Nikon Auto Extension Rings PK and PN fit between the camera body and lens either individually or in combination. The extension rings have no optics. The PK and PN rings are used in manual focus mode and are considered non-CPU lens attachments.
The PK-13 Auto Extension Tube allows a lens to focus closer than its normal set minimum focusing distance. The PK-13 provides 27.5mm of extension. The rings can be used alone or in combination.
An AI Nikkor will preserve automatic diaphragm and meter coupling functions, letting you compose, focus and meter at wide apertures.