Calculation method of depth of field

The following is the formula for calculating the depth of field. These include:

δ-diameter of allowable dispersion circle

F- the shooting aperture value of the lens.

F- lens focal length

L focal length

Δ l1-foreground depth Δ L2-background depth of field Δ l-depth of field Δ l1= f Δ l 2/(f 2+f Δ l)

(1) Back Depth of Field Δ L2 = f Δ L2/(F2-f Δ L)

(2) the depth of field δ l = δ l1+δ L2 = (2f2fδ L2)/(F4-F2 δ 2L2)

As can be seen from formulas (1) and (2), the back depth of field >: the foreground depth.

It can be seen from the formula of depth of field that the depth of field is related to the aperture used by the lens, the focal length of the lens, the shooting distance and the requirements for image quality (expressed by the size of the allowable dispersion circle). These main factors affect the depth of field as follows (assuming all other conditions remain the same):

(1), lens aperture: the larger the aperture, the smaller the aperture value (f), the smaller the depth of field; The smaller the aperture, that is, the larger the aperture value (f), the greater the depth of field;

(2) Lens focal length: The longer the lens focal length, the smaller the depth of field; The shorter the focal length, the greater the depth of field;

(3) Shooting distance: The farther the distance, the greater the depth of field; The closer the distance, the smaller the depth of field.

Different manufacturers and different membrane areas have different numerical definitions of allowable dispersion circle diameter. Commonly used: the diameter of the dispersion circle of a frame of 24 mm x 36 mm150px 225px4 "x 5" is 0.035mm 0.0817mm0/46mm, which is about1106mm of the diagonal length of the negative. The premise is that the picture is enlarged to a 5x7 inch photo, and the observation distance is 25~750px.

5. Some calculation examples There are some online calculators on the Internet. Interested users can refer to:

Photographic optical calculator

The Windows version of the downloadable counter is in f/Calc.

(1) and 200/2.8 focus at 5m, the depth of field of f/2.8 is:

δ= 0.035 mm

F = 200mm mm

F=2.8

L = 5000mm mm

δ l1= 60mm

△ δL2 = 62

δL = 122 mm

Conclusion: When shooting with f/2.8, the clear range of this lens is 4.94m-5.062m, and the depth of field is very shallow.

(2) When 200/2.8+2x = 400/5.6 is focused at 5m, the depth of field of f/5.6 is:

δ= 0.035 mm

F = 400mm mm

F=5.6

L = 5000mm mm

δ l1= 30mm

δL2 = 3 1 mm

δL = 6 1 mm

Conclusion: When the main lens is shot with f/2.8, the depth of field of this lens is half that of (1).