False color

False Color as implemented on the SmallHD field monitor; reference color scale at the bottom.

False color is an exposure tool that colorizes specific levels on the image. Tonal regions are color coded with easily recognizable colors. Sometimes these regions include only the near-clipping extremes and the most common tonal range of skin, but the whole range can be split in intervals and colorized. A legend for the different color codes is often included in the frame for reference.

False color can be thought of as an extension of zebras. Zebras can be left active for monitoring during recording. But false color is generally used to tune exposure in advance, as composition is difficult to judge with colorization on. Overreliance on false color with its neatly predefined tonal zones can force shooters into exposure patterns. This tool should be used for reference only.

Through-the-lens digital spotmeters

Isolating a portion of the image for evaluation can be a big help. Problematic or important areas can be quickly judged on their own or in respect to the whole image. This is especially useful when paired with tools that display the exact levels. Constant aperture zooms can be used to isolate an area of interest while preserving the exposure. This is not always applicable though.

Some cameras (Canon C-series, for example) include a spot waveform, which imposes the waveform of a smaller rectangular image region on the waveform of the whole image. Magic Lantern includes a simpler and more straightforward spotmeter. It indicates the averaged brightness level of a small rectangular area in the center of the picture. This is a powerful exposure tool when coupled with some good knowledge of the underlying transfer curve. Finding out reflected light (luminance) ratios is then just a matter of pointing and measuring shadows and highlights.

Good old light meters

Any overview of exposure tools is not complete without a few words on light meters.
For years light meters have been indispensable to the cinematographer due to film’s lack of immediate feedback. They are of two basic varieties: reflected light meters and incident light meters. Both functions are often found in a single body. Incident light meters measure falling (incident) light, or illuminance. Reflected light meters measure light reflected from scene surfaces, or luminance. This is an important difference: incident light readings are thus scene independent, unlike reflected light readings. Cinematographers, in general, tended to rely on incident readings. The spotmeter is a variant of the reflected light meters. Unlike standard reflected light meters, which are usually reading reflected light within an angle of 20 to 30 degree, spotmeters measure reflected light in a small angle (sometimes as small as 1 degree). This makes them very useful for isolating surfaces and for contrast ratio calculations. Light measurements are converted to photographic exposure terms based on the assumption for average scene brightness (18% reflection). So creative use of light meters usually requires exposure compensation based on the creative intent.

Light Meters: Gossen Luna-Lux SBC, Kenko KFM-1100, Sekonic L-758DR

All digital cameras feature internal light meters. These are through-the-lens reflected light meters. On photo cameras they usually default to sophisticated evaluative (matrix/multizone) metering modes. Matrix metering is great for still images, but it is hard to consistently use for matching exposure between shots. An experienced video user can get more mileage from the spot metering mode of the camera.

With the rise of digital video and the associated exposure tools external light meters are falling into obscurity. Digital video guys tend to discard them as unnecessary and old fashioned. Nevertheless, they can still be useful to a knowledgeable video shooter. Unlike any of the exposure tools mentioned above, external light meters measure either the scene reflected light or the incident light directly, and not by processing the captured image. So predicting the transfer of scene luminance into image levels requires intimate knowledge of the tonal characteristics of the recording medium, and – in the case of incident readings – some experience of the reflectivity of various materials. That’s how shooting film works. A somewhat underestimated property of incident readings: being independent of the reflective properties of scene elements, they can allow for consistent light measurement and, consequently, for consistent exposure without the need for a grey card or any other scene reference. And as they don’t need a camera to function, external light meters are also handy for location scouting and setting lights up. This is their most common use nowadays.

Incident readings are taken with the meter at the subject, pointed towards the camera. Reflected readings are taken with the meter at the camera, pointed towards the subject.

It is worth noting that light meters are not created equal. With different brands, varying calibration constants may lead to variations of up to 1/5 stop in measurements. This is a bit annoying, but ultimately doesn’t really matter – meters should always be tested (and possibly corrected through meter correction factors) with the specific transfer curve used.

A further article will explore the application of these exposure tools.

