The Kenko tubes are excellent.

In fact, in a number of ways they are BETTER than the Nikon tubes (PK-13, 27.5mm and PK-12, 14mm and PK-11A, 8mm).

The Kenko are better because they cost less per tube... you get three in the set for $130: 12mm, 20mm and 36mm. The Nikon tubes are only sold individually and cost $89 or $99 each... To get close to the 68mm of total extension possible with all three of the Kenko, you'd have to spend at least $287 (two 27.5mm and one 14mm, not a very good selection of sizes... other combos are possible, but will cost even more). On price alone, the Kenko are winners by a large margin.

Using them singly or in various combinations, the three Kenko tubes give you seven different possible amounts of extension, for use with a wide variety of focal lengths. I've used them on everything from 20mm to 500mm lenses.

But even more importantly, the Kenko tubes are fully usable with ANY F-mount lens. They have electronic contacts to fully support autofocus and aperture control with "G" lenses (lenses that don't have a built-in ring to set the aperture). The Nikon extension tubes have no electronic contacts, so there is no autofocus and - most importantly - THERE IS NO MEANS OF SETTING THE APERTURE on any "G"-series lenses that lack the aperture control ring (of which there are many). This isn't just "loss of automation". With the Nikon tubes, you will be stuck using a "G" lens wide open all the time, unable to stop it down for increased depth of field (important with close-ups and macro). So it's not just a lack of automation that you can work around... the Nikon tubes are for all practical purposes unusable wit "G" lenses. Not so with the Kenko, thanks to the electronic contacts built into their tubes. I honestly don't know why Nikon has so stubbornly kept making their "old school" extension tubes and never updated them to fully function with their modern lenses. It wouldn't be a big deal to just add the electronics to their tubes, too.

So the Kenko tube functionality IS NOT the same as the Nikon. In fact, the Kenko tubes are BETTER.

And the Kenko tube quality is pretty much equal to the Nikon or Canon tubes. They are well made with similarly strong latch mechanisms. I don't know that it matters very much, but someone told me the Kenko are are plastic now (I use an older set that's definitely metal... cosmetically a little different, but functionality and quality identical to sevearl much more expensive Canon tubes I use, too). Well engineered plastic can be fine, anyway. I think the Nikon tubes are still metal... but I'll leave it to someone else to confirm (I don't have any of the Nikon tubes on hand right now... I had a couple of them stolen during a break-in recently).

Besides the Kenko, here are cheaper third party tube sets... such as the Vello set for around $80, also incl. 12mm, 20mm & 36mm (some other brands instead provide 11mm, 21mm & 31mm). These also have the electronic contacts to provide full functionality with any F-mount lens. While not bad, these are definitely made of lighter plastic and don't appear to have as strong latching mechanism. I'd be hesitant to use them with heavier cameras and lenses.

There are even cheaper"econo" versions of some of the third party, Chinese tubes... typically selling for under $50 (sets of either 12, 20, 36 or 11, 21, 31mm). These use plastic bayonet mounts (like some entry-level "kit" lenses do). They might be okay for occasional use with the lightest of lenses. But I wouldn't expect them to hold up over time or trust them with a more valuable lens.

Heck, there are even dirt cheap extension tubes... under $25... some even $15 or less. These are very simply "dumb" tubes, without any electronic contacts or even the "AI" mechanical linkage such as the Nikon tubes have. With these super cheap tubes, AF and aperture control will be completely manual. PLUS, the lens aperture will actually stop down as it's set to a smaller aperture, dimming your viewfinder along with it (in other words, open aperture isn't maintained until the moment of exposure, the way it is with the Nikon "AI" or "auto indexing" tubes).

All three types of the cheaper, Chineses tube sets sell under a BUNCH of different brand names. I would wager there are actually only two or three manufacturers and all the other brands are just relabeled from those.

I highly recommend the Kenko tubes... Or the Vello for someone who will never be using them with heavy lenses and is on a tight budget.

Tubes are MUCH easier to use than the bellows someone recommended above. Bellows are essentially a variable length extension tube... but they also are rather bulky, fragile and easily damaged, and they have no support for AF or aperture control (same as the "dirt cheap" tubes above). Also, bellows MINIMUM extension is typically around 40mm, while their MAXIMUM is usually around 150mm. With many lenses, this will make for VERY high magnification. With bellows I'd highly recommend a tripod and a precise geared focusing rail. Note: There are some bellows with electronic connectivity to support AF and aperture control with modern lenses and cameras... However, they are quite expensive.... upwards of $500 typically. See "Novoflex" brand. I don't know if they are offering anything that's Nikon F-mount compatible right now.

There also are "helicoids", which are another type of variable extension tubes... the range of extensions possible isn't as great as bellows, but the idea is similar. They essentially are two rigid tubes, one fitted inside another, that allow for some adjustment of the length of extension. I don't see any of these available for Nikon at present, but - if memory serves - typically these give a range of approx. 40mm to 65mm extension.