Chapter 5

Video Contrast Ratios

Help, They Don’t Match!

“Contrast ratio” refers to the maximum allowable difference between the brightest area of the picture and the darkest. While film can accommodate ratios as high as 100 to 1 (i.e., the brightest object may be 100 times brighter than the darkest object), television is commonly considered to have a contrast ratio of 20 to 1. Current technology has improved this ratio to 30 to 1, and 50 to 1 in HDTV. The use of CCDs and improved circuits are the reasons for this increased range. From a practical standpoint, you should still work with the 20 to 1 ratio, however. This gives you a 5 f-stop range. If you try to use a contrast ratio higher than the standard you are working with, your images will degrade. It is best to work with a lower contrast ratio than the medium’s standard you are currently working in.

Limited contrast ratio is frequently used as an excuse for not being able to produce aesthetically pleasing pictures. That is hogwash. You can create a great sense of depth and produce very good-looking pictures working within the limits of the transmission system. The earlier problems of extreme highlights causing pictures to bloom and distort have been almost completely eliminated because of CCDs. CCD cameras can handle highlights 6 to 8 f-stops above 100 IEEE units and can shoot in the “night mode” (0 Lux). Unless you are shooting live outdoor events, over which you have no lighting control, there is no reason to tax the system with contrast ratios it is incapable of handling.

Reflective Surfaces

In Chapter 3, you read about the reflective aspect of light as it related to the quality of that light. Two types of reflectors were discussed: those used in lighting instruments to gather and intensify light and those placed in front of lights to redirect and/or change the quality of their output. There is a tendency to regard these as the only forms of reflectors, but, in actuality, everything we see is a reflector. If it were not a reflector, we could not see it. We would have that mythical black-body spoken of in Chapter 1. The problems of contrast ratios arise with different materials having different degrees of efficiency in reflecting light.

Just as you should be concerned about the type of reflector that is contained in the instrument you use and about the type of reflector board you select, you must be concerned about the reflective quality of the subject matter that you are lighting. When you consider every subject you see as a reflector, you will tend to make better subject selections for your scenes. Your function as a lighting director is to put less light on highly reflective subjects and more light on those subjects that reflect light less efficiently, to bring the reflective difference between the two extremes within the 30 to 1 figure. When the degree of light placement is specific enough to reduce excessive reflective differences between various subjects, you will not exceed the 30 to 1 ratio of the system’s transmission capability, and you will still make all the pictorial elements visible and recognizable to the television viewer.

A Classic Problem

A classic example that comes to mind from my own experience involves a rather simple situation. At an early and uninformed period of my career, an appliance dealer said he wanted to show his best console color TV operating in a living room setting. It sounded simple. The television arrived at the studio—a bulky affair with a highly polished walnut cabinet. The first problem was to get the engineering staff to feed it a closed circuit RF (radio frequency) signal. This story dates me—but I’ll continue.

The television was placed against the back wall of a corner setting that was formed by two light blue flats from the scene shop. That looked bare, so a picture was hung on the opposing wall and the ubiquitous rubber tree plant was placed in the corner alongside the console. I used a number of scoops to light the scene. After all, I did not want a lot of ugly distracting shadows cast on the wall by the plant, and I knew that the soft light of the scoops would not cast distracting shadows. By now you are probably laughing because you know the results of such an approach.

When I looked at the floor monitor of the live camera feed, the console cabinet had no detail. It was just a dark form silhouetted against the light blue walls. The image of the closed circuit program was completely washed out, and there were several hot spots on the curved surface of the TV’s picture tube. Since the cabinet was an important aspect of the shot and it had no detail, I reasoned that I needed more light, and I pulled the scoops down lower on their pantographs to increase the intensity of light on the dark cabinet. Naturally, the light-blue flats got brighter, and the ratio between wall and cabinet remained the same. The hot spots on the picture tube moved a little, but they got even hotter. I was not lighting the scene; I was illuminating it.

