Aperture use skills

Aperture adjustment technology and application skills

Abstract: Correctly mastering the adjustment technology of aperture is an important part of shooting ideal pictures. This paper discusses the importance of aperture adjustment of color camera and the automatic and manual aperture adjustment technology, and puts forward several aperture adjustment methods under special circumstances. Keywords: aperture adjustment brightness

For all kinds of color TV programs, whether the reproduction brightness is handled properly or not will directly affect the expression of TV program content and the quality of TV production. The fidelity of the reproduced image is one of the important indexes of a color camera, which depends not only on the camera itself, but also on the illumination and method of the lighting source and the aperture adjustment technology. It can be said that aperture adjustment is the key to shooting an ideal picture.

1, the function of the aperture

Aperture control is one of the basic functions of the camera, and the brightness of the scene facing the camera changes greatly. Therefore, it is necessary to adjust the aperture anytime and anywhere to ensure the image quality, otherwise it will destroy the color balance of the picture and lose the change of the degree level.

The brightness of scene reproduction is not only related to its own characteristics, but also related to the illumination of lighting sources. Because the intensity of light and shade that the CCD device in the camera can bear is limited, and this limit is far less than the range of light and shade of natural scenery. In order to adapt to different lighting conditions, a variable circular diaphragm (commonly known as aperture) is installed in the zoom lens of the camera to limit the size of the incident beam, and the amount of light entering is controlled by changing the aperture of the diaphragm. The larger the aperture, the more light enters, and vice versa. The amount of light entering is proportional to the area of the aperture. In addition, the actual brightness projected on the CCD device is also related to the focal length of the lens. Under certain illumination, the brightness is low when the focal length is long and high when the focal length is short. Usually, the ratio of the actual aperture of the diaphragm to the focal length is used to represent the actual transmittance of the lens (that is, the relative aperture is expressed as a fraction of the f value). The value of f is F 1.4, F2, F2.8, F4, F5.6, F8, f1,F 16, F22, which increases in multiples of the square root, and the aperture area corresponding to each file is reduced by half, so is the amount of light entering, that is In order to meet the requirements of F value, the aperture and zoom ring in the zoom lens are linked. As long as the aperture f value is the same, the brightness projected on the CCD device is equal. In addition, the aperture also affects the performance of depth of field, with a small aperture and a large range of depth of field. Because a series of aperture values control the amount of light entering the lens, the camera can be flexibly selected and work normally under a wide range of brightness changes.

2. Adjustment principles and methods

The camera has two control modes: automatic aperture and manual aperture. The basic principle of automatic aperture is: according to the illumination of the shooting scene, using the feedback of the video signal, the aperture is enlarged or reduced accordingly, and the appropriate light input is maintained, so as to obtain the specified output signal intensity and keep the white level of the video signal of the camera at the specified value. When the illumination of the shooting scene is high, the video signal amplitude of the camera will increase. If it exceeds the specified signal level, the automatic aperture circuit will generate corresponding voltage to control the lens aperture, thus reducing it until the video signal output amplitude meets the requirements. If the illumination of the scene becomes low, the automatic aperture circuit will adjust the aperture to make it larger and increase the output amplitude of the video signal.

The captured light image is transformed into three primary colors of red, green and blue through a dichroic prism, and is converted into primary color signals through corresponding CCD devices. After the red, green and blue video signals pass through the signal processing circuit, they are sent to the non-addition circuit at the same time, and the maximum level is selected as the input video signal for automatic aperture control, and then sent to the detection circuit, and its maximum amplitude or average value is measured through peak detection or average detection. It is converted into corresponding DC voltage (as the shooting environment is ever-changing, the output of the detection circuit mostly adopts the compromise value of average value and peak value), and finally it is sent to the comparison circuit. The DC voltage output by the detection circuit is compared with the reference voltage, and its error signal is used to control the motor of the aperture to adjust the aperture. If it is higher than the reference voltage, the error signal is negative, thus reducing the aperture until the error value is zero; On the contrary, the error signal is positive, and the aperture becomes larger, so that the video signal remains at a predetermined standard level.

At present, professional or broadcast cameras adopt automatic aperture adjustment, manual aperture adjustment and instant automatic aperture adjustment to cope with the brightness changes of scenes under different conditions.

Automatic aperture adjustment means that the aperture selection switch is placed in the automatic position (A), and the camera tracks in time according to the brightness of the shot scene, and automatically adjusts the aperture to control the amount of light entering the lens, so as to avoid losing the gray level in the bright part of the image and destroying the color balance, and adapt to the brightness change of the scene in general. Manual aperture adjustment is to put the aperture selection switch in the manual position (M) and adjust the aperture ring according to the needs of the picture, which can achieve good results for the special effects of backlight, sudden change of brightness, strong contrast and picture requirements. Real-time automatic aperture adjustment means that when the aperture selection switch is set to the manual position (M), the instant automatic aperture adjustment button is continuously pressed, and the aperture will temporarily switch to the automatic aperture adjustment mode, and the aperture is adjusted by using the automatic aperture adjustment circuit to detect the brightness change of the measured scene. After releasing the button, it will return to the manual state, and the aperture will be fixed at the adjusted value.

