Your verbal explaination is great, Mark. But there is a bit of a problem with your math. Simply put f-stop is a light transmission factor, not and not the same thing as aperture size. I know I played with that same formula for a long time, and it did not work until I realized that. Change f (f-stop) to a (aperture diameter) and it works fine.
Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ----- Original Message ----- From: "Mark Cassino" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]> Sent: Thursday, July 24, 2003 8:38 AM Subject: Re: Shallow DOF with 6X7 lenses (was: 6x7 lenses - Brotherhood comments solicited) > At 04:28 AM 7/24/2003 -0700, Steve Larson wrote: > >Why do 6X7 lenses have a shallow depth of field even stopped down > >all the way? Chris mentioned it with the 165/2.8. Are they all like that? > > I could understand it if it was at close focus with tubes WO, but for a > >landscape shooting at infinity? Please enlighten me. > >Steve Larson > > The simple answer is that 6x7 lenses don't have a difference in DOF > compared to 35mm lenses, but you are using the lenses differently to > accommodate the larger format. > > DOF is a function of magnification and aperture size. You really learn > that with macro work where the DOF at any given magnification (say 1:1) is > the same regardless of the focal length used to take the shot. > > With the larger negative on a 6 x 7 you need more magnification (a longer > lens) to frame up a shot the same way as you do with 35mm. Let's say you > are shooting a landscape with the lens set to infinity. You get it nicely > framed up on a 35mm with a 50mm lens. Deciding to shoot that scene on your > 6x7, you find that you need a 110mm lens (or so) to frame it up the same > way. That's because the lens is projecting the image onto a larger piece > of film, so you need more magnification. That increase in magnification > results in a decrease in DOF. > > The reverse hold true when shooting with small formats - e.g. digicams with > their really tiny sensors. A frame filling shot that would be 1:1 on 35mm > is more like 1:4 on the digital, and so the DOF is seemingly larger. But > again, the magnification is the driver. > > If my explanation is not clear, play around with the math. The basic > equation is: > > d = 2fc*(m+1)/m^2 > > where d=dof, f = f stop, c = circle of confusion size, and m = magnification. > > - MCC > - - - - - - - - - - > Mark Cassino > Kalamazoo, MI > [EMAIL PROTECTED] > - - - - - - - - - - > Photos: > http://www.markcassino.com > - - - - - - - - - - > >
