Archived posting to the Leica Users Group, 2004/02/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 04:16 PM 2/16/2004 -0800, you wrote: >I promise that I'll get the LUG archives search engine working again, soon. Too late! So to add to John Collier's truth, here's my DOF treatise, more than you wanted to know! COC = Circle Of Confusion Image Size = magnification For a given COC, a 50mm lens 10 feet away from the subject has the EXACT SAME DOF as a 100mm lens 20 feet away from the subject. A 50mm lens at f/1.4 focused at 10 feet has from 9'7" to 10'5" in focus. A 100mm lens at f/1.4 focused at 20 feet (exact same image size on the film as the 50mm example), the DOF is from 19'7" to 20'5". The depth of field is EXACTLY the same. EXACTLY 10 INCHES IN BOTH CASES for the given COC. DOF has EVERYTHING to do with IMAGE SIZE (magnification,) which is a combination of the focal length you are using AND how far away you are from the subject. If the "image size" is the same, the DOF is the same for a given f/stop, REGARDLESS OF FOCAL LENGTH and REGARDLESS OF THE FILM SIZE. This is a simple fact. *** MORE *** Depth Of Field is a function of f/stop and image size. An 80mm lens on a 4X5 has precisely the same DOF as an 80mm lens on a Hasselblad, which has exactly the same DOF as an 80mm lens on a Leica, in every situation. One does not give less or more DOF than the other. Take two Hasselblads, one with an 80mm lens, the other with a 160mm lens. Take the 160mm Hasselblad exactly twice the distance from the subject as the 80mm Hasselblad is (this will make the photographed image - its size - on the film exactly the same for both cameras) and the DOF in the image in both cameras, at exactly the same f/stops, will be exactly the same. Conversely, if you photograph with the 80 and 160 from exactly the same distance and f/stop, the DOF in the 160 image will be considerable less than the DOF in the 80 image. The 160 image size will be twice as large as the 80 image as well. One manufacturers 80mm lens has the same optical characteristics as any other manufacturers 80mm lens. Or 100mm lenses, or 150mm lenses. It's not a function of manufacturer or film size, it is the function of image size and f/stop. It is the law of optical physics. There are a few minute fudge factors here but are totally insignificant. An 80mm lens on a 35mm camera has the same DOF as an 80mm lens on a 6x6 camera which has the same DOF as an 80mm lens on a 4x5 camera which has the same DOF as an 80mm lens on a digital camera. Regardless of film size actually used. The 6x6 and 4x5 80mm has more "coverage" than the 35mm format, but the image size - magnification (regardless of the film size) is identical, therefore the DOF is identical. What f/stop you use and where you focus in your scene will determine how much is in focus. From the exact point of focus, DOF extends 1/3 forward (toward the camera) and 2/3 back (away from the camera). If you photograph a persons face, at wide open (f/2.8 or f/3.5) and focus on their eyes, there is a good chance that the end of their nose will be out of focus. If, however, you focus on the tip of their nose, their eyes will probably be in focus. Except for perhaps Pinocchio. As you stop down, more comes into acceptable focus, increasing 1/3 front and 2/3 back. Back DOF increases twice as fast as front DOF. And John Collier is, of course, correct. It is all visually relative to the sharpness of your lens and sharpness of your film. Old time cameras, cheap P&S cameras, etc, all seem to exhibit a greater DOF than does a good Leica and a Summicron. It is because the old and cheap bad lenses limit your ability to determine where the sharpness boundary is. Sharp lenses give a clear delineation of where this boundary is. The DOF scales on good modern lenses are about two stops off. Lenses and films are much better than when these scales were devised. Shoot at f/16, read the scales for f/8. The same goes for hyperfocal charts. Digital sensors.... they're another story. JB - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html