There are many sizes, shapes, and qualities of telescopes and binoculars available. An understanding of the following optical principles can help a person select the best equipment for the job and for the money. Telescopes and binoculars collect much more light than the eye. They then magnify the resulting image with an eyepiece. Curved surfaces of lenses or mirrors focus a large amount of light by bending or reflecting incoming light rays so that they come together at the focal point.
Focal distance is measured from lens or mirror to focal point.
Excessive curvature decreases performance for two reasons. High curvature, high quality lenses or mirrors are very difficult and costly to manufacture, so these components are of lower quality in inexpensive telescopes. In telescopes with larger lenses or mirrors, excessive bending or diversion of light is avoided by focusing light at larger distances from the primary. Large focal lengths reduce the need for drastic curvature. Large focal ratios provide high magnification and small fields and are best for planetary viewing.