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From the reading I have done on extension tubes, I understand that some are just 'dumb' tubes while others have electronics that may allow for metering and autofocus to still work. Further, from this question I gather that not all manufacturers or extension tubes are created equal.

What should I look for when shopping for an extension tube set?

  • Does Pentax sell their own or officially support a certain brand?

  • Are there certain known manufactures to look out for?

How big a difference is there really between a cheap extension tube set and an expensive one?

  • Does autofocus really work with 'smart' extension tubes?

  • Does it make a difference (for the auto features) what lens is being used?

  • Are electronic extension tubes even worth it, or should I just put the money toward a macro lens?

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2 Answers 2

up vote 6 down vote accepted

Pentax does make their own extension tubes, but they are not AF-capable. You can get a set of tubes (12, 19 and 26mm) or separate 50mm and 100mm tubes that are auto-aperture, as well as a helicoid (adjustable) 26-46mm tube that requires setting the aperture manually on the lens. If you want extension tubes strictly for shooting macro (and want to stick to the Pentax brand name), then the Extension Tube Set K will get you where you want to go.

Since the KA mount is compatible with the old K mount, there is a large catalog of "dumb" tubes out there to choose from, some selling for under ten bucks brand-new.

There is at least one reputable third-party manufacturer, Kenko, offering autofocus-capable tubes for the Pentax mount. If you just want a tube to get a little closer than the close-focus limit of one or more of your lenses, then it's worthwhile getting tubes with AF coupling. You'll lose infinity focus with the tube mounted, of course, and you may find that some lenses are very badly behaved inside their design limit, but when a tube and lens combination works well, it is a glorious thing indeed.

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I'm not positive, but I suspect that the AF-capable tubes only work with screw-drive AF lenses. That's fine for most lenses, but if you have an SDM-only lens, they may not provide AF functionality. –  coneslayer Apr 21 '11 at 13:30
    
You can also come across the bellows set made by Pentax on sites like Craigslist from time to time. I usually end up not buying it and then kicking myself later... –  John Cavan Apr 21 '11 at 13:40

Auto-focus is quite useless in macro shooting anyway - the depth of field is very thin and it's hard for AF to hit. Focusing (at least roughly) is done by adjusting distance between camera and subject.

More important is ability to control aperture from camera - most modern lenses do not have an aperture ring. You'll want to have the aperture wide open for focusing and small aperture when taking the actual shot (to have a deeper DOF).

Also, an extension tube with electric connections would let you record lens information along with your photos.

Optically, there is no difference between advanced and cheap extension tubes.

The cheapest extension tubes work well with old vintage lenses that would not gain from any connections anyway.

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3  
Extension tubes are not just for macro shooting. They're often used to focus inside the close-focusing distance of longer lenses for wildlife and beauty shots as well, so autofocus does matter, at least on the shorter tubes. –  user2719 Apr 21 '11 at 12:36

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