Shooting with extension tubes is very different from using a macro lens. With a macro lens; the close you can focus, the larger the subject will be (more magnification).

I have a Sigma 180/3.5 macro and find it very nice, but a combination of 36mm extension and a 70-300 VR is nicer – due to allowing more adjustment to working distance and magnification.

The focusing ring becomes a bit of a “working distance adjustment” and the zoom becomes a magnifying adjusment. Focusing is done by moving the camera front or back. Auto focus is hardy of any practical use – move it away from the shutter button or you’ll go nuts.

Now…
From experience I’d say that to be able to shoot at reasonable working distance and usable DOF for a single exposure you need to keep extension down below the focal length you have chosen. Better yet at half the focal length or less.

For multiexposure and focus stacking techniques you need a sturdy tripod and a focusing rail.

With a Nikkor 16-85 you will be very close to your subject;
36mm extension and f=50mm will allow about half of an AA battery to be depicted almost all in focus along the image diagonal, while leaning the lens against the table just beside the battery. That is the closest focusing distance, focusing ring at close range end.

Turn the focusing ring the other way, to the infinity-end and you’ll be at 10cm distance with the front element, and the entire battery visible in the frame.

en.wikipedia.org/wiki/Macro_photography – tells a bit more details.
en.wikipedia.org/wiki/Lens_(optics) – more on lenses in general

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