Something About The Depth of Field

First, I have to introduce you a few concepts before move on the calculation of the depth of field.

1.Focus
A point toward which light rays are made to converge after passing through a convex lens.

2. Circle of confusion
Optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.It is the largest blur spot that will still be perceived by the human eye as a point.

3.Depth of Field 
Distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Changes by changing the focus, aperture value or the distance between the subject and the camera.  

The bigger the lens aperture, the smaller the depth field and vice versa.

The longer the lens focus, the smaller the depth of field, and vice versa.

The farther the distance, the bigger the depth of field. 

4.How to calculate the depth of field 
δ: diameter of circle of confusion
f: focal lengh
F: aperture value of the lens
L: focus distance
ΔL1: front depth of the field
ΔL2: back depth of the field  
ΔL: depth of the field

                FδL²  
ΔL1= ——————    (1) 
            f² + FδL  

                FδL²
ΔL2=  ——————    (2)
             f² - FδL 

                                         2f²FδL²

ΔL =	ΔL2 + ΔL2  = ———————

                                      f⁴ - F²δ²L² 

 The back depth of th field > front depth of field