel rays focus at the same distance, which is only true if there are no aberrations. Therefore, when using a lens with a spherical aberration, the best approximation for the focal length is the distance at which the difference between the paraxial and marginal rays is the smallest. It is for this reason that a smaller aperture on a camera lens gives a sharper image and greater depth of field.