Public security technology exploration * Partition exposure method for criminal photography flash lighting can use the partition exposure method. Using the partition exposure method, combined with the reflectivity of the on-site object, through the scientific control of light distribution, selection of appropriate exposure and development degree, it is possible to obtain a tone that meets objective or anticipation and requirements, so that the subject can accurately reproduce the criminal The partition exposure method in flash photography is based on the stable illumination intensity and exposure of the flash, and the quantitative analysis of the exposure status of the flash photography is based on the partition exposure theory created by American photographer A. Adams. When using this kind of light source for criminal photography, it is possible to obtain a prediction of the photographic result before making a decision on the amount of light distribution and exposure; or after making specific demands on the photographic effect, it is possible to control the light distribution and the exposure amount. The theory of accurately reproducing the partition exposure is based on the photographic performance of the photosensitive material, and the brightness value of the scene measured by the light meter is compared with the tone value obtained by the photosensitive material after receiving a certain amount of light, according to the advance Imagination and requirements, adjust the exposure to a value that satisfies this imagination and requirements It is an effective tool for controlling photo tones. Its specific features are: 1 to divide the brightness range of the subject and the range of brightness produced by the emulsion in the film and photographic paper. 18% of the medium gray is the V zone, which is the shadow of the exposure indicated by the reference arrow of the light meter. Therefore, for each additional exposure, a zone is added, which is VI, pass, Vffl and K zones. They are all exposed at the exposure transition level above the reference arrow of the light meter. The tone reflected on the photographic paper is gradually lighter; the underexposed levels below the reference arrow of the light meter are W, I, 11, respectively. In the 1st and 0th zones, the tone is stepwisely deepened. The difference between the brightness of the two adjacent zones is the difference between the brightness values. The tone formed by each exposure zone is known and a certain tone is included from the 0 zone to the IX zone. The brightness range is equivalent to the amount of light metering from less than five levels to over four levels. In the performance of the photo tone, the 0 area is the maximum blackness developed by the photographic paper after full exposure, and the 1 area is the object that cannot be represented. Any texture and texture of dark black, the area can only show part of the texture Grayish black, while the XI zone is a pure paper-based white zone. It is slightly textured. The white tone can clearly express the texture and texture of the object. The tone values ​​are I, W, V, VI and W zones. The light meter measures the brightness range of the scene and the brightness value of each point. After the exposure is determined, the tone effect of each part in the photo can be predicted. The part where the brightness value of the scene is consistent with the exposure value is always expressed as the V area, and the other parts of the scene are in different different tone areas. 4 According to the brightness of the scene and the tone of the scene. The degree of lightness, by controlling the exposure amount, can make the tone value of the obtained photo consistent with the actual reflection value of the scene or the pre-imagination and requirements. 5 It can be aimed at the brightness range of the subject and the tolerance of the film and the photo paper. Degree, by controlling the degree of development, the range of image tones formed is controlled within the optimal performance range of the photosensitive material. The tolerance of black and white film and photo paper is usually 128:1 and 32:1. If the brightness range of the scene is five, the film will be washed according to the development formula; if the brightness range of the scene is seven, six or four, In the third stage, the flushing can be reduced by 40% 20% or 50% by 10% respectively (% of development time, the tone is compressed or expanded in the five-level range. Of course, the magnitude of this control is also limited in the development time. At 60%, it will have a serious impact on the performance of the shadow layer level; when it is 50% higher than normal development, the film grain will become thicker; when it exceeds 100% of normal development, the performance of the highlight portion will be impaired. The expansion and compression of the tone is generally no more than three levels. Before explaining the flash exposure area, it is necessary to first understand such problems: 1A. Adams's partial exposure method is to use the spot meter to measure the scenes. Based on the brightness value of the point and the brightness range of the entire scene, combined with the photographic performance of the film and the tone effect to be performed as a basis for determining the exposure, when using the light meter to measure the exposure value of the flash illumination Usually the illuminance of the position of the subject is measured. 