No, Mark, f-stop is focal-length divided by aperture-diameter. So, by definition focal-length divided by f-stop is aperture-diameter (that is, a 100mm lens at f4.0 has a 25mm aperture).
Actually, that formula in the original form had the value f = aperture. The problem with that is most of us erroneously use f-stop and aperture interchangeably, so when we see f = aperture we, without thinking it through, tend to stick in f-stop. It took me weeks to figure out why my calculations weren't matching the tables I had. When I finally had an "AH-HA!" experience and changed it to aperture diameter, everything worked. That is why I am so conscious of the difference now, aperture-diameter really sticks in my mind after all that. The formula you quote is factored down into the simplest form. Most DOF formulas you encounter have f-stop, focal-length, subject-distance, and enlargement-ratio in them. They, of course, all factor down to aperture-diameter, and magnification. The formula you show is the one that proves aperture-diameter, magnification, and COC are the only things that actually affect DOF. Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ----- Original Message ----- From: "Mark Cassino" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, July 24, 2003 12:27 PM Subject: Re: Shallow DOF with 6X7 lenses (was: 6x7 lenses - Brotherhood comments solicited) > At 09:32 AM 7/24/2003 -0400, T Rittenhouse wrote: > > >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. > > I'd be interested in playing around with aperture diameter in the equations > to see how it works. > > How do you determine the aperture diameter (short of dismantling your > lenses and measuring)? F-stop is focal length divided by the diameter of > the front of the lens. I've seen lenses where the aperture is directly > behind the front element, and for those you could just divide the focal > length by the f stop an have the aperture diameter. But with the aperture > located in the rear of the lens it is smaller than the front element in any > case - so how do you determine it's size? > > - MCC > - - - - - - - - - - > Mark Cassino > Kalamazoo, MI > [EMAIL PROTECTED] > - - - - - - - - - - > Photos: > http://www.markcassino.com > - - - - - - - - - - > >