It was time for another approach. What I needed were instruments with greater control over spill and the ability to intensify light in certain areas while cutting back in others. By using a series of highly spotted Fresnels, I was able to bring out greater cabinet detail and reduce the glare on the face of the picture tube, but I still had a number of hot spots on the screen. The rubber plant seemed to soak up light and required a Fresnel of its own to bring out any detail in its leaves and make it look lively. Other Fresnels were used to light the background. It took a great deal of time, and the production manager was screaming that we had to get ready for the news block.

How To Solve The Problem

Today, I would use my knowledge of physics to analyze the reflective surfaces involved, select instruments and accessories accordingly, and make short work of such an assignment. The biggest problem would be the TV. Of course the TVs today are totally different, so probably none of this would apply.

The highly specular faceplate and the extremely dark cabinet are at odds with each other. Putting enough light on the cabinet from a single bright source would wash out the closed circuit program and cause an extremely bright hot spot on the cathode ray tube (CRT). Since precise control of light is needed, I would select a Leko that would allow me to shape the light pattern to conform to the shape of the cabinet and keep excess light off the back wall. With enough light to bring out the cabinet detail, I would hang a rectangular French flag in the light path about 3 feet in front of the Leko so that it cast a dark, well-defined shadow on the CRT. This would remove the hot spot and the problem of contrast of the closed circuit playback. One instrument would light the TV. Another carefully focused Fresnel would take care of the rubber plant, and two special instruments could light the background. The use of a Leko with a window pattern gobo projected on the back wall would create the illusion of an off-screen light source and add depth to the flat walls with its pattern of light and dark stripes. The adjoining wall could be lit with a carefully placed Fresnel, and if necessary, a dot could be placed to remove a possible hot spot on the CRT. The lighting time would be 15 minutes; the results, believable and excessive reflective ratios of subject, controlled.

Avoid Noise

In addition to staying within the contrast limitations of the television system, you must also put sufficient base light on the entire set so that no areas produce video that is below the signal-to-noise ratio of your camera. We tend to worry more about highlights in lighting for video than we do about the dark areas. These areas can produce noise in the video if they are not lit to a minimal level. Areas from which there is no light reflected toward the camera will not reproduce as black on the waveform or picture monitor. They will be filled with creepy-crawly salt and pepper and colored splotchy noise. The “night shot” cameras I mentioned earlier in the book were designed to let the consumer record their child’s birthday party in candlelight. However, even if we were creating that exact same scene in the studio, we would still use conventional lighting.

Table 5.1: Contrast to f-Stop Conversion

If the brightest element in the scene is 100 IEEE units or peak white level, the rest of the picture will obviously ride up and down as manual or automatic adjustments are made. Because we are working with a 30 to 1 contrast range, 30 steps down from that peak amplitude is the noise thresh-old of the system. The noise does not go up or down by adjustment of controls; it just sits there. And to make matters worse, every time your nondigital master tape is dubbed down a generation, the threshold level of this noise is raised by a contrast step or two.

You can use the camera as an expensive light meter to determine the contrast range in a scene. Zoom in to a tight close-up of the brightest area of the scene and determine the f-stop for 100% amplitude on the waveform monitor or until the zebra stripe appears in your viewfinder. Now repeat the measurement looking at the darkest area of the scene. The f-stop range you identify translates into a contrast ratio by a base 2 power law (see Table 5.1).

Now you can see how improper lighting translates into noisy pictures. If your scene has a 5 f-stop range and a highlight suddenly appears and drives the auto iris down 2 f-stops, your remaining contrast range is now no better than 8 to 1. Any scene elements below that threshold will be driven into the mud. All you see on the screen is system noise.

On the other side of the coin, if your scene has only a 2 or 3 f-stop range from peak white to black, the picture is going to look flat and washed out, with very little color saturation, if the auto iris opens up in search of some highlight.

Lighting Ratios

Lighting ratios are determined by comparing the intensity of the key light to that of the fill. (If you are uncertain about key and fill lights, turn to Chapter 6.) Generally, only a small area of the scene, such as an actor’s face or body, is involved in computing a lighting ratio. Once that ratio has been established for a given position on the set, it should remain the same throughout a series of shots in that area. While the actual position of the key and fill light may change in height or horizontal angle to accommodate movement from shot to shot, the ratio between the two should not change so that the shots will match when spliced together. You may wish to change the intensity of the key and fill to control the depth of field in a series of shots. These changes will present no problem when the shots are cut together in post so long as there is a proportional increase or decrease in both the key and fill to maintain the same ratio between the two.

