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BOOKS AND ARTICLES Exposure, dynamic range, white balance and zebra
A. How the camera "sees" - dynamic range The camera captures the surrounding reality differently than the human eye sees it. What is the main difference? In the number of distinguishable shades of light from the darkest to the lightest. Here we come to the conceptdynamic range". This is the difference between the lightest and darkest part of the image that the camera can capture. In fact, this is the resolution of the camera in terms of the contrast of the perceived light. Our eye has a much greater dynamic range than any video camera (nature has tried, because it depends on vision our lives, and only the life of the operator depends on the quality of shooting :) ) This means that with strong contrast lighting, we are able to see both details illuminated by the brightest light and details in deep shadows. various estimates range from 800:1 to 1200:1, while fixed film - approximately 100:1, video - 40:1.
Imagine that you are in a room where a table lamp is on and bright daylight shines through the windows. You can easily see all the details that are in the shade, objects illuminated by a lamp, and a tree outside the window. If you shoot this picture with a video camera, then its dynamic range will not be enough to capture both objects in the shadows and details in the brightly lit areas of the room. There will be either details in the shadows and overexposure in the window and from the lamp, or black shadows and the absence of details in them and a well-designed window and a lighted table. This feature of video cameras leads to the fact that in order to get a natural-looking and beautiful picture, you have to artificially lower the contrast of the image using staged light. With our room example, we would need to raise the shadows to such an intensity that the camera "sees" the dark objects and at the same time does not lose the bright details in the window and on the table. The importance of the right light in your shooting cannot be overestimated, because in addition to the purely technical side just described, there is also an artistic one. When you shoot, you literally paint with light, and it creates the mood of your film, sets the accents and creates subtle subliminal hints. In the future, we will touch on the artistic laws of the work of staged light in more detail, but now it is important that we understand the technical side of the matter. B. Exposure 1. Essence: aperture, shutter speed and how they affect the result Exposure is the main control parameter when shooting. exposure is a combination of aperture and shutter speed. Diaphragm determines how open the aperture that allows light to the sensor/film is. Exposure determines how long the hole is open.
Diaphragm measured in relative units (f / 2.8, 3.5, 4.5, 5.6 ... 22, etc.), the essence of which is not so important. Just remember that the smaller the number, the larger the hole and the more light hits the sensor. Exposure measured in fractions of a second (1/2s, 1/30s, 1/250, etc.) This is the amount of time the shutter is open. The rest of the time it is closed, and the light does not hit the film. If we are talking about digital cameras, then there is no physical shutter, and the shutter speed is implemented electronically. Both the one and the other parameter affect the amount of light transmitted to the sensor, right? Aperture and shutter speed together Exposition - control the degree of exposure of the sensitive sensor or film. The greater the exposure, the more light enters the frame and the stronger it affects the sensor - the brighter the image in the frame. Too much light - the frame is overexposed. Too little - underexposed. The optimal exposure value gives us a well-exposed frame with detailed details in the shadows and without overexposure.
So, how to use all this in practice? All digital cameras have a built-in light meter. This is a device that, according to various principles, calculates the optimal exposure value for a given scene and automatically sets either both parameters (automatic mode) or one of the parameters (the second is set by the operator - mode shutter or aperture priority). You can always see the result in real time on the monitor of your camera. How to choose the right exposure values? Of course, you can be guided by your own eye and simply judge the correct exposure by the monitor, but shutter speed and aperture have important artistic features.
