Depth of Field Formula

Equation for calculate depth of field is,

$$t=\frac{2·N·c·(m+1)}{m^2-{\left(\frac{N·c}{f}\right)}^2}$$

where,
N = Focal number (f-number)
c = Circle of confusion
m = Magnification
f = Focal length of the lens
t = Depth of field