<Dd> N ≈ v N − v F 2 c . (\ displaystyle N \ approx (\ frac (v_ (\ mathrm (N)) - v_ (\ mathrm (F))) (2c)) \, .) </Dd> <P> The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate . In most cases, focus and f - number can be determined with sufficient accuracy using the approximate formulas above, which require only the difference between the near and far image distances; view camera users sometimes refer to the difference v N − v F (\ displaystyle v_ (\ mathrm (N)) \, - \, v_ (\ mathrm (F))) as the focus spread (Hansma 1996, 55). Most lens DOF scales are based on the same concept . </P> <P> The focus spread is related to the depth of focus . Ray (2000, 56) gives two definitions of the latter . The first is the tolerance of the position of the image plane for which an object remains acceptably sharp; the second is that the limits of depth of focus are the image - side conjugates of the near and far limits of DOF . With the first definition, focus spread and depth of focus are usually close in value though conceptually different . With the second definition, focus spread and depth of focus are the same . </P> <P> If a subject is at distance s (\ displaystyle s) and the foreground or background is at distance D (\ displaystyle D), let the distance between the subject and the foreground or background be indicated by </P>

The main factor affecting depth of focus is the