ND = near distanceH = hyperfocal distanceD = distance focused uponF D = far distanceEXAMPLE: What is the depth of field of a 155mm (6.1inch) lens that is focused on an object 10 feet from thecamera lens using f/2.8? (Note: In a previous examplethe hyperfocal distance for the lens was found to be 554feet.) By the formula, the nearest sharp point isdetermined as follows:ND = 9.8 feetThus the nearest point in sharp focus is 9.8 feet from thelens that is focused on an object at 10 feet, using f/2.8.Also by the formula, the farthest point in sharp focuscan be determined as follows:FD = 10.2 feetTherefore, the far point in sharp focus is 10.2 feet whenfocused on an object at 10 feet, using f/2.8.Consequently, the depth of field in this problem equalsthe near distance subtracted from the far distance(10.2 - 9.8 = 0.4-foot depth of field). Thus all objectsbetween 9.8 and 10.2 feet are in acceptably sharp focus.When this depth of field is not great enough to cover thesubject, select a smaller f/stop, find the new hyperfocaldistance, and apply the formula again.When the only way you have to focus is bymeasurement, the problem then becomes one of whatfocus distance to set the lens at so depth of field is placedmost effectively. There is a formula to use to solve thisproblem.P =^{D x d x 2}D + dWhere:D = distance to farthest point desired in sharpfocusd = distance to nearest point desired in sharpfocusp= distance to point at which the lens should befocusedSubstituting the figures from the previous examples,D= 10.2 feetd = 9.8 feetP= lens focus distanceThen:P = 10 feetTo obtain the desired depth of field at f/2.8, we set thelens focus distance at 10 feet.If the preceding explanations and formulas haveconfused you, here is some good news! Most camerasand lenses have depth of field indicators that show theapproximate depth of field at various distances and lensapertures. Figure 1-30 shows that with the lens set at f/8and focused at about 12 feet, subjects from about 9 feetto about 20 feet are in acceptably sharp focus. Bybringing the distance focused upon to a positionopposite the index mark, you can read the depth of fieldfor various lens openings.Keep in mind that a depth of field scale, either onthe camera or on the lens, is for a given lens or lens focallength only. There is no universal depth-of-field scalethat works for all lenses.In conclusion, the two formulas used to computedepth of field serve for all distances less than infinity.When the lens is focused on infinity, the hyperfocaldistance is the nearest point in sharp focus, and there isno limit for the far point.CONJUGATE FOCIObject points and their corresponding image pointsformed by a lens are termed conjugate focal points. Thedistances from the optical center of the lens to thesepoints, when the image is in focus, are termed conjugatefocal distances or conjugate foci (fig. 1-31).1-26