2 For most portable small electronic flash users, since the ordinary light meter is ineffective for the flash, the exposure data of the flash photography is usually calculated by formula: where F is the flash photography. The aperture coefficient value of the time exposure, GN is the index of the flash, L is the illumination distance of the flash to the subject; or the electronic flash with the automatic flash device is obtained by looking at the exposure meter on the flash holder without having to perform such an exposure calculation due to It reflects the comprehensive amount of light reflected by the illumination range, and it is impossible to examine the amount of light of a certain part of the scene. Therefore, the flash exposure area described in this paper is not discussed. The exposure value is based on 3 and A. The partition exposure theory proposed by Adams is different. In most cases, the main parameter for determining the amount of flash exposure - a flash index - is not based on the reference value provided by the reflective light meter, but with the incident light meter. The difference between the provided exposure reference parameters is that the reflective meter gives the view The amount of light reflected after receiving the light reflects the brightness state of the scene, and regardless of the reflectivity of the scene itself, the exposure based on the brightness value is the V area exposure, which always restores the brightness of the scene to a reflectance of 18 The medium gray tone and the incident light meter show the light intensity of the environment in which the scene is located. It is based on the reduction of the medium gray board with a light reflectance of 18% to the medium gray level with a light reflectivity of 18%. That is to say, the exposure value determined based on the illuminance value is only the reduction of the portion where the scene reflectance is 18% to the V-region tone. The amount of light reaching the film and the development are different due to the difference in the degree of reflection of the respective points of the object. The density produced afterwards is also different. The parts with high reflectivity fall on the overexposed levels, and the parts with low reflectivity naturally fall on the underexposed levels. The final tone of the printed photos is also shallow. Different, so as to reproduce the image that matches the real scene. Therefore, the determination of the exposure area during flash photography must consider at least two factors, one is the reflectivity of each part of the subject, and the second is the selected Light value combined with the theory of partition exposure and the characteristics of flash photography, the area of ​​the flash exposure can be divided into such a division: the exposure given by the flash exposure estimation formula or the exposure value measured by the flash illumination meter at the position of the subject. As the V area exposure, the shadow of the middle gray part of the subject's light reflectivity of 18% (P = 18%) is defined as the V area tone, and a zone is added for each additional level of exposure. Each time the body reflectance is doubled, it is also called a zone; similarly, each zone is reduced by one area for each first exposure, and the area of ​​the object is also reduced by one time (the object's reflectivity can be reduced by one). Under the same natural illumination, the measured brightness value of the standard gray board is used as the benchmark. The actual situation is that the highest reflectivity of the scene scene usually does not exceed 95%, and the lowest reflectivity is usually not less than 2%. Under the light, the reflectivity of snow is about 9%, the reflectivity of white wall is about 70%, the reflectivity of yellow skin and natural wood is about 20%, and the reflectivity of black velvet is about 2%~4. % Therefore, in the case of light (at this time light) Therefore, according to the above exposure area, the relationship between the exposure and the tone is as shown in Table 1. If the exposure value is changed, the corresponding tone value of each part of the scene is also changed and changed. It is stable and known. Before the exposure is determined, It can be predicted that the actual result of this change will increase the first-order exposure by F=GN/L and reduce the change of the tone caused by the first-level exposure. See Table 2 for the partial exposure method of flash illumination in criminal photography. In criminal photography, when using flash lighting, there are generally single-lamp flash dual-lamp flash and multi-lamp flash. Several forms of single-lamp flash photography and single-lamp front flash single-lamp side flash single-lamp diffuse flash and single-lamp flash mixed with ambient light When discussing the partition exposure in four ways, we only look at the situation of flash illumination, and ignore the influence of other light except when discussing the combination of single-lamp flash and ambient light (1) single-lamp flash 1 Single-lamp front flash single-lamp front flash is one of the most common and convenient lighting methods in criminal photography. However, this lighting method gives the subject a minimum light ratio (1:1), showing a three-dimensional effect. The shadow is hard and lacks texture. The resulting image contrast mainly depends on the difference in the reflectivity of each part of the subject. Therefore, the range of the tone is usually within six levels, and even only two to three levels are in most cases. Reflecting the normal tone of the field object, the exposure can be given according to the exposure calculation formula and for the object with small contrast, such as the shoe print of electrostatic adsorption extraction, etc., according to the development control law (increasing the development can increase the contrast of the film, reduce Development reduces the contrast) and the principle of exposure and development control (the low-tone part is mainly controlled by exposure, while the high-tone part is controlled by both exposure and development), and the exposure and the tone are combined. Get better contrast and control the subject's tone to the best performance of film and photo paper. 