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.
Do you happen to have the actual formula that you used?
A couple ting - the formula I'm using (which comes from the "Close Up Photography" volume in the Kodak Workshop Series) clearly uses the f-stop number (i.e. "11" for f-11) in the example. I've tested it with macro setup up and it seems to work. If you simply substitute the aperture size the logic of the equation completely falls apart. So I assume you must of been using some other equation.
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.
The equation I cited has f-stop, magnification (which is the product of focal length and subject distance) and circle of confusion size (which is picked to support the enlargement ratio.) So the same factors are in there.
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.
If you don't mind showing me how aperture diameter (as opposed to f stop) fits in, I'd like to see.
- MCC
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Mark Cassino
Kalamazoo, MI
[EMAIL PROTECTED]
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Photos:
http://www.markcassino.com
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