High Key

The lighting ratio between key and fill is important in establishing the mood of a shot. If the key light measures 160 foot-candles and the fill light provides 80 foot-candles at the subject’s location, you have a high-key setup with a 2 to 1 lighting ratio. Take special note that there is an inverse relationship between the term “high” key and the “low” ratio used to achieve it. In highkey shots the ratio of key to fill on the subject is low, like 2 to 1 or 3 to 1. High-key ratios are usually used in comedy and nondramatic situations. Backgrounds in high-key shots are generally brighter than those in low-key setups. The overall effect of high-key lighting is brighter shots with less texture. The look is similar to what you would expect when shooting outdoors on an overcast day.

Though it has been stated that backgrounds are usually brighter, by definition, in high-key shots, they do not figure into the calculation of a lighting ratio. They only come into play when exposure ratios are calculated (see “Exposure Ratios” later in this chapter.) There is nothing to prevent high-key lighting from being employed in front of a black backdrop.

Low Key

Low-key ratios are used in more dramatic situations in which the inverse ratio of the key to fill is high. An example of dramatic low-key lighting would be a situation in which the key light measures 200 foot-candles and the fill measures 25 foot-candles. In this case, the ratio would be 8 to 1. The result is deeper shadows and a more textured look. Lighting ratios deal only with modeling light on the subject, but as a general rule, low-key shots are involved in scenes in which the background is at a lower intensity than the background used for high-key ratios on the subject. More specular light sources are used for both the subject and the background when low-key ratios are being used.

If a lighting ratio has been determined by the lighting director, simply take a meter reading from the key light—let’s say it’s 400 foot-candles. If the lighting director wants a 4 to 1 ratio, the fill light must be 100 foot-candles. Aim the light meter at the fill light and move the light until the reading is 100 foot-candles.

When shooting video film-style, it is important to maintain the same lighting ratio throughout a series of connected shots in order to preserve continuity and make the shots cut together smoothly during post. Generally we think of continuity as something that deals with props and costumes being in the same positions from shot to shot (and this form of continuity is very important), but continuity of lighting ratios is of equal, perhaps greater, importance. With multicamera, real-time television production, such as sporting events or award shows, continuity is automatically maintained. When video is shot film-style, lighting each shot separately as filmmakers do, failure to keep track of lighting ratios can become a big problem in post, especially if a series of connecting shots is made over a period of 2 or more days.

The lighting ratio alone is not the only factor that determines the mood of the shot. In Chapter 7, we will examine a number of stock setups that can be used to convey mood and time of day. Changing the position and direction of the key can have just as great an influence over mood as the ratio involved can have.

Determining Lighting Ratios

There is a variety of ways to determine an appropriate lighting ratio for any scene. The biggest influence should be the subject matter of the script and the way it is being dealt with. The director’s intent and the elements of the set should also influence your approach. You may be asked by the director for a 2 to 1 high-key setup, or you may be told to work at a given f-stop in a high-key or low-key situation. Whatever the criteria, you will need to know the proper techniques for metering your setup.

Depending on the size of your crew and the setup involved, you have a couple of choices regarding metering techniques. You can use your incident light meter, with the flat diffuser disk or the hemisphere dome. If you are using a setup in which the fill light is positioned in front of the actor, instead of being off to the opposite side of the key light, the fill will also contribute light to the key side of the subject. In such cases, you should turn off all the set lights except the key and fill and take a meter reading using your incident meter with the hemisphere dome in place. Place the meter in front of the actor’s face and point it toward the camera. Take your reading. Say it is 250 foot-candles. That is the key, or “K” value, that will be used in the formula that follows. Next, have an assistant turn the key light off, and take a reading. This time it is 85 foot-candles. That is the value of your fill, or “F” value. Now let’s determine the lighting ratio (LR). Using the formula LR = K/F, LR = 250/85, or LR = 3 to 1 (approximately).

