Doubler

n. person or thing that doubles, person or thing that increases by two

A teleconverter is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens. For example a 2× teleconverter enlarges the central 12×18  mm part of an image to the size of 24×36 mm. Teleconverters are typically made in 1.4×, 2× and 3× models.
See more at Wikipedia.org...

Noun
1. an electronic device that doubles the voltage or the frequency of an input signal
(hypernym) electronic device

v. double, understudy, line, pass, round

(n.)
One who, or that which, doubles.
  

(n.)
An instrument for augmenting a very small quantity of electricity, so as to render it manifest by sparks or the electroscope.