Panasonic AG-DVX200 Tech Brief - Volume 6 - V-LOG L Based Production - Page 6

on every single shot. When exposing using ETTR, skin tones may end up being recorded brighter or darker in every scene, simply based on where the highlights happen to be in that particular shot, and every shot will need to be corrected to bring the skin tones back to a reasonably consistent level so that your footage will intercut cleanly and seamlessly. And, depending on just how bright the highlights are in any given scene, ETTR may result in a scenario where the skin tones and midtones are signi cantly underexposed in an e ort to catch and preserve all the highlights. at might make for nice highlights but it might also result in noisy skin tones and midtones, since in post production you may have to stretch the skin tones up out of the darker (and noisier) sections of the sensor. Generally, cinematography is (and should be) more about the subject than it should be about the highlights; excessive attention to the highlights may mean compromising other aspects of the footage, so a strict "ETTR" approach is not always going to provide the overall best results in a video project. Exposing For Middle Gray An alternative method of exposure would be to expose for middle gray. Video systems are frequently referencing "middle gray" or "18% gray." 18% gray is a photographic and lm standard, it's a shade of gray that re ects 18% of the light that hits it. It is frequently incorporated into test charts, and you can easily buy an "18% gray card" at photography stores. 18% re ectiveness represents approximately the average overall brightness of many scenes, and camera autoexposure systems are typically designed to expose to where the scene represents approximately 18% re ectance levels. In Ansel Adams' "Zone System", middle gray is known as Zone V. When exposing for middle gray, you'll nd the zebras and the waveform monitor vastly more useful than the histogram. In this section I'll refer to exposure levels in terms of IRE levels. In conventional video gammas, middle gray is usually exposed properly at somewhere around 50 to 55 IRE. However, not so in VLOG-L. In VLOG-L, middle gray is properly exposed at 42 IRE. If you happen to have a gray card in your scene, it should show up on your waveform monitor at approximately 42 IRE for "proper" exposure (note, here I am using the term "proper" in an idealized, mathematical way; the artistic merits of the scene may very well dictate that the exposure needs to be higher or lower than this). e VLOG-L gamma curve maps the following brightness levels to the following IRE levels: 0% re ectance (black): 18% re ectance (middle gray): 90% re ectance (white): absolute clipped superwhite: 7.3 IRE 42 IRE 61 IRE 80 IRE If you are using test charts, you will likely have access to 18% "middle gray" and 90% "white"; many gray cards sold in photographic stores will have 18% gray on one site, and 90% white on the other. 90% re ectance doesn't necessarily indicate "pure white" or the brightest object that can be seen or recorded; rather it is (as its name suggests) a white where 90% of the light that hits it is re ected. e 6