Actually, DOF is controlled primarily by the aperture opening IRRESPECTIVE of the focal length. A 1" diaphram opening on a 50mm lens gives the same DOF characteristic as a 1" opening on a 300mm lens. It's modified by lens design but defined by the opening size. That's why f1.4 on 50mm is close the same DOF as 2.8 on a 6x7 lens.
It also confuses us when going between formats. Collin -- -------------------------------- Toward the end of his life, George Truett was asked what he would do differently if he had his life to live over again. He said, "I would have spent more time with a few people." quoted from Discipleship Call Perspective http://disciplescall.gospelcom.net/perspective/ --
