New answers tagged extension-tubes
Pentax had a helical extension tube. It is dumb, but it allows changing the tube's length by simply twisting the ring. One can vary the magnification, & if using a zoom, by altering focal length, can refocus. I bought one MANY moons ago & for me it's been quite useful.
Fotodiox is selling am "aperture control enabler" now that's designed to do this for macro reversed lenses. You could use it pointing the standard direction on a manual camera body if you got the right filter thread adapters, though it will mess up the flange-focal distance, so it's probably only useful if you're using a macro lens anyway or your image ...
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