Exposure Tools for Digital Video, Part 2

The instant feedback of digital video is its greatest advantage over film. On a shoot, this feedback relates to exposure more than anything else. Exposure errors can be fixed immediately (no need for rushes or dailies) and exposure choice can be feedback informed and guided. A calibrated field monitor can be of great help, but it needs both good viewing conditions and excellent rendering capabilities in order to offer a decent representation of the recorded video. Exposure is often hard to judge based on monitor feedback alone. But a number of tools can process the captured signal and display it in various ways to aid the exposure decision. This article is an overview of the most popular of these exposure tools, and their strengths and weaknesses.

Most of these exposure tools take as their input the image data after the transfer curve is applied, so they directly represent the recorded video. In some cases – mostly with higher-end equipment – the display data may differ from the actual recorded data. This is usually done when log space video is recorded. The flat image is not very useful for monitoring, so it may have some LUT applied for preview and exposure purposes. This LUT will generally simulate to some degree the final look of the video.

These exposure tools are available through the camera firmware, on field or studio monitors, or through specialized software fed with the camera signal. In any case, they need a screen to display info on, be it a camera LCD, an external display, or just a computer screen. They are at their most useful when combined with good knowledge of the camera transfer curves (gammas).

Histogram

This histogram indicates overexposure with large clipped areas

An overly dark image but without truly crushed blacks

The histogram is familiar to most photographers and Photoshop users. The histogram is a simple chart diagram. It displays the distribution of image pixels over the possible range of the video signal domain. The more pixels falling into a particular value bin, the higher its column in the histogram. The histogram doesn’t really contain any information about what part of the image falls where. But it is useful for quickly identifying the presence of clipped highlights or crushed blacks, as well as determining the overall key of the acquired image. A histogram skewed to the right suggests a high key image, a skew to the left suggests low key. Histograms usually come in two modes – luminance (brightness) and RGB – which are often switchable according to the preferences of the camera op. RGB histograms are actually compound histograms featuring a histogram for each of the three color channels, either overlaid or in parade. They help identify color channel over-saturation or color balance issues.

The main strength of the histogram is that its graphic representation can be scaled down without significant information losses because it is the overall shape that delivers meaningful exposure information. This allows to get it overlaid on the actual image, tucked somewhere to the side.

Zebras

Zebras marking blacks near the crush point

Zebras are a video camera invention and one of the first exposure tools to appear in video cameras. The concept is simple. Colored stripes (hence the name) are overlaid on the actual image, marking the position in the image of any clipped whites or crushed blacks, or both. Unlike the histogram, zebras localize and visualize the problematic areas so it is easier to judge if the out-of-range pixels are important or can be painlessly sacrificed. Depending on the implementation, zebras may have their warning triggering clip/crush points customizable; they may blink or feature other improvements.

Waveform monitor

Sample input image

Luminance waveform of the above image (in IRE units)

The RGB parade shows the blue channel is clipped

The waveform monitor (WFM) is one of the most useful tools in video production. Both during shooting and in post. Traditionally, the WFM is a monitoring device that displays the level of a video signal over time. In analog video this means voltage. In the digital world the WFM concept has been expanded to handle discrete values. In a sense, the digital waveform monitor is an extension of the histogram. But instead of binning pixels from the entire image, the waveform monitor bins pixels from each image column separately. These bins are then represented by pixels in a column of the output of the WFM. On the vertical axis of the waveform monitor are the values defining the bins. These can be the actual video space values, values scaled from 0 to 100%, or IRE units. The horizontal axis runs over the columns of the input camera image. The intensity of the output pixel corresponds to the saturation of its respective bin. The more pixels from the corresponding input column falling into a value bin, the brighter the bin’s pixel on the WFM.

The waveform monitor can do pretty much any job the histogram can handle, plus more. Because each image column is processed separately and there is one-to-one correspondence between image columns and WFM columns (assuming a non-scaled WFM image), any offending areas are easily horizontally localized on the waveform monitor, and then in the actual camera image. This is especially useful for faces, grey cards or just large suspicious areas. Moreover, important values on the vertical axis can be signified with lines parallel to the horizontal axis for quick judgment of the image tonal distribution. This is a powerful tool when coupled with good knowledge of the transfer curve used.

Waveform monitors can be used with a variety of input color spaces. Luminance is generally enough for exposure and is one of the channels of the typical YCbCr digital video stream. RGB, YRGB and others can be used for variety of tasks, most notably for color correction. These are either displayed in parade, or overlaid in different colors.

The exposure tools overview continues with false color, spot displays and light meters.

Exposure Tools for Digital Video, Part 1