linear, gamma, log
CAMERA EXPOSURE INDICATORS
Since exposure meters won’t always give the most accurate guidance when the gamma or other aspects of image (such as knee or toe, for example) have been changed, other exposure guides are needed. For-tunately, professional cameras have several exposure tools. As we’ll see in the section Philosophy of Exposure, this has changed with the newest generation of cameras.
ZEBRAS
In the viewfinder or appearing on the camera operators monitor, zebras have long been a useful tool for judging exposure. Unlike his-tograms or goal posts, they show you specifically within the frame where there is overexposure or exposure at the specific level you are looking for, such as skin tone.
Zebras indicate a particular IRE level in the scene; however they are useless unless you know what IRE level the zebras are set to indi-cate. Aside from being useful in judging specific tones for exposure, zebras excel at indicating overexposed areas.
Professional cameras offer the choice of selecting what IRE level you want to set the zebras for and generally have two sets of zebras which can be used in a variety of ways. A fairly common use is to
Figure 5.12. (Left) Normal exposure on an HD camera. Notice the proper skin tones, the correct color satura-tion in the chart, and how all seg-ments are shown on the gray scale.
These images are from a latitude test on an HD camera.
Figure 5.13. (Right) At just two stops overexposed, the skin tone is blown out, the color saturation is severely reduced and there is no separation visible in the lighter tones on the gray scale. Clipping of the highlights appears on the wave-form monitor as a flatline at the top of the signal. This is at only plus two stops of exposure and already it has exceeded the latitude (dynamic range) of the HD camera. This is an older camera; more current cameras have more dynamic range.
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target one set of zebras for midtones (such as typical skin tone) and the other set for dangerous overexposure, such as 90, 100 or even 108 IRE. Cameras use different methods for differentiating the two sets of zebra stripes (and usually offer the option of turning one or both of them off ). In some cases, one set of stripes is diagonal and the other set is the opposite diagonal; some are horizontal, some are
“crawling ants” and so on. Figures 5.14 and 5.15 show a method used by Sony — the sets of zebras are diagonal lines tilted in opposite directions. It is important to be sure you know what the protocols are for the camera you are using and be sure you know what levels the zebras are set for. If you are using zebras, it will be an important part of your camera prep and setup procedures to check what levels they are set to.
HISTOGRAM
The histogram is probably the simplest tool around for analyzing an image. It is somewhat primitive but can be very effective, espe-cially for a very quick look at the overall exposure of a shot. It is also nearly ubiquitous: even the simplest consumer still cameras have histogram displays. It is basically a distribution diagram of the tones in our scene: on the left hand are the dark tones and on the right side of the diagram our light tones. (Figures 5.16 through 5.18.)
A normal scene properly exposed will have a more or less even dis-tribution of tones from dark to light and won’t be too much pushed up against the left or right sides. The distribution of tones in the middle will change based on the scene; it’s the light or dark tones pushing up against the extreme limits that you really want to watch out for.
TRAFFIC LIGHTSAND GOAL POSTS
These are particular to Red cameras and provide very general but useful information. They are basically warning systems to alert the operator to percentages of the picture that are either in clipping or in “noise” at the bottom end — meaning underexposed. They may seem simplistic, but are in fact well suited to the new exposure
Figure 5.14. (Top) Normal exposure with the zebras (70% and 100% in this example) and shown on the waveform monitor.
Figure 5.15. (Above) The same scene very overexposed with zebras and the waveform. On this Sony camera, the two types of zebras are shown as opposite diagonal angles.
needs of shooting RAW and log. Even though this method of shoot-ing allows a good deal more room for error, that doesn’t mean that making errors in exposure is any more of a good idea than it was with traditional HD recording.
GOAL POSTS
Red’s explanation of their Goal Posts, (Figure 5.19) is this: “In order to quickly balance the competing trade-offs of noise and highlight protection, Red cameras also have indicators at the far left and right of their histogram. Unlike the histogram though, these are not affected by the ISO speed or look setting, and instead represent RAW image data. The indicators are depicted as vertical bars to each side of the histogram, and are often referred to as the “goal posts,” since the aim is usually to achieve a histogram which doesn’t hit either side.
The height of each goal post reflects the fraction of overall pixels that have become either clipped (on the right), or near the capabili-ties of the camera to discern real texture from noise (on the left). The full scale for each goal post represents a quarter of all image pixels. In general, the left goal post can be pushed up to about 50% height and still give acceptable noise, but even a small amount on the right goal post can be unacceptable, depending on where this clipping appears in the image.”
TRAFFIC LIGHTS
In addition to the Goal Posts, Red provides another indicator of the crucial indicator of clipping — the so-called Traffic Lights, shown in Figure 5.20. These indicators show what color channels (Red, Green, Blue) have some areas that are being clipped. Some people see these indicators and assume they mean Go, Caution, Stop. Not so at all. Unfortunately, calling them traffic lights reinforces this mis-conception.
