What is the depth of field?

Depth of field (DOF) refers to the distance range between the front and back of an object measured by imaging in front of a camera lens or other imagers. The distance from the aperture, lens and focal plane to the subject is an important factor affecting the depth of field.

After focusing, the distance between the clear images before and after focusing is called depth of field.

There is a certain length of space in front of the lens (before and after the focus). When the subject is located in this space, its image on the negative is just between the same diffusion circles. 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.

Chinese name: depth of field

Mbth: depth of field

Pinyin: jǐngshn

Three elements: aperture, lens and the distance from the focal plane to the subject.

Definition: the range that is still clear before and after the focus

Depth of field camera

When the lens of a camera is clearly focused on an object, the point on the same plane perpendicular to the lens axis at the position opposite to the lens center can form a quite clear image on the film or receiver, and the point in a certain range before and after the lens axis can also form a relatively clear image point acceptable to the eyes. The distance between all the scenes before and after this plane is called the depth of field of the camera. When the light rays with parallel optical axes enter the convex lens, the ideal lens should be that all the light rays converge at one point and then spread out conically. The point where all the rays converge is called the focal point.

Before and after the focus, the light began to gather and spread, and the image of the point became blurred, forming an enlarged circle called the diffusion circle.

In reality, images taken for viewing are observed in some way (such as projection, enlargement into photos, etc.). ). The image perceived by human eyes is closely related to magnification, projection distance and viewing distance. If the diameter of the diffusion circle is less than the resolution of human eyes, the blur produced by the actual image can not be recognized in a certain range. This unrecognizable dispersion circle is called allowable dispersion circle. There is an allowable dispersion circle before and after the focus.

Based on the photographer with a camera, the distance from the focus to the near allowable dispersion circle is called the front focal depth, and the distance from the focus to the far allowable dispersion circle is called the back focal depth.

Three elements:

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) The distance between the subject and the background:

Changing the distance between the subject and the background will not change the depth of field, but only determine whether and to what extent the background is blurred.

(4) The distance between the object and the lens:

The farther the distance, the deeper the depth of field; The closer the distance (not less than the minimum shooting 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:

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

δl 1- deep prospect

Δ δL2—— Back Depth of Field

δl- depth of field

Exploration depth δl 1 = fδl 2/(fδ2+fδl)

Back depth of field Δ L2 = fΔ L2/(F2-fΔ L)

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:

draw

24 mm x 36mm mm

150 pixel x 225px pixel

4 inches x 5 inches

Dispersion circle diameter

0.035 mm

0.0817mm

0.146mm

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).

Heart algorithm:

Fuzzy level method is a fast centroid algorithm for quantitative estimation of hyperfocal distance and shallow depth of field. That is, the traditional focal length is replaced by the standard height multiple of the short side of the field of view, and the fuzzy level is introduced, which is combined with the mental arithmetic of hyperfocal distance and shallow depth of field.

Theory:

Ambiguity X = diameter of dispersion circle r/ short side dimension of sensing element L× 100%.

Note: L300dpi is the printable size of the short side at 300dpi. The brightness of the diffusion circle is uniform, and the energy of Airy disk is concentrated in the center. When the diameter of Airy spot is twice the diameter of dispersion circle, the fuzzy effect is almost the same. ? [ 1]?

What is the degree of ambiguity at infinity X∞? = Ambiguity at focal length X0.5 times

= diameter of dispersion circle/dimension of short side of sensing element × 100%

=f/(F× 1700N) × 100%

Where the definition of n is shown in the figure on the right.

When f=24mm, F=5.6, N= 1, X∞? =X0.5=0.25%, and the corresponding fuzzy degree R∞? =R0.5=0. Remember the benchmark of f24F5.6N 1, and the rest can be estimated quickly. For example, f35F2.8N0.5 corresponds to R∞? = r0.5 = 2.5f35f11n0.5 corresponds to R∞? =R0.5= -0.5 .

The background is not infinite. When R∞≥2.5, the background 5m away from the focal plane can usually be blurred.

As long as the sharpness at infinity is guaranteed (R∞≤k), the background will be sharp, and as long as the foreground is far more than 0.5 times the focal length, the foreground will be sharp.

example

As shown in the picture on the right, N=5.6 (blue dot), f= 17, F=5.6, APS-C. Due to perspective, the length of the cylinder at the blue dot is half that at the red dot, indicating that the distance between the blue dot and the camera is twice as long as that at the red dot. Focusing on the blue dot plane, the ambiguity degree r of infinity and red dot is the same, R∞? =R0.5= -3. If F=4 is set, then R∞? = r 0.5 =-2.5； F=8, then R∞? =R0.5= -3.5. Three aperture settings can obtain a large depth of field. At this time, the best aperture of the lens should be used (depending on the design and processing error of the lens).

Influencing factors:

1, lens focal length

2. The distance of the subject

3, the size of the aperture

4. The size of the photosensitive element (related to the radius of the allowable dispersion circle)

Relationship:

1, the larger the aperture, the smaller the depth of field, and the smaller the aperture, the greater the depth of field.

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

3. The closer the subject is, the smaller the depth of field is, and the farther the subject is, the greater the depth of field is.