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*The formulae used for the calculations above are based on Harold M. Merklinger "The INs and OUTs of FOCUS".  The depth of field is approximately equally distributed in front of and in back of the distance to the subject.

The formulae are:

g = N*CoC

Where N = F-stop number and CoC (the circle of confusion) assumed to be 1/30mm for 35mm slide format, then g = the allowable focus error.

The nearest point of the depth of field (D₁):

D₁ = ( f²D + gfD - gf² ) ÷ ( f² - gf +gD )

The furthest point of the depth of field (D₂):

D₂ = ( f²D - gfD + gf² ) ÷ ( f² + gf - gD )

Where f = focal length of lens, D = distance to the subject

Finally:

D₂ - D₁ = DOF

 

As mentioned above, the calculator uses a CoC of 1/30mm.  There appear to be a wide range of CoC's in use by camera manufactures and photographers.  Based on my review of online and off line reference materials, I've found a CoC range from 1/16mm to 1/333mm for 35mm film format.  Canon uses .0325mm which is about 1/31mm.  Anyway, you may find that the results produced by other online calculators vary significantly from the calculator above.  Some of this discrepancy is caused by usage of different CoC values.  However, there are a number of online calculators that are using approximate formulae for arriving at DOF.  These approximate DOF calculators will not be valid for macro photography DOF but should work fine for landscape photography. The calculator on this page is valid for both landscape and macro photography because it employs precise rather than approximate formulae.