Panasonic AG-DVX200 Tech Brief - Volume 6 - V-LOG L Based Production - Page 5

ere are two general schools of thought when exposing raw footage or logarithmic gamma footage: exposing for middle gray, or Expose To e Right (ETTR). Let's discuss ETTR rst. Exposing To e Right (ETTR) Exposing To e Right is a technique based on using a histogram for exposure. A histogram shows the distribution of brightness in the image, and the further the image is shi ed towards the highlights, the further right the image moves on the histogram. Proponents of the "Expose To e Right" technique argue that the darker tones and shