extension ring

extension tube

noun
  1. photog a spacer element that can be fixed between the camera body and the lens to increase the distance between film and lens and allow closer focus than would be possible without it
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012