What is the calculation method of depth of field?

Depth of field refers to the range of the distance between the front and back of an object measured by imaging in front of a camera lens or other imager.

Introduction to depth of field

After focusing, a clear image can be formed in the range before and after focusing, which is called the depth of field. There is a certain length of space in front of the lens (before and after focal length adjustment). When the subject is located in this space, its imaging on the negative is just between the two diffusion circles before and after focusing. The length of the space where the subject is located is called the depth of field. In other words, the blur of the image presented by the subject on the negative plane of this space is within the limited range of the allowable circle of dispersion, and the length of this space is the depth of field.

Three elements of depth of field

Aperture, the distance between the lens and the subject are important factors affecting the depth of field;

1, the larger the aperture (the smaller the aperture value f) and the shallower the depth of field, the smaller the aperture (the larger the aperture value f) and the deeper the depth of field.

2. The longer the focal length of the lens, the shallower the depth of field, and vice versa.

3. The closer the subject is, the shallower the depth of field is, and the farther the subject is, the deeper the depth of field is.

Calculation formula of depth of field: see figure.

As can be seen from the formula, the back depth of field >: the foreground is deep.

(1), lens aperture:

The larger the aperture, the shallower the depth of field; The smaller the aperture, the deeper the depth of field;

(2), lens focal length:

The longer the focal length of the lens, the shallower the depth of field; The shorter the focal length, the deeper the depth of field;

(3), shooting distance:

The farther the distance, the deeper the depth of field; The closer the distance, the shallower the depth of field.

When shooting, the process of adjusting the camera lens to make the scene at a certain distance from the camera clearly image is called focusing, and the point where the scene is located is called focus. Because "clarity" is not an absolute concept, the scene can be imaged clearly in front of the focus (close to the camera) and at a certain distance behind it. The sum of the front and back ranges is called the depth of field, which means that the scenery in this range can be clear. The depth of field is first related to the focal length of the lens. The lens with long focal length has small depth of field, while the lens with short focal length has large depth of field. Secondly, the depth of field is related to the aperture. The smaller the aperture (the larger the numerical value, for example, the aperture of f 16 is smaller than f 1 1), the greater the depth of field. The larger the aperture (the smaller the value, for example, the aperture of f2.8 is larger than that of f5.6), the smaller the depth of field. Secondly, the depth of field in the foreground is smaller than that in the back, that is to say, after precise focusing, the scene in front of the focus can be clearly imaged, while the scene in the back of the focus is clear.

The spatial depth that can be seen clearly at the same time is called the imaging spatial depth of the eye, that is, the depth of field.

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. Usually used are:

The allowable dispersion circle of a 35mm photographic lens is about11000 ~11500 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).