Pentax lenses and extension tubes

greenspun.com : LUSENET : Pentax 67 SLR : One Thread

I plan on trying out my 90mm LS and 200mm lenses with the pentax extension tubes..here is my question...since most lenses are optimized for infinity, is it not best to use the appropriate size tube ( and still get the magnification you want ) while still trying to keep the lens at infinity focus ?

-- dan colucci (dcolucci@aol.com), April 30, 1999

Answers

Keeping the lens at infinity with tubes for macro work will not make any difference in optical performance compared with having the lens focused close. Optical performance is slightly degraded on non-macro lenses when used up close because they are not corrected for spherical aberration at those distances. Whether you separate the lens from the film plane via an ext tube or by lens barrel focusing, the result will be the same. Focusing for these two lenses is accomplished by moving the entire optical barrel, there are no independantly moving elements. SR

-- Steve Rasmussen (srasmuss@flash.net), April 30, 1999.

I have been using the helicoid extension tube with the 45, 90 f4 (not LS) and 300 lenses. Results have been good and the variation avalable with each lens is nice to have. DOF is miniscule unless well-stopped down. Lack of automation is no problem since I stop down to see DOF anyway.

-- Tom Goodrick (tgoodrick@earthlink.net), May 14, 1999.

Moderation questions? read the FAQ