How *Auto* Are Pentax Auto Extension Tubes?

Question

While asking for extension tubes for a Pentax DSLR, Pentax replied:

As requested, please find the following extension tubes available. Product 30535, auto extension tube set B Product 30551, life-size auto extension tube B 50mm w/case Product 30556, life-size auto extension tube B 100mm w/case

After searching for these products online I found different information regarding them. One forum post somewhere said the only auto part is that the meter at the widest aperture and stop down to the smallest. Is this true?

My intuition is that extension tubes stop down to whatever aperture is set in the camera. Perhaps they would be able to autofocus too but I do not care about that :) Only that there is still control over aperture without a lens with an aperture ring.

Does anyone know how the automatic aperture and metering work with these tubes?

Answer 1

As far as I know, the Pentax-brand tubes rely on an aperture set on the lens, so while they'll work with manual-focus K-mount lenses and most of the FA-series, they won't let you work properly with the D-series lenses that don't have an aperture ring. They are "auto" in the sense that they allow you to focus and meter wide-open, then stop the lens down to the set aperture mechanically, but they don't appear to have the contacts necessary for electronic aperture control. (The newer digital-intent lenses are essentially stopped-down to minimum aperture mechanically all of the time, as you would have to do with a lens having an aperture ring if you wanted to use the aperture setting on the camera body.)

Kenko used to make a proper set of automatic tubes for the Pentax that gave full electronic aperture control and screw-drive autofocus, but while they still show up on the used market from time to time, they appear to have been discontinued. There are, of course, any number of off-brand (Chinese) tubes out there claiming to be auto, but they tend not to give away too many secrets in their ads and placements, and I'd suspect they're just K-mount mechanical linkages since I can't see anything that looks like electrical contacts in any of the pictures.