Opteka Auto Focus DG Macro Extension Tube Set for Select Nikon SLR Cameras Extension tubes are designed to enable a lens to focus closer than its normal set minimum focusing distance. Designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with Nikon SLRs Contains three tubes of different length, a 13mm, 21mm, and 31mm Glass-free; Lightweight 1 Year Opteka Warranty