Wrong! You are confusing aperture (diameter of the opening) with f-stop (focal length divided by aperture). Also since DOF is normally figured for 8x10 prints viewed at 10 inches you can use 0.25mm (1/100 inch) as your COC for all calculations.
At f8 a 500mm lens has 10x as large an aperture as a 50mm so your DOF will be 1/10 that of the 50mm. Ciao, Graywolf ---------------------------------------------------------------- ----- Original Message ----- From: Mark Cassino <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, February 19, 2002 11:37 AM Subject: Re: DOF with close up lenses > 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 . - 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 .
