Extension tubes enable a lens to focus closer than its normal set minimum focusing distance
The tubes can also be used for manual exposure control
Nikon Auto-Extension Rings Nikon extension rings can be used singly or in combination to vary the reproduction ratio of lenses from 20 to 300mm. They are mounted between the camera body and the lens. Through the lens exposure control automatially compensates for lens extension. Manual and aperture-priority auto-exposure control modes are possible Manual focus only. Auto extension ring with AI diaphragm coupling. Extension: 14mm. For Nikon F5 and F100.