Step 3 adjust your skills

The adjustment of aperture is a basic operation for photographers in the process of shooting. The photographer carries the camera on his shoulder, operates the zoom and recording switch with his right hand and adjusts the focus ring with his left hand. It is necessary to focus on composition, and it is difficult to take into account the change of environmental brightness when shooting in the field. Because of the complexity of shooting environment and conditions, photographers often use automatic aperture control to deal with the brightness changes of shooting scenes.

Automatic aperture adjustment skills under special conditions

Automatic aperture does not need to measure and adjust the aperture in advance, which solves the contradiction that it is difficult for photographers to grasp the change of ambient brightness at any time and adjust the aperture in time during shooting, and helps to concentrate on finding the shooting angle. Especially when shooting scenes with different brightness such as pushing, pulling, shaking and moving slowly, the aperture can be automatically adjusted according to the brightness change of the shooting scene. For close-ups of people or scenes with little contrast, good exposure quality can be obtained when shooting in forward light, side light or backlight. However, the automatic aperture adjustment circuit determines a compromise value between average detection and peak detection of video signals according to general shooting conditions. In practical use, due to the limitation of equipment conditions and working environment, the ratio of the two can not be adjusted at will, and it is also impossible to detect the area ratio of the brightness contrast between the foreground and the background (that is, the ratio of the high brightness area to the low brightness area in the same picture). In the same brightness contrast environment, if the area ratio is different, the adjustment of the aperture is different, so it is difficult for the automatic aperture to adapt to the situation that the brightness contrast in the same picture is too large.

In the environment where the brightness of the subject is low and the surrounding scenery is high, if you use automatic aperture to shoot, the subject will be underexposed and lose color. For example, when shooting trees with cloudy background, because the brightness of the sky is very high, the brightness of the trees is very low, the exposure of the trees is seriously insufficient, and the color is completely lost and turned black. The green tree will be a black and white image. In the environment where the brightness of the subject is high and the brightness of the surrounding scenery is low (for example, the brightness of the actors on the stage is usually 3 ~ 4 times higher than the background), the subject is overexposed, and the color is completely lost and turned white. When shooting scenes with different average brightness by pushing, pulling, shaking, moving and other techniques, the aperture will be enlarged or reduced accordingly with the brightness change of the scene, which will make the brightness of the picture change constantly and produce a flicker effect. For example, when the lens is turned from outdoor to indoor, the picture will darken first and then light up; In the process of shooting a dialogue scene, if someone wearing white clothes enters, the aperture will narrow, as if the brightness in the environment suddenly drops, thus affecting the artistic effect of the picture. It can be seen that the automatic aperture is not suitable for use in any environment, and it has the following shortcomings: ① Because the aperture is controlled by the average brightness, some scenes will be obviously underexposed when shooting scenes in Gao Fancha or shooting against the light, and the tone level of the picture will be destroyed; (2) When showing a moving scene, the movement of some light-colored objects will make the aperture big and small, and the brightness of the picture will not reach a stable state; (3) Because the feedback signal is used for comparison and amplification, and then the size of the aperture is changed by driving the motor, when the brightness of the scene changes, the aperture also changes, which affects the artistic expression effect. Therefore, recently produced cameras adopt the following methods to improve the quality of captured images.

(1) Set the automatic aperture detection window. Adding a correlation detection window in front of the automatic aperture detection circuit, that is, only detecting the average value or peak value of the video signal in the window and ignoring the scenery outside the window, is beneficial to improving the image quality of backlight shooting. However, when shooting a moving bright object, if the object enters and exits the detection window at this moment, the brightness of the window area will change, which will lead to the change of the aperture, which will lead to a great change in the brightness of the picture.

(2) Add backlight and spotlight functions. In a complex environment, it is difficult for photographers to master the manual aperture adjustment technology, and some cameras have added the automatic aperture compensation function of backlight and spotlight. When shooting in a bright backlight environment, press the backlight compensation key, and the aperture will increase by about one notch on the basis of the automatic aperture. When shooting in front of a high-brightness spotlight, press the spotlight compensation button to get an image with reduced brightness level, and avoid the white-hot effect caused by high brightness.

(3) Additional automatic aperture fine adjustment function. The camera equipment used in the studio often has the function of automatic aperture fine adjustment. On the basis of automatic aperture, choosing the ratio of video peak value to average value and adjusting the aperture with remote controller is helpful to improve the image quality, but it can only be used if shooting conditions and working nature permit.

Manual aperture adjustment skills under special conditions

When shooting demanding TV programs under special conditions, most people use manual aperture and adjust the aperture according to actual experience. In order to reduce the change of light and shade to an imperceptible range, the following methods are usually used to determine the aperture value.

