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Depth of focus
by Wikipedia
Depth of focus is a lens optics concept regarding the tolerance of placement of the plane of film in relation to the rear element of the lens. While the phrase is sometimes erroneously used to mean depth of field, it should be stressed that depth of focus and depth of field are not the same. Depth of field is a measurement of how much distance exists where the subject in the frame will appear to be sharp. Depth of focus, however, is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus. In some ways it can be viewed as the flip side of depth of field, occurring on the opposite side of the lens. Where depth of field often can be measured in easily seen quantities such as inches, feet, or meters, depth of focus is extremely precise in demands, usually in the thousandths of inches or hundredths of millimeters. Since the measurement indicates the tolerance of the film's displacement within the depth of focus, depth of focus is sometimes referred to as "lens-to-film tolerance."
The same three factors which determine depth of field also determine depth of focus, but confusingly, these factors may or may not act differently with depth of focus. Depth of field increases with smaller apertures, farther subject distance, and wider focal length. Depth of focus increases with smaller apertures, closer subject distance, and longer focal length. Because depth of focus decreases with wider lenses, the precision of depth with which the lens is seated becomes much more critical. In motion picture cameras, different lens mount and camera gate combinations have exact flange focal depth measurements to which lenses are calibrated.
In addition, the choice to place gels or other filters behind the lens becomes a much more critical decision when dealing with wider lenses. Placement of items behind the lens will alter the optics pathway, shifting the backfocus. Therefore oftentimes this must be done in concert with stopping down the lens in order to compensate enough to make any shift negligible given a greater depth of focus. It is often advised in 35 mm motion picture filming not to use filters behind the lens if the lens is wider than 25 mm.
A rough formula often used to quickly calculate depth of focus is the product of the focal length times the f-stop divided by 1000. The precise formula for depth of focus is two times the f-stop times the circle of confusion times the quantity of one plus the magnification factor. However, the magnification factor depends on the focal length and exact focus the lens is set to, which can be difficult to calculate. Therefore, the first formula is often used as a guideline, as it is much easier to derive.
Source: http://en.wikipedia.org/wiki/Photography
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