Almost, guys. DOF is based on the diameter of the aperture.
"f" is a "function" of relative light availability, not DOF. It is an exposure function only. Take your 50/1.4 lens. (roughly speaking) Lens diameter x 1.4 = film plane diagonal. A lens of f1.4 on a 4x5 would be about 4.5" in diameter! Put that same 1" aperture on a 4x5, 150mm lens, and it's (again, roughly) f5.6. DOF is the same, but light transmission changed. As a consequence, with the same aperture, shutter speed is longer with the same opening -- because the film plane is larger and needs more light to cover it. To get roughly the same DOF between formats, shooting 35mm @ 1/125 @ f8, you'd be using 1/8 @ f32 on 4x5 or 1/30 @ f16 on a 6x7. Roughly 2 stops difference from format to format. Don't confuse aperture with DOF. The relationship is only an indirect one. Collin - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
