Good post about depth of field, Dave. Let me add a few things. The "circles of confusion" Dave mentioned are out-of-focus points of light on the film, and if these can be recorded with a diameter of 1/100th inch or less, we see them as being sharp or in focus because human beings have very unsharp eyesight. So, as you close down the lens aperture to smaller f/stops, these circles of confusion are reduced in size on the film, thus appearing sharp and in focus, even though the laws of physics state that only objects in a single plane can be in focus at one time. So we get increased "apparent" depth of field because of our poor visual acuity.
The other way to reduce the size of these circles of confusion (when viewed from the side, they actually are cones of light passing through a point of focus in the camera), is to reduce the magnification of the subject. Moving the camera farther away from the subject, or using a wide angle lens from the same camera position, yield smaller subjects--reduced magnification--on the film, and, in turn, greater or wider apparent depth of field. My students used to ask me which would produce the most DOF, using a wide angle lens up close or using a telephoto from a greater distance away. The answer is neither, provided that the subject were the same size on the film with either lens stopped down to the same aperture. The trick is the magnification of the subject--big magnification by a long lens or big magnification by moving in closer will yield identical DOF at the same aperture, although the two images will be very different from each other in their respective perspective. Maximum DOF=wide angle lens focused at infinity with f/stop closed down. Minimum DOF=telephoto lens focused at its closest distance with f/stop wide open. Use any combination of lens focal length, focus distance and f/stop to control the DOF for any photo. In some instances it is preferable to shoot a photo with a non-telephoto lens or to be farther away from the subject in order to get increased DOF with a smaller image size, and then later enlarge the image to obtain the desired size and to remove unwanted background. If you need more DOF than that, it is time to get out the view camera and "Schliempflug" the camera back by tilting it to optimize the laws of physics as the light passes through the lens. John B. Corns John B. Corns --> SPORRS: Serious Photographers of Railroad Related Subjects
