Lenses are different from other transparent objects because of their ability to focus light. There are many types of lenses and the spherical lens is one of them. The spherical lens is a lens whose surface is the shape of part of a cylinder or sphere. The unique quality about this lens is that it does not bring parallel rays of light to a common focus but rather the focus depends on the distance from the center of the lens and the rays that pass through the edges of the lens come to the focus closer to the lens than the light passing through the center. This creates a focus shift; an issue that photographers are quite familiar with. Photos taken with the diaphragm wide open are out of focus because the image is dominated by light that passes through the lens far from its center. This is because focus falls far behind the film plane.
This is a very dominant imperfection of the spherical lens. The collimated light, refracted by the spherical lens doesn’t converge at the same focal point after it goes through the lens. This happens because the refraction of light rays increase with the distance to the optical axis of the lens. The effect of spherical aberration is a blurred image. With that in mind, how do you improve focusing quality of the spherical lenses?
Add an aperture
This solution works on the principle that when you reduce the incoming beam, the focusing of the spherical lens will be improved by reducing the spherical aberration effect. One of the simplest ways of reducing the spherical aberration is to add an aperture, stop or hole in front of the lens. With the placement of the aperture, you will be able to block the rays that end up blurring the image. The main disadvantage to this solution is that you will get significantly reduced light throughput of the spherical lens.
Adjust the focal length
Focus shift in photography results when the focus point falls behind the film. This is a problem that can be solved by adjusting the focal length. The variation of the position of the lens will affect the diameter of the focus.
Using aspheric lens
This is a lens whose surface profile is not part of a cylinder or sphere. This lens is used to correct spherical aberration which is common on the spherical lenses like double-convex and plano-convex lens shapes. The lens works by focusing the light to a small point thus creating virtually no blur and in so doing improve focus. The aspheric lens is designed to reduce spherical aberration by adjusting the conic constant and the aspheric coefficients of the curved lens surface.
Apart from the major problem of the spherical aberration, the spherical lenses have many uses. They are used to form images, to focus and also to collimate or expand light. They are used in virtually all optical systems from the deep-UV lasers to the low-cost condensers that deliver light to the detector. When shopping for the spherical lens, it is good to remember that lens performance will be determined by the material, shape and the manufacturing quality.