2 Single-lamp side flash and diffuse flash. In criminal photography, single-lamp side flash illumination is usually used for Three-dimensional traces, such as the addition of fingerprints, dust footprints, mud prints, etc. This kind of flash method has strong illumination direction and can give a strong stereoscopic effect of the subject. When the subject is in the shadow of the backlight, there is always no light, and it always has a 0-zone tone, so that a large contrast can be obtained. Of course, since the stereoscopicity of the scene trace is usually small, and sometimes the trace and the carrier The contrast between themselves is also very weak. Even for dark-colored carriers, for some traces of medium-gray, in the case of exposure and development giving a "normal" value, the tone of the highlights is at most medium-gray, and the contrast is still limited. In actual photography, it is first necessary to adjust and control the exposure area and the degree of development depending on the contrast between the trace and the carrier itself. The specific control principle and foreseeing effect, see Table 4, single lamp diffuse reflection flash, with the reflection of the reflective screen, the illumination area is greatly increased, and the light quality is softened. By adjusting the position of the reflective screen, the smooth side light can be obtained. A variety of lighting effects such as top light is one of the more common lighting methods used in single-lamp flash photography. In criminal photography, this lighting method is suitable for indoor scenes with small environments and some small and medium-sized physical evidence, such as on-site legacy cases. In the case of this kind of illumination, if there is no flash meter, it is difficult to calculate the correct exposure. At this time, use F=GN/L+L2 (where L1 and L2 are flash to The distance between the reflector and the reflector to the subject) can be obtained as a rough estimate, and then the first to second light mice are used according to the reflectivity of the reflector to use the automatic type electronic flash for diffuse reflection flash, because of the photosensitive element Still facing the subject, it can automatically compensate for the light loss caused by the flash, without having to calculate separately. Although the exposure to the diffuse flash is difficult to accurately partition, but in practice With reference to the principle of partition exposure, through the accumulation of experience, it is not difficult to produce ideal photos of ambient light, including natural light and the inherent light in the scene environment. Under the ambient light, the brightness range of the on-site object includes not only the reflectance ratio of each part. It is also affected by the light ratio caused by the difference of the illumination direction and the illumination intensity. The relationship is that the original object brightness range ratio = the object reflection ratio is different from the X-body object light ratio than the inherent ambient light. The photographer's will is transferred, and the light is often unsatisfactory than the contrast and styling effect. The flash is light and strong, convenient and flexible. The amount of light can be controlled by adjusting the size of the aperture during exposure, and the small and medium-sized shooting at close range. When the object is used, it can be used to improve its shape and unreasonable light ratio relationship to obtain the desired tone performance. After the additional flash illumination, the brightness range of the subject will change as follows: the ratio of the brightness range of the illuminated object = original The brightness range of the object is higher than that of the X additional illumination light. The reflectance of the scene is shown in Table 1. The flash is measured by the V area (Fz-). The relationship between the reflectivity of the scene is 7 (middle) according to the F=-exposure tone value. Table 2: The reflection rate of the scene and the change of the tone when the flash exposure changes. The reflection value of the scene reflection rate of the F+1 is V F- 1 time tone value V Note: In the table, P is 18% medium gray, F+1 indicates that the exposure value is increased by one level F, and F-1 indicates that the exposure value is lower than F. The exposure area selection and development control the object's reflectance range used for the exposure value (F = - the degree of development is normal, the range of the resulting tone is shown in Table 4, the exposure area selection and the development control trace and the carrier reflectance range of the single-side flash) - Exposure value (f=~gn) Development degree Normal tone range Table 5 Application of flash and natural light mixing and exposure selection Object shadow part Reflectivity Ten object bright part Reflectivity Guest reflection rate than object original light ratio than object original brightness The range of the tone area to be represented is the flash position and the active side of the main side, the main positive and the auxiliary plus flash after the flash is added. The brightness range is combined with the flash. The principle of combining the ambient light flash with the ambient light is when the subject is in Sidelight side backlight or backlight When the contrast is too large, the flash light is distributed on the front side to make auxiliary light; when the subject is in direct or scattered light, the contrast is too small, and the flash is distributed on the side of the light, and the main light is in the actual exposure control process. Depending on the brightness range of the original object, the light distribution is determined, taking into account the amount of ambient light and the amount of flash light. The overall consideration of the combination of the object range and the final tone result is to select the exposure method: first, the subject is measured with a light meter. The brightness and brightness range of each point, determine the range of the tone that the subject should represent. According to the tone value to be expressed, first adjust the aperture according to the amount that the flash should give, and then adjust the exposure time according to the exposure that the ambient light should give. The specific application method and effect relationship list description, see Table 5 / 3 shows that the reflective rate of the part