Figure 5.16. (Top) Histograms can be a very useful tool for judging exposure. The top figure shows a fairly normal distribution of high-lights, midtones and shadows.
Figure 5.17. (Middle) An underex-posed shot — all tones are pushed to the far left.
Figure 5.18. (Bottom) An overex-posed shot, with all tones pushed far to the right.
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Notice for example that the third color is not yellow, but blue. The real meaning is that when about 2% of the pixels in a particular color channel have reached clipping, the corresponding light turns on. It’s one of those things that are wonderfully precise in scientific terms but still require a great deal of human interpretation to be used prop-erly. Good news, we have delayed the takeover by the machines for at least a few months! Sleep well, Sarah Connor.
According to Red: “When about 2% of the image pixels for a par-ticular color channel have become clipped, the corresponding traffic light for that color channel will light up. This can be helpful in situ-ations where just the red channel has become clipped within a skin tone, for example. In that case, the right side goal post would be much lower than it would appear otherwise, since all three channels haven’t become clipped.”
Adam Wilt has this to say about these tools “The traffic lights seem interesting but in the end, how valuable is it really to know that
‘2% of pixels in that channel’ are clipping? In my Red work, I find them useful as ‘idiot lights’: I can tell at a glance if there’s a possibility I might be getting into trouble. They don’t replace a careful study of the histogram; what they do is say, ‘hey, buddy, you might want to take a closer look here...’ and they say it even when I’m too busy to be focusing on the histogram, because I’m focusing instead on move-ment and composition.
“Same with the Goal Posts, they are halfway between the see-it-in-a-flash ‘idiot lights’ of the traffic lights and the study-intensive his-togram. They show me (a) I have stuff in the scene that exceeds the range of the capture system at either or both ends of the tonal scale, and (b) by comparing their relative heights, I can see quickly if I’m losing more shadow details, or more highlights, or if I’ve balanced the losses equally (assuming that’s what I want, of course).
“I use ‘em all: traffic lights as a quick-and-dirty warning, goal posts as a highlights/shadows balancing indicator, and the histogram or an external WFM to see in detail what the signal is doing. The traffic lights and goal posts don’t show me anything I can’t get from the histogram or WFM, but they show it to me very quickly, with a minimum of focus and concentration required on my part to inter-pret the results. It’s nice to have choices.”
FALSE COLOR EXPOSURE DISPLAY
Many pro cameras and field monitors now offer the choice of false color, which displays different tonal ranges coded in various “false”
colors (Figure 5.21). As with any color code system, it’s worthless unless you know the key. Although different camera companies use their own set of colors, they usually have some commonalities. False
Figure 5.19. (Top) Red’s Goal Posts are also very helpful in judging exposure. They are the two vertical stripes at the far right and left of the histogram. The height of the bar on the right indicates what percentage of pixels are clipped or in noise. The bar on the left indicates what per-centage of pixels are in noise. Full scale of these indicators is only 25%
of total pixels, not all of them as you might expect.
Figure 5.20. (Above) Two views of the Traffic Lights on a Red camera.
It is important to note that they are not red, yellow, green as you would expect in real traffic lights. They are not the equivalent of Go, Caution, Stop. What they actually mean is that when about 2% of the pixels in a particular color channel have reached clipping, the corresponding light turns on.
color displays can usually be turned on or off either in menus or with assignable buttons on the exterior of the camera that can be assigned to any one of several different camera controls depending on the wishes of each individual user). Once you get to know them, they can be a useful guide to exposure, but they can also interfere with viewing the scene while operating the camera. For this reason, many cinematographers use them only when lighting the scene and while determining the proper exposure for the scene and not while shoot-ing the scene, when they might be a distraction for the operator.
RED FALSE COLORS
Red cameras have two false color selections: Video Mode and Exposure Mode. Video mode false colors are shown in Figure 5.22. Exposure Mode is a more simplified method which is used with RAW image viewing. Most of the image will appear as a grayscale but purple will be overlaid on any parts of the image that are underexposed and red will be overlaid on any overexposed regions of the frame. Since this applies to the RAW data, it indicates over and underexposed regions of the frame regardless of the current ISO or look settings.
Since it is simpler, it can be less confusing to look at than video mode false colors; choosing which mode to use for viewing can depend on whether it is used only in preparing the shot (while lighting) or if an exposure mode or focus assist (peaking) is going to be used while operating the actual take.
