It's funny, when I saw your first mail on this, my first thought was that 1000 was too little. The book of Hawkins ans Avon (the source of "my" formula) mentions, that the CoC comes from assumptions as to which angle the human eye can detect - the CoC from a viewing distance where this angle will appear.
I see your point of using D, so I kept your formula om my computer. all the best Jens Bladt mailto:[EMAIL PROTECTED] http://hjem.get2net.dk/bladt -----Oprindelig meddelelse----- Fra: graywolf [mailto:[EMAIL PROTECTED] Sendt: 10. april 2004 21:11 Til: [EMAIL PROTECTED] Emne: Re: Zone Focus - what is it? I do not have a 70mm lens to check against. However, I just checked my M50/1.7 at f22 my formula gives 2.2 meters, however the DOF scale give 1.5 times that or 3.3 meters. That would be using a 1500 multiplier which is being conservative, but matchs your .021 CoC figure. Where the 1000 comes from is that for most people the eye can distinguish a 1/100th inch (.25mm) point viewed from 10 inches (250mm), 10in/0.01in, or 250mm/.25mm = 1000. The 1500 (your .021mm) comes from what almost no one can distinguish. Which value to use is simply a choice of how critical you want to be (most people, or everybody). 10 inches is the closest distance most people with normal vision can focus. Thus we come up with an 8x10 print viewed at 10 inches for our basis. The CoC on film is that number divided by the magnification required to make an 8x10 print. In the case of 35mm film, about 8x. 0.25 / 8 = 0.03mm my formula, or 0.17 / 8 = 0.021mm yours. It seems to me easier to forget the film and figure for the print thereby reducing a couple of variables and being able to use the same formula for all formats up to 8x10 film, even digital. So using 1500 * D for a 70mm at f22 should give a number matching yours. It does not, it gives 4.77 meters. I have been thinking for some time of doing some experiments to prove or disprove the various formulas for DOF, and hyperfocal distance, that seem to be out there. However, 1000 * D has always worked to my satisfaction. Do note that I have seen a lot of formulas, especially on the web, that have N (f-stop) in them when it should have been D (aperture). Comes from mistakenly using thinking they mean the same thing, I believe. I know that for years I tried using such a formula and it never seemed to work, until I had that "Ah ha!" experience, and plugged in aperture where it said f-stop. Actually that particular formula was correct, but where it said A=aperture, I applied my own ignorance and inserted f-stop. I am sure others have done the same, and then clarified it as A=f-stop in their version. Now I use D for "aperture Diameter", N for "f-stop Number to avoid that confusion. Your formula is: F^2 ------- that gives 5.4 meters for the M50/1.7 at f22 mentioned above. 0.021*N -- Jens Bladt wrote: > So in my example with 70mm f22 H = 1000 * (70/22) = 3182mm (3.2m) which is > considerably less than 10.6m...? -- graywolf http://graywolfphoto.com/graywolf.html
