Each shape has the ability to either reduce or enlarge an image. Inherent in all lenses are aberrations . These can be spherical or chromatic aberrations. In both cases using a single element lens will result in some type of aberration. These lenses are often called objectives.
Spherical aberrations are most common in poorly made lenses.
If a lens is not ground with great care, images will not be brought to an even focus across the entire image plane. Chromatic aberrations caused by different light wavelengths being refracted at different angles. This would leave color halos at the edges of specimens. Old or poorly made lenses often suffered from chromatic aberrations. Apertures One way to reduce the problems of aberrations is to limit the area of the lens being used.