Telephoto lenses are constructed using internal lens groups.
Telephoto lenses allow photographers to get close to their subjects without having to be physically closer to them. There are some instances when it is not possible or safe for a photographer to get up close with subjects. Shooting sporting events as well as wildlife usually require photographers to use a telephoto lens. Telephoto lenses are constructed with internal lens groups that magnify the subject.
Positive Lens Groups
The positive lens group consists of the lens elements contained at the front of the lens. The focal length of a lens is determined by the distance of the optical center from the focal plane of the film or sensor when the lens is focused on the subject at infinity. In using more than one lens the positive lens group reduces the physical length of the lens.
Negative Lens Groups
Also known as the telephoto group, these lenses are found at the rear of the lens. These lenses take the image collected in the front group and perform the magnification of the image that eventually appears on the film or digital sensor.
Multiple Lens Elements
Both the positive lens group and the negative lens group will have multiple lens elements that are designed to reduce aberrations or other distortion if the image was only collected through one lens. In some cases longer telephoto lenses have the option of using an internal filter known as a drop-in filter. On smaller lenses these filters are normally attached to the front of the lens. With longer lenses the front element is too large or too far away to allow the photographer to adjust the filter, in the way a photographer adjusts a polarizer filter.
Tags: lens group, positive lens group, Telephoto lenses, front lens, internal lens, internal lens groups