Chapter 14: Lighting and Exposure: Beyond the Basics
Now that you are familiar with the fundamental concepts and techniques of exposure and lighting, we can turn our attention to slightly more intricate issues related to image control. In this chapter we look a little deeper at how a video sensor responds to light, and the various options that cameras offer for pushing the video signal beyond the limits of Rec. 709 standards. We’ll also look at two advanced metering options commonly used on professional film sets. The information in this chapter might seem a bit intimidating at first, but honestly, there is nothing terribly complex here. The essential thing is to take this material off the page and get it into your hands and practice. It only takes a bit of hands-on experience with a camera, working and experimenting with these concepts, tools, and techniques, to get them firmly into your image control arsenal. So…let’s get started.
High Resolution and Color Figures
Dynamic range is the total range of luminance values a video sensor can render with detail before falling off into pure white or total black (represented by the dotted lines). SD video had a dynamic range of about 6 stops (top). HD video formats improved that range to around 8 stops (center), and these days it’s not uncommon to find sensors with 12-14 stops of dynamic range (bottom).With the gamma curve graph from Premiere Pro’s Lumetri color grading tool, we can see how changing gamma angle effects an image. A gamma angle of 45° represents a perfectly proportional increase in contrast response to exposure (top). Flattening the gamma curve angle reduces contrast (center) and a steeper angle emphasizes contrast (bottom). Cameras contain multiple built-in picture presets in which the sensor’s gamma curve has been intricately tweaked to achieve a variety of looks.Rec. 709 clips hard as it reaches its dark and bright exposure limits (solid line a). But most standard picture profile settings include slight plus black gamma (b) and video knee compression c) allowing for more visible detail in the dark and bright areas. This effectively extends dynamic range a stop or 2.Video knee compression can extend the seeing range of the sensor into the brightest areas of the scene. Note how more detail is visible outside the window after video knee is engaged (right). (Go to the companion website to see this figure in high res.)Plus black gamma can boost the darkest areas of a scene, pulling more detail out of the shadows, practically creating an artificial “toe” (a celluloid film term for areas of underexposure close to pure black); note how, after the black stretch option is engaged, some detail can be seen inside the arch (right). ( Go to the companion website to see this figure in high res.)Comparing the generic gamma curves for Rec. 709 (a), Cine Gamma (b), and Log Gamma (c) reveals how by flattening the contrast of the image, the dynamic range of the sensor is extended. This gives you more latitude when shooting, but it also requires color grading.A LUT is a small piece of color correction code that, when applied to Log footage, reverses the flat gamma. There are a wide variety of LUTs available in your nonlinear editing program (NLE), and each will give your footage a different visual look. Pictured here are the Fuji Eterna 50D and Lumetri Cinescape LUTs applied to the same S-Log footage. Notice the different color and contrast qualities each brings to the image. See the color insert.Built-in camera or monitor LUTs are usually very basic video looks. But you can download creative LUTs from commercial websites and load them into your camera or monitor. Pictured is Filmconvert’s Fuji Eterna Vivid 8543 film stock emulation LUT (left) applied to Log footage (right). See the color insert.Taking readings from subjects far away from the camera is an easy task with a spot meter, but the readings have to be interpreted before setting the f-stop. Setting the aperture for the sky at f/22, for instance, will darken it to middle gray and underexpose everything else. Can you tell what f-stop was chosen to record this image?The waveform monitor is an important exposure tool on a set. Many external field monitors come with a built-in WFM function, like this Atomos, that can display an image, or the waveform, or even overlay the waveform onto the image as seen here. Notice the bare light bulb clip above 100 IRE (top arrow) and the middle gray card expose at 50 IRE (bottom arrow).Beyond knowing the parameters for white, black, and middle gray, it’s helpful to break down the IRE scale into broader luminance “zones,” like highlights (with detail), mid-tone range, and shadows (with detail).Most scenes comprise a range of exposure values. In this shot, the darkest area reads f/2, and brightest reads f/16, giving us a contrast range of six stops. How this scene will be recorded depends on the sensor’s dynamic range and our f-stop selection. Setting the aperture to expose correctly for Sandor (top), Chip (center), or Nicole (bottom) causes other areas in the image to fall outside the sensor’s dynamic range. Go to the companion website to see this figure in high resolution.This overhead shows the lighting strategy that addresses the exposure problems with the shot. We now can shoot our scene with the knowledge that all critical areas will fall within the film’s latitude.The final scene shot at f/11 and incorporating some simple lighting, which keeps Sandor, Chip, and Nicole within the film’s latitude, yet with a range of appropriate exposures for a naturalistic look. Go to the companion website to see this figure in high resolution.Exposing “correctly” for the incident light in the vestibule allows all of the luminance values to fall where we’d expect and with visible detail from white to black. The gray sweater reads between 35 and 40 IRE (top), the white shirt registers around 80 IRE (center), and the black jacket falls around 10-15 IRE (bottom).In our “fixed” scene the final exposure is f/11 (intentionally underexposing Nicole by three stops). Now, the gray sweater falls to around 18 IRE (top), the white shirt registers just above 40 IRE (center), and that black jacket crushes at 0 IRE out of our dynamic range (bottom). Though underexposed, the “white” shirt and gray sweater retain detail, but the black jacket contains no visual information at all.
