Welcome to diffraction wonderland. Been there, got the t-shirt. Even at f32 images are A LOT less sharp than at f16.
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Sorry Lee, you are wrong on this. When I first started working at Kodak and I was studying optics, I asked this question based on experience. One of the optical engineers sat down with me and Smith's book on optics and started with two equation. Substituting one into the other, with in two lines, the focal length drops out of the equation. I wish I had taken notes on it, but the bottom line is that for the same image size [magnification] the focal length is not a variable and the theoretical depth of field is the same for all lenses.
Search my posts, I have covered this in great detail in the past.
Steve
I have to agree. The 420mm lens I am using gives a long working distance between lens and subject only. DOF does not change. I checked my SPSE handbook and lens focal length is not used in the equations.
OK, let's try this one on for size: using the Schneider DOF spreadsheet calculator and two lenses, a 50mm focused at 50 ft, and a 100mm lens focused on the same subject, but from 100ft away to provide equal magnification. ( http://www.schneideroptics.com/softw...Calculator.xls )
The Schneider DOF calculator (which I keep set for my preference of a CoC of 0.028mm) yields:
50mm lens focused @ 50 ft @ f:5.6
Near 25.56 ft
Far 1132.62 ft
100mm lens focused @ 100 ft @ f:5.6
Near 67.66 ft
Far 191.54 ft
100mm DOF distance from 50mm shooting position (subtract 50 feet from near and far DOF)
Near 17.66 ft
Far 141.54 ft
So the 100mm lens DOF extends from about 18 ft in front of the 50mm lens to 141 feet in front of the 50mm lens. The 50mm lens DOF extends from 25 ft to 1132 feet from that same reference point. This is at the same aperture and same subject magnification with the same plane of best focus.
The results reflect the same kind of relative front/rear shift in depth of field that Ctein shows in his photographic results and his chart when going from longer to shorter focal lengths.
Arthur Cox, pages 76-77, Photographic Optics, 15th ed, Focal Press, poses the question:He then goes on to show the depth of field with a 2 inch lens enlarged by a factor of four has greater depth of field than a contact print from a negative shot with an 8 inch lens, both shot at f:4, with the shorter lens having a DOF that is just over 4 times greater than the longer lens when both are focused on an object at 10 feet.Quote:
"A scene is to be photographed in such a way that the depth of field is to be a maximum. Is it better to use a long focus lens and make a contact print, or to use a short focus lens and make an enlargement?
The answer is in favor of the short focus lens."
This agrees with Ctein's assertions. I searched for a while for your applicable APUG post Steve, but failed to find it. APUG needs a better search engine.
Lee