Several Basic concepts of slr

1. focus)

When the light parallel to the optical axis is injected into the convex lens, the ideal lens should be all the light gathered at a point, and then spread out in a cone, which gathers a point of all the light, it is called focus.

2. circle of confusion)

Before and after the focus, the light starts to accumulate and spread, and the image of the point becomes blurred to form an expanded circle, which is called a dispersion circle.

In reality, watching the captured image is observed in some way (such as projection, posting large photos, and so on, the visible image is closely related to the magnification, projection distance, and viewing distance. if the diameter of the dispersed circle is smaller than that of the human eye, the Blur produced by actual images within a certain range cannot be identified. This unrecognized dispersion circle is called a permissible circle of confusion ).

Different manufacturers and different film areas have different allowable dispersion circle diameter values. It is generally used as follows:

Frame	24mm x 36mm	6 cm x 9 cm	4 "x 5"
Dispersion Circle Diameter	0.035	0.0817	0.146

35mm the allowable dispersion circle of the photographic lens, which is about 1/1000 ~ of the diagonal line length of the negative film ~ About 1/1500. The premise is that the image is enlarged to 5x7 inch, and the observation distance is 25 ~ 30 cm.

3. depth of field)

There is a allowable dispersion circle before and after the focus, and the distance between the two dispersion circles is called the depth of field, that is, before and after the subject (focusing, the image still has a clear range, that isDepth of field. In other words, the depth of the front and back of the subject and the blur of the image displayed on the negative are within the limited range of allowable dispersion circles.

The depth of field varies with the focal length, circle value, and shooting distance of the lens. For fixed focal lengths and shooting distances, the smaller the aperture, the larger the depth of field.

Based on the camera photographer, the distance from the focus point to the nearest allowable dispersion circle is called the foreground depth, and the distance from the focus point to the distant distance from the allowable dispersion circle is called the rear depth of depth.

4. Calculation of depth of field

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

Delta	--	Allowable dispersion Circle Diameter
F	--	Lens focal length
F	--	Shot aperture Value
L	--	Focus distance
△L1	--	Deep prospects
△L2	--	Rear depth of field
△L	--	Depth of field

Fdelta L2   Foreground deep Delta L1 = ------ (1)   F2 + F Delta L

Fdelta L2   Rear depth of field △l2 = ------ (2)   F2-F Delta L

2f2F Delta L2 Depth of field △l = △L2 + △l2 = ------     F4-F2 Delta 2L2

We can see from the formula (1) and (2) that,Rear depth of field> foreground depth.

From the calculation formula of the depth of field, we can see that the depth of field is related to the aperture, focal length, shooting distance of the lens, and the image quality requirements (as shown in the size of the allowable dispersion circle. The effect of these main factors on depth of field is as follows (assuming that none of the other conditions changes ):

(1) Lens Aperture:
The larger the aperture, the smaller the depth of field; the smaller the aperture, the larger 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 larger the depth of field;

(3) shooting distance
The longer the distance, the larger the depth of field. The closer the distance, the smaller the depth of field.

5. Some computing instances

Downloadable counters for WindowsF/Calc: English Chinese patch

All downloaded files are compressed using Winrar.
We recommend that you download and install the original installation file, and then copy the Chinese file to the corresponding path.
Email from Chinese:Baby.Leos@163.com

(1) f/200/2 depth of field when the focus is 5 MB:

Delta	=	0.035
F	=	200
F	=	2.8
L	=	5000
△L1	=	60mm
△L2	=	62mm
△L	=	122
Conclusion	:	When shooting with f/2.8, the clear range is from 4.94m ~ 5.062 m, very light depth of field.

(2), 200/2. 8 + 2X = 400/5. 6 when the focus is 5 MB, f/5.6 depth of field:

Delta	=	0.035
F	=	400
F	=	5.6
L	=	5000
△L1	=	30mm
△L2	=	31mm
△L	=	61mm
Conclusion	:	The depth of field is half of (1) when the main lens is shot with f/2.8 in combination with the 2X addition mirror.

 

 

Depth of field is calculated based on the aperture coefficient, distance, and allowable diameter of the dispersion circle.
Schematic

 

Next we will introduce a foreign Online Calculation formula:
Http://www.dofmaster.com/dofjs.html

This is clear.