Depth of Field, Depth of Focus, and Film Flatness
Ok, I think we can all agree that depth of field is an important concept in theory and in reality when using a large format camera.
Now, in both theory and reality there is one plane of sharp focus in any photograph. The location and orientation of this plane depends on the distance between the optical center of the lens and the film plane and the orientation of the subject plane, the film plane and that of the lens board (the Scheimpflug Principle). What appears to be in sharp focus in front of and behind the plane of sharp focus is really a function of the size of the circles of confusion. If these are small enough then those parts of the image will appear to be sharp when the photograph is viewed from a normal viewing distance. This area of apparent sharpness in front of and behind the plane of sharp focus is considered the depth of field. Depth of field is dependent upon the reproduction ratio (the relative size of the image on the groundglass/film plane and its size in reality) and the f-stop being used. As the reproduction ratio approaches 1:1 (closeup work) the depth of field will decrease. As the f-stop number becomes larger (meaning the size of the aperture gets smaller) the depth of field will increase.
What is depth of focus? Depth of focus is the space behind the lens where the film plane position can vary slightly and still not cause any loss of sharpness. Sometimes, people have felt that this space can makeup for a slight variation in film position and lack of film flatness. Depth of focus does increase as the lens is stopped down. To some extent, the increase in depth of focus matches the increase in depth of field as the lens is stopped down. Depth of focus also increases as the camera and lens get closer to the subject: i.e. as the reproduction ratio begins to approach 1:1. Depth of focus is not affected by focal length of the lens. However, if the subject area completely fills the depth of field area the depth of focus becomes so small that there is no tolerance for any variation in the film’s position, thus depth of focus can not really be used to make up for an incorrectly aligned plane of focus (mis-aligned film and lens board planes) or a poorly positioned piece of film.
So, what does this all mean. Well depth of field is an important concept in theory and reality. I think we can all agree on this opinion. However, depth of focus really becomes an academic concept that is not of much use to use when we are making a photograph IMHO. In most situations the depth of focus is so small as not to be an effective aid to us. One thought about the importance of depth of focus is that of the four primary books on large format technique, only one of the books even mentions depth of focus.
So, how does this relate to film flatness in various film holders. Well in my 30 years of using large format and using every type of holder I could find, and in my knowledge of hundreds or working large format photographers doing the same, an unsharp image due to poor film flatness in a holder is an extremely rare commodity. Many studies have been done looking at the effective film flatness of various holders, whether or not they have a pressure plate, etc. and there are differences certainly. But the key question is do these minute variations make any difference to a photographer making a photograph. Rarely and only in the most extreme conditions is the answer. We can debate these minute differences, we can continue to do studies, etc., etc. but in reality they all work except in an occasional damaged holder situation or damaged camera back scenario.. Camera and film holder mfg's have worked with this tolerance requirement for years and years and years and have effectivley solved this probelm so we generally do not have to worry about it. Relax and photograph
Steve simmons (www.viewcamera.com)