NIKON AF-I TELECONVERTER TC-20E 2x
2× rear teleconverter for Nikon F
US 5,253,112 A, Example 1 — Kenzaburo Suzuki, Yoshinari Hamanishi
7 ELEMENTS / 6 GROUPS | 2× REAR CONVERTER | ALL SPHERICAL
A dedicated converter: it mounts only on lenses made to accept it. A teleconverter has no aperture stop of its own, so it is shown attached to a host lens, where the lens's stop stays the system stop and the focal length and f-number both grow by the converter's factor.