This technique is time and labor intensive because of the need to first switch all set lights off except the key and fill and then eventually switch off the key light. This is because the hemisphere dome will accept light from a wide angle, as much as 180°, and you do not want other set lights to contaminate your reading. That same factor also makes it possible for you to get an accurate reading of the combined key and fill lights to get a true value for the intensity of light that will be striking the actor on the key side. Once that value is known, you must measure only the fill light to calculate the correct ratio.

A metering technique that is much quicker and less labor intensive involves the incident light meter and the flat diffuser disk. In this case, the reduced angle of acceptance eliminates the need to kill all the set lights except the key and fill. To determine the lighting ratio, point the flat disk directly at the key light and move the disk around slowly. You will notice a slightly higher reading in one specific position. Use that high reading as the K value in the formula. Next, aim the flat disk directly and the fill light from the position of the actor’s face. To avoid turning off the key, you can use your hand to shadow the key light from the disk. As before, move the meter slightly to determine the highest reading from the fill light, making sure you continue to prevent the key light from falling on the disk. Make note of your F value and compute as before.

Exposure Ratios

The exposure ratio (ER) is the figure that indicates the brightness differential between two or more areas of the same scene. It is not the intensity difference between two instruments, the key and the fill. If you want, you can think of it as a lighting ratio on a grander scale. A typical example would involve measuring the intensity of light on the acting area and making another measurement of the intensity of the background. If the background is lit fairly evenly, only one measurement is required for the entire background. If the background is lit unevenly as the result of windows or practical lamps on set, these areas should be measured separately.

For example, if an effect light is projected on the back wall of the set to simulate sunlight streaming through a window on screen right, that area of the background will be brighter than the screen left area of the back wall that does not have a projected effect. We would expect screen left to be darker because of the absence of effect projection and the natural falloff characteristic of light. To illustrate, we will call the screen left section of the background area 1 (A1), the screen right area of the background area 2 (A2), and the acting area center screen area 3 (A3) (see Figure 5.1).

Because exposure ratios involve the intensity of larger areas lit by more than one or two instruments, meter readings are always taken using the spherical diffuser. You want to measure the effect of all the instruments that contribute to the intensity of a given area. When you meter an area to establish and calculate exposure ratios, use the spherical diffuser with all set lights for the master or wide shot on at operational levels.

You want to be able to determine the ratio between various areas that will be involved in the scheduled shots. The formula is ER = Al/A2. The intensity of each of these areas involved should be measured while aiming the meter at the camera with all the set lights operational.

If the shooting schedule calls for a master wide shot, two medium shots, and a close-up, as shown in Figure 5.1, you will need to meter the three areas involved. You do not have to maintain the same foot-candle readings in the two areas involved in each of the narrower angle shots, but the ratios must remain the same. You may want to decrease the light in area 3 and area 2 to reduce the depth of field for the close-up of actor B. The intensities can be lowered, but the ratios between the areas involved must remain the same to maintain lighting continuity.

When shooting indoors, where you have complete control over lighting ratios and exposure ratios, you should have no trouble staying within the 30 to 1 contrast ratio of the television system. Problems may be caused by the nature of the subject, such as extremely shiny objects. Those reflections can be reduced by treating their surface to control the potential problems.

Problem highlights from highly reflective subjects can be controlled by reducing the light that strikes them with dots, fingers, flags, scrims, or nets, or by spraying them with dulling spray, hair spray, or certain types of a light dusting of spray paint of the appropriate color. Matte silver gaffer tape can also be used effectively on some silver subjects to reduce glare. Naturally, any paint you spray on a subject must be tempered by the disposable nature of the subject. Shiny bald spots and high foreheads on talent can be treated with Arrid Extra Dry deodorant spray or baby powder to reduce glare and stop perspiration; the unscented variety is preferable. For purposes of tact, glue a plain paper wrapper around the can before taking it on set, and tell the talent you are using a special material to improve his on-camera appearance, if he asks any questions. Outdoors, the use of butterflys, scrims, and fill light can bring contrast ratios under control. We will examine these accessories in detail in Chapter 7.