(1) Automatic determination method: determine the plane position, focus and compose. Firstly, the automatic aperture mode is adopted, and the aperture will be detected according to the average brightness of the scene and stay at a corresponding F value. Then, before shooting, turn the aperture mode selection switch to the manual position, and the aperture will no longer change with the brightness of the scene. In the feature film "Choice Without Regret", which won the second prize of the national party member Audio-visual film Red Star, the author used this method to shoot the scene of people visiting the host, avoiding the brightness change caused by people walking back and forth and affecting the picture quality.

(2) Compromise method: For technical shots such as pushing, pulling, shaking and moving, the picture content is usually a continuous process. First, compromise the aperture value of the starting frame in automatic aperture mode and the aperture value of the left picture, and then shoot in manual aperture mode. At this time, the picture quality of the starting frame and the left picture is slightly reduced, but the overall effect of the picture tends to be consistent. In the feature film Scholar, which won the third prize of the national party member audio-visual film Red Star, the author set the aperture selector switch to the automatic position (A), and the aperture value was F 1 1 when measuring the amplitude, and it was F5.6 when measuring the amplitude. After shooting, the aperture value is set to F8 by manual aperture method, and a good picture effect is obtained.

(3) Left picture orientation: Because the left picture is usually the destination and summary of the performance content, for the technical lens with high picture effect and push and pull, the average brightness of the left picture composition should be used to determine the aperture value, and the manual aperture method should be used for shooting. In the four episodes of TV series "Pioneer of Special Zone", when shooting the scene of the protagonist tackling a scientific research project, the author should not only shoot the working environment of the characters, but also show their concentration, and use the push method to determine the aperture value by taking the close-up of the face as the left picture, which has achieved good performance results.

(4) Face measurement method: In the natural environment, regardless of the light intensity, the skin reflectivity of yellow people is about 70%, which is equivalent to the medium level of brightness change from bright to dark in the scene. When shooting a character scene, the aperture value is determined by this intermediate gray level, which is beneficial to the reproduction of the dark part of the picture. First, turn the aperture mode selector switch to the automatic (A) position, and then turn the aperture to the manual (M) position based on the close-up of people, and then shoot.

In addition, when shooting a scene with strong contrast, we should make full use of the zebra spectrum provided by the camera as a reference standard to adjust the aperture to ensure that the meaningful parts in the picture are exposed accurately and the colors are saturated. In strong sunlight, when shooting the foreground but the background is blurred, the automatic aperture mode will narrow the aperture and increase the depth of field. Therefore, a neutral gray light barrier can be added in front of the lens, so that the light of each wavelength is weakened in the same proportion, the light input is reduced, the aperture is increased, and the depth of field is reduced.

Adjustment skills of simulated shooting

Adjusting the aperture according to the shooting environment can realistically restore and reproduce the tone level of the scene, but sometimes in order to facilitate the organization of shooting, the night scene can be simulated in the morning, afternoon or evening.

Polarizer can weaken or eliminate polarized light, so it can be used to control and select light, weaken the strong light in the sky and darken its color tone, that is, it can be used to shoot pictures simulating night scenes during the day. The method is: install a polarizer in front of the lens so that the direction of the lens is 90? Angle, the polarization direction of the polarizer (that is, the direction that allows light to vibrate) points to the sun, and at the same time, the amount of light entering is controlled by manual aperture to obtain appropriate picture brightness.

(1) Simulate the moon scene. In the afternoon, when the sun is slanting, shoot against the light. After determining the position, focusing and composition, carefully rotate the polarizer and adjust the aperture at the same time, so as to darken the brightness of the scene, open the light and dark levels, turn the foreground of the picture into a black silhouette, and the outline of the foreground is blurred, showing the effect of "moonlight night". When the sun hides behind thin clouds, it can set off the atmosphere of "moonlit night". Because the strong light caused by the sun in the scene becomes the strong light caused by the moonlight, the picture has a strong sense of nature and realism. The author used this method when shooting the feature film "Cultivating Hope in the Field of Hope", when the host visited the villagers at night.

(2) Simulate the night scene. Shoot in the morning and evening. After determining the position, focusing and composition, adjust the polarizer and aperture, and then set the color temperature filter to 3200K to make the overall effect of the picture blue. When shooting in the morning, try to make the scene in a dark background, such as the shadow of buildings. When shooting at night, we should make full use of street lamps and car lights as companions of "night scene" and control the distribution of light and dark areas in the background at the same time, so as to achieve approximate results.

In the process of simulated shooting, we should also pay attention not to use skillful shooting methods such as shaking, moving and following, otherwise the angle between the light and the camera will change, and the color of the sky in the picture will also change, thus affecting the visual effect. At the same time, color monitors should be used to monitor the picture effect on the spot and adjust it in time to deepen the artistic conception of the picture.

To sum up, when shooting people's scenery, because the camera faces a wide range of brightness changes, all video signals have to be amplified, so it is inevitable to change in a certain dynamic range, and it is impossible to fundamentally solve the problem of average brightness change of the picture. However, as long as you master the basic principle and application technology of aperture control, be familiar with the functions of the equipment, and deal with it flexibly according to different situations, you can reproduce the shooting scene realistically and improve the artistic expression, thus achieving the most ideal requirements of TV creation.