COMPARING RED EXPOSURE MODES
Table 5.2 shows the specifications of the three Red exposure modes
— Video Mode, Zebra Mode and Exposure Mode. Zebra and Video Mode are based on IRE or RGB values, which is a relative scale based on the output signal sent to the monitor, not necessarily the actual values of the recorded scene. Red says “It is most useful when assessing direct output via HD-SDI, or whether the current ISO and
‘look’ are a good starting point for post-production.”
Figure 5.21. False color display in Adam Wilt’s Cine Meter II iPhone app.
Unlike most cameras, the color code is user adjustable — the value range can be selected for each color.
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Red puts it this way: “As with other IRE-based modes, zebra mode is only applicable for the current ISO and look settings (such as with HD-SDI output) — not for the RAW image data. If anything is changed in post-production, the indicators won’t be representative of the final output tones. In those situations, zebra mode is there-fore more of a preview and output brightness tool than an exposure tool.” Despite this, they can still be useful in some circumstances. In general, most exposure tools are measuring the image with the vari-ous adjustments to the look already applied.
RAW in this context, is an absolute scale based on the output of the sensor. It is not necessarily related to the viewing image in terms of brightness values. Red says:“This is most useful when trying to optimize exposure and looking toward post-production.” This is, of course, the basic concept of shooting RAW vs. shooting video that is more or less “ready to go.” Shooting with the image adjusted to roughly where you want it to be can be thought of as “HD style”
shooting, where gamma, knee and black values are adjusted with
“the knobs.”
Shooting RAW/Log, as we have talked about, is not oriented toward producing a final image, but is more about producing a “dig-ital negative” that has great potential for fine-tuning and adjustment further down the line. As we will see, this is the fundamental con-cept of the ACES workflow as well. As we have discussed elsewhere, the downside of this is that the images are not directly viewable and this makes using exposure tools like zebras and histograms pretty much useless — they can still be used as approximations but they are not really accurate.
With a Red camera the exposure mode you choose will determine what type of false color schemes you see displayed in the viewfinder.
Red sums up their overall recommendation for using these tools:
Figure 5.22. False colors can give more precise exposure informa-tion about individual porinforma-tions of the image, but only if you know the code! At top is the grayscale includ-ing superblack and superwhite. Below that is the color code for Red camera false colors, then Alexa’s color code and finally the Red camera Exposure Mode at bottom.
Red Camera False Colors: Video Mode
Red Camera False Colors: Exposure Mode Arriflex False Colors
rple
Red
Purple
RED EXPOSURE MODES Basis Levels Adjustable?
Exposure Mode RAW 2 No
Video Mode IRE 9 No
Zebra Mode IRE 1-3 Yes
Table 5.2. Red’s Exposure modes and specifications. Notice that Video Mode and Zebra Mode are only appli-cable in IRE (Rec.709, not RAW).
“First, in Exposure Mode, use the purple and red indicators to adjust your lighting or lens aperture. The strategy is usually to achieve an optimal balance between clipping from overexposure and image noise from underexposure. With most scenes, there can be a surpris-ing range of exposure latitude before excessive red or purple indica-tors appear.
Then if necessary, in video or zebra mode, use the other tonal indicators to fine-tune how the scene will appear via HD-SDI, or to adjust your suggested look when sending footage for post-produc-tion. The zebra and video modes are also an objective way to assess LCD preview brightness under varied ambient light.”
ARRI ALEXA FALSE COLORS
The Arri Alexa has similar false color codes but they are a bit simpler (Figures 5.22 and 5.25). Arri’s color code has fewer steps which some find makes it easier to read in the viewfinder or monitor. Green is middle gray (38% to 42%) and Pink is average Caucasian skin tone (52% to 56%). Red (which they call White Clipping) is 99% to 100%
and Purple, which they call Black Clipping, is 0% to 2.5%.
Figure 5.23. (Top) A frame from a Red camera in standard mode.
(Courtesy of the Red Digital Cinema Camera Company).
Figure 5.24. (Below) The same frame in Exposure Mode: red indi-cates clipping (overexposure) and purple shows parts of the frame in noise (under nominal exposure).
(Courtesy of the Red Digital Cinema Camera Company).
Figure 5.25. Numerical values and colors of the Arri Alexa False Color system. (Courtesy of Arri).
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F-LUT
FLUT or F-LUT is specific to Red cameras. It stands for Floating Point Look Up Table. According to the Red company: “FLUT™ is a new control (for very fine ISO/mid grey adjustments without inducing clipping) and an underlying technology that facilitates all of the above, and indeed was necessary to make the new color and new gamma curves work.” It was devised by Graeme Nattress, a mathematician who is a prolific innovator in the field of image pro-cessing software. He advises “FLUT is fine control over ISO, so ISO 320 +1 is the same as ISO 640. Similarly ISO 800 -1 is the same as ISO 400. Generally set rendering intent with ISO then fine-tune, if