On 8 July 2012 02:40, Stan Halpin <[email protected]> wrote: > I would put the fixed extension tubes on the camera, then the bellows, then > the hellicoid, then the lens.
For a moment I had the same thought, but then I remembered that in extreme lens extension, especially for macro work, the first preference is to focus by moving the whole rig back and forth so you don't disturb the reproduction ratio. The second preference is to move the focal plane, which in this case means the camera body ,which the helicoid tube behind the bellows achieves. Only if all other options are impossible should you rack the lens out, as doing so often means having to make gross corrections to the camera position, as well as risking contact between the lens and the subject. Personally, I think John's rig is probably exceeding best practice for this lens. He's got the lens at about 3.5x focal length extension (231.5mm ~ 251.5mm of tube & bellows plus the 1x focal length inherent in the lens itself) Extending the lens helicoid itself is doing some unknown thing, but it's not adding extension to the main focusing group of the lens, just extending the FREE element, which in reverse position may or may not serve any purpose. For big magnifications an enlarging lens often does a better job because of it's conventional symmetrical design. Modern camera lenses with floating elements and such can quickly get out of their design/performance envelope when they're put into non-standard configurations. That said, a wide angle lens reversed can give good results at spectacular magnifications, although YMMV. regards, Anthony -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
