Secondary lens

The resulting combination has a greater focal length than the primary lens alone, and the same absolute aperture, so its f-number is increased accordingly.

Some teleconverters are dedicated or matched to particular primary lenses, normally one or more lenses by the same manufacturer as the teleconvertor, and when used in this combination deliver quality comparable to the primary lens.

Others are general-purpose, and in use typically offer less image quality then a primary lens of the same quality, but allow a camera kit to offer a wider variety of focal lengths with a smaller number of lenses, saving in both cost and weight.

It is not normally possible to construct a similar wide-angle adaptor, as the angle of view of the primary lens is matched to the film format for which is designed, and unwanted light is typically stopped by baffles.

However active adaptors are available containing a (not very powerful) teleconverter, which makes the combination mechanically possible, albeit with a small amount of cropping at the image edges and loss of connection of the camera to any lens automation.