is only half of the gray light reflectivity of the middle gray plate, and so on; the number in the P exposure selection column, F + 1/2 indicates that The selected aperture value is increased by half a degree compared with the normal calculation value, and the aperture value selected by F-1 is further reduced by one level than the normal calculation value, and the analogy is; 1/4 of the natural light exposure selection column indicates total total As long as the exposure of the light is 1/4 of the metering value, the aperture is determined according to the value of the flash exposure, and the exposure time is further increased by two steps according to the metering value, and two or more flash lamps can be used for illumination. More flexible modeling of the scene subject, control of the light ratio relationship, to make up for the lack of single-lamp flash illumination, in addition to the flat material to remake the flat material need to match the flat light, when the dual-lamp flash illumination, generally a light is located in the subject The side of the front side of the side is used as the main light, and the other light is placed at the camera position to illuminate the light as auxiliary light. As shown in the figure, we can calculate the F main) and F according to the formula of the formula exposure value. Supplementary), and compare the relationship between F and F), and find the ratio of light to F (auxiliary) = GN (auxiliary) / L (auxiliary) = 56 as an example, indicating that when F (main) changes, light Compared with the relationship between F and F (auxiliary), see Table 6 in Table 6 for the ratio of light to the relationship between the light and the auxiliary light from the L (auxiliary) auxiliary lamp. ((Auxiliary) auxiliary light refers to the GNC auxiliary ) It can be seen that only when the amount of main light exceeds the amount of auxiliary light can a larger light be obtained than usual. Criminal photography has certain requirements for depth of field. Therefore, in the specific shooting, always determine the aperture factor F value. When performing light ratio control, refer to the requirements of the above table and light ratio, according to L=GN/F To adjust, the positional relationship between the primary and secondary lamp distances can also be adjusted according to the following formula: the shadow zone that clearly expresses the texture and texture of the subject on the photo is the I-W zone, which enables the subject to begin to appear. The darkest exposed area of ​​the texture is the area. The area that can show sufficient shadow is the selective exposure of the I area. The brightness of the subject itself should be considered based on the minimum exposure required to make the shadow part of the subject appear appropriate. Reflectivity and the best tone performance area, the ratio of light to K is 1:2~1:4. In the choice of exposure, it is necessary to take into account the shadow requirements of shadows and highlights, combined with auxiliary lights and main lights. The amount of giving is comprehensively grasped and the specific selection method is as follows. See Table 7 for multi-lamp flash illumination. In addition to two lamps, the auxiliary light is added, and the effect light such as contour light and background light is added in the exposure control. , still with the amount of light of the main and auxiliary lights and The current tone is a pre-requisite factor. The method is basically the same as the above. The exposure processing of the contour light and the background light is essentially adjusted after the exposure amount is determined according to the main and auxiliary light factors, and then they should be given to the pass or area exposure. The amount of background light usually controls the amount of light in the V zone. The usual method is to control the amount of the profiled light to match the main and auxiliary light in the double-light flash illumination of the VI or Table 7 and to select the object reflectance range in the exposure zone.士-p flash illumination light than the main light illumination amount auxiliary light illumination amount exposure value shadow zone range table, with the aperture coefficient F set by the auxiliary lamp as the reference base amount, which is determined by F (auxiliary) = GN (auxiliary) /L auxiliary, the main light of the main light, F+1 and F+1~1, respectively, the amount of light given by the main lamp is one level and one level and half of the amount of light given by the auxiliary lamp, that is, by F main) = GN main) The aperture obtained by /L (main) is smaller than the first one and the first half of the F auxiliary. The F in the residual analog value column indicates that the aperture value set by the auxiliary light is directly exposed, and F-1 and F-2 are respectively It means that the aperture value at the time of exposure should be smaller than the aperture set by the auxiliary lamp. The secondary flash is the most common and most used in modern criminal photography. The ideal artificial light source, especially the small electronic flash lamp, has more and more perfect automatic functions, and is easy to use. Many of its characteristics are not replaceable by other artificial light sources, but in practice, the exposure is rarely controlled accurately. Especially in the control and grasp of the tone, due to the more use of its automatic function, although the shooting efficiency is improved, and large exposure errors are avoided, the autonomy is lost, and in a sense It doesn't really reflect the original appearance of the criminal scene. When using flash lighting for criminal photography, you can use the partition exposure method, combined with the reflectivity of the on-site object, by selecting the appropriate exposure and scientific control of the degree of development. The correct tone is the guiding significance for criminals to avoid blindness in photography work and to ensure accurate reproduction of the scene.
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