The main artistic tool of the operator in terms of exposure is diaphragm. The more the aperture is opened, the less depth of field. The more closed the aperture, the more objects in the frame will be sharp - depth of field increases with decreasing aperture. That is, if you want to take a close-up of a person with a blurred background, you need to keep your aperture open - maybe around 3.5 - 5.6 if you're shooting with a 50mm APS-C lens. If you are shooting a long shot of a landscape and you want everything that enters the lens to be sharp, then you cover the hole, say, to f / 8 on a 28 mm lens and choose a shutter speed so that the frame is exposed normally. Or use ND filters to control the flow of light - if you do not want to use a shutter speed that is not optimal for your tasks. How does the resulting image change excerpt? Everything here is quite logical and easier to understand than the issue with aperture and depth of field. Let's first talk about photography (that is, we shoot one frame) - this will make it easier to explain the essence. Shutter speed affects the clarity of fixed moving objects. Let's say you're shooting a static frame and you have a fast moving car in the frame. At a shutter speed of 1/30 s, the shutter opens exactly at this time, and the car manages to drive a certain distance. As a result, the frame captures the initial, final and intermediate phases of the car's movement - it comes out blurry. That is, shutter speed "freezes" a certain piece of time, and all changes in the position of objects in the frame that occurred at that time are reflected in the frame in the form motion blur. There are various artistic possibilities here.
If you shoot a fast running athlete with a very slow shutter speed (let's say 1/1000), then every detail in the frame will be clear, you will get a frozen moment of reality, with particles of sand flying into the air from the impact of a sneaker, etc. Now imagine that you put your camera on a tripod at night over a busy highway and opened the lens for 25 seconds. Cars shining with headlights will literally draw glowing lines of their trajectory in your frame, and you will get a night road filled with lines of light.
Or a simple example - shooting fountain jets. The jets are made up of many droplets moving at high speed. If you shoot a fountain with a shutter speed of 1/30 for example, then the fountain will consist of monolithic jets. If you set the shutter speed to 1/500, then the jets will break into thousands of drops, because each of them does not have time to fly even a millimeter in 1/500 of a second and thus "hangs" in the air separately from its comrades.
Okay, with one frame we figured out (we discussed the photo frame). What features does video excerpt? Let's think of a video as a series of photographs that change at 25 frames per second (25 fps is the standard frame rate when PAL shows a progressive, that is, non-interlaced, broadcast). The standard video shutter speed for 25 fps is 1/50 s. That is, each frame is exposed for 1/50 of a second. This is not a very short time, and many objects usually have time to make a certain movement, which results in a slight blur of moving objects on the frame. When the movement of the object is fast, and it is blurred more, when it is slow - less. This motion blur from frame to frame helps to convey motion in the resulting video and the video comes out smoothly. This is one of the differences between photography and video - chances are you wouldn't be all that happy with most video freeze frames with fast moving objects, but when put together, these frames capture motion well. What happens if we shoot with a very fast shutter speed, like 1/250 or 1/500? Each frame will be virtually devoid of motion blur. If we're running at 25 progressive frames per second (the standard when we're shooting in a PAL country), we'll get some strobing video if there's camera movement or objects being filmed. This visual strobe comes from the fact that 25 frames per second is not enough to create a completely smooth illusion of movement, and now there is no help in the form of motion blur from frame to frame. However, this is not always bad. In some cases, such an effect can be used for an artistic task. For example, in Saving Private Ryan, Steven Spielberg used a fast shutter speed for combat scenes and, with the help of a subtle strobe, conveyed the state of extreme concentration and lightning-fast action of soldiers in battle, a state in which a person on adrenaline perceives everything around with increased "clarity". Another example. When shooting for special effects, it is sometimes necessary that the objects on each frame are absolutely clear, so that it is possible to cut them out of the background and use them separately, and add motion blur later. Then a short shutter speed is also used. But if you do not have such a goal, then you should adhere to the classic shutter speeds when shooting, and if necessary, reduce the amount of light, use neutral filters (a darkened plate made of special glass). 