I originally replied to this a few days ago, but learned that some changes in my email setting meant that my posts didn't go through - hopefully (thanks Doug!) this will now...
Depth of field is a function of magnification, so it doesn't matter how you get to the magnification, DOF is the same. If you keep this formula handy: DOF = 2fc(m+1)/m^2 (that's "m squared" in the denominator). Where - m = magnification f = aperture c = the size of the circle of confusion you want to use to define apparent sharpness, generally considered to be 0.1 to 0.025 mm. (The circle of confusion is this: how big can an out of focus "point" be and still look sharp in you final enlargement. So, we're saying that a theoretical "point" (which of course has no dimnesion) could be out of focus and so grow to 0.033 mm in size and we'll still consider that sharp.) So at 1:1 with an aperture of f8 and a target circle of confusion of 0.033mm, DOF is 0.528 mm. It doesn't matter if you get there with a 50mm or with a 500mm, extension, diopters, or teleconverters - DOF is the same. (Those other things change angle of view, perspective, distance to subject, and exposure factors though.) So... figure up what your magnification is with the diopters and go from there. For me, this was the single most important lesson gleaned from many readings of John Shaw's "Closeups In Nature." It really cuts through some of the confusion regarding DOF and focal length. Hope this helps - MCC - - - - - - - - - - Mark Cassino Kalamazoo, MI [EMAIL PROTECTED] - - - - - - - - - - Photos: http://www.markcassino.com - - - - - - - - - - - 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 .
