ND  =   near  distance H  =   hyperfocal   distance D  =   distance focused upon F D =   far  distance EXAMPLE: What is the depth of field of a 155mm (6.1 inch) lens that is focused on an object 10 feet from the camera lens using f/2.8? (Note: In a previous example the hyperfocal distance for the lens was found to be 554 feet.)  By  the  formula,  the  nearest  sharp  point  is determined  as  follows: ND  = 9.8 feet Thus the nearest point in sharp focus is 9.8 feet from the lens that is focused on an object at 10 feet, using f/2.8. Also by the formula, the farthest point in sharp focus can  be  determined  as  follows: FD =   10.2 feet Therefore, the far point in sharp focus is 10.2 feet when focused   on   an   object   at   10   feet,   using   f/2.8. Consequently, the depth of field in this problem equals the near distance subtracted from the far distance (10.2 - 9.8 = 0.4-foot depth of field). Thus all objects between 9.8 and 10.2 feet are in acceptably sharp focus. When this depth of field is not great enough to cover the subject, select a smaller f/stop, find the new hyperfocal distance,  and  apply  the  formula  again. When  the  only  way  you  have  to  focus  is  by measurement, the problem then becomes one of what focus distance to set the lens at so depth of field is placed most effectively. There is a formula to use to solve this problem. P =^{D  x  d  x  2} D  +  d Where: D = distance to farthest point desired in sharp focus d = distance to nearest point desired in sharp focus p= distance to point at which the lens should be focused Substituting  the  figures  from  the  previous  examples, D= 10.2 feet d = 9.8 feet P= lens focus distance Then: P =   10  feet To obtain the desired depth of field at f/2.8, we set the lens focus distance at 10 feet. If the preceding explanations and formulas have confused you, here is some good news! Most cameras and lenses have depth of field indicators that show the approximate depth of field at various distances and lens apertures. Figure 1-30 shows that with the lens set at f/8 and focused at about 12 feet, subjects from about 9 feet to about 20 feet are in acceptably sharp focus. By bringing  the  distance  focused  upon  to  a  position opposite the index mark, you can read the depth of field for  various  lens  openings. Keep in mind that a depth of field scale, either on the camera or on the lens, is for a given lens or lens focal length only. There is no universal depth-of-field scale that works for all lenses. In conclusion, the two formulas used to compute depth of field serve for all distances less than infinity. When the lens is focused on infinity, the hyperfocal distance is the nearest point in sharp focus, and there is no limit for the far point. CONJUGATE  FOCI Object  points  and  their  corresponding  image  points formed by a lens are termed conjugate focal points. The distances from the optical center of the lens to these points, when the image is in focus, are termed conjugate focal distances or conjugate foci (fig. 1-31). 1-26