2. ISO sensitivity The film comes in different sensitivities. The more sensitive the film, the less light you need to use in order to get a frame. With sensitivity, however, spurious noises get mixed up with the image. Sensitivity is measured in specific units and there are various standards for its measurement. Today, the standard is used almost everywhere. ISO, so we will operate with these units. For a rough understanding, on the fingers: 80 ISO is low sensitivity and practically no noise, 400 ISO is medium sensitivity with a minimum of noise, sensitivity of 800 ISO and above can be considered high. Prosumer and television cameras usually have a fixed, unchanging sensitivity that can be measured in the same ISO units. It is possible to increase the sensitivity of the sensor by applying the so-called GAIN - signal amplification. At the same time, noise in the image grows, which is undesirable, but when shooting reportage, sometimes there is no other way out. DSLR и digital movie cameras have the ability to set the sensitivity of the sensor within a wide range, which is very good for work. In addition, due to the large sensor, noise even at very high sensitivity is minimal, which allows you to work with a minimum of light, the value of which is difficult to overestimate. 3. Features of digital video (overexposure) There are certain features of how digital signal passes too large values. When we have too much light in certain parts of the frame, overexposure. In the case of film and even analog video, such areas flow to full blown gradually and some graphic information can be saved in them. In the case of digital video, the possible grid of values simply overflows and the entire area becomes (conditionally) white, that is, the maximum brightness. No information (details) in overexposed areas is saved. This is a limitation of the technology and must be taken into account when shooting. In general, in the case of digital photography, it is better to underexpose a little and then pull the result during processing, than to overexpose and never return information from the lost areas of the image. 4. "Zebra" All professional cameras have a tool to help you judge exposure. Yes, you can see the result on the camera monitor, but where does plain white end and where does overexposure begin? How to correctly determine the correct exposure of the frame, if the sun hits the monitor, for example, or the camera allows you to adjust the brightness and contrast of the monitor and you get completely different pictures with the same exposure? Where is the truth?
And the truth in "zebra". This is a function that turns on the shading of areas of the image on the monitor when they exceed certain brightness limits. Let's say you can make a zebra appear when the brightness of an image area reaches 100%. Then you will always see which parts of the frame you have overexposed and will be able to make decisions based on accurate information. It is even better when it is possible to turn on two "zebras" at the same time. One will show, for example, a brightness level of 70% (it is useful to bring light objects to this level), and the second - 90-100% (white objects in direct sunlight, glare, etc.) Using this tool, you will always confident in their exposure. 5. White balance Light can be of different colors. It depends on the light source. The light from a candle is a rich yellow color, ordinary incandescent lamps also shine with yellow light, the sun is closer to blue. Therefore, there is a concept color temperature light source. Color temperature is the temperature, measured in Kelvin, of an ideal black source (yes, sounds like a spherical horse in a vacuum..) that emits light of a particular hue. Color temperature of incandescent lamp - 3200K (yellow tint), sunlight - 5600K (bluish tint). As the color temperature rises, the light changes from "warm" (yellow) to "cold" (blue).
Our eye easily adapts to light of different color temperatures, so we hardly notice the difference. However, the camera needs a point of reference - what kind of light is considered white? Imagine that we are shooting a sheet of white paper under the light of an incandescent lamp. If the camera is set to the color temperature of the sun, the paper will come out yellow. We know the paper is white, but the camera needs to be told that and give it a point of reference. We do this using the "white balance" function.
It's usually done like this. A sheet of white paper (or a gray card) is placed under the main light source (lamp or sun). The camera is aimed at this sheet in such a way that the white completely fills the frame. A button is pressed and the camera is balanced and you get the right colors. There is also the possibility to use preset presets - usually these are 3200K и 5600K for incandescent and solar lamps, respectively. This is convenient when there is no time to set the white balance manually (mainly in reportage and documentary shooting). There is also an automatic mode in the cameras, but it often cannot cope with difficult lighting conditions and it is better not to use it at all. What to do when you have mixed light sources? For example, sunlight from the window and an incandescent lamp in the room. In most cases, it will be best to bring these sources to the same color temperature, that is, either hang huge yellow color gels (soft light filters) on the windows, or, easier, use a blue filter on a yellow light fixture. Artistic solutions using different color temperatures of light are possible and will definitely be in your practice, you just need not be afraid to experiment. Author: Alexander Cherny; Publication: anti-gorod.com Oleksandr Cherny is a freelance editor, filmmaker and cameraman based in Kyiv, dealing with commercials, music videos and feature films. His portfolio includes work for brands such as Tuborg, Chernihivske, MTS, Kyivstar, LG, Pepsi and others. Alexander maintains a professional blog at http://achorny.livejournal.com/, he can be contacted at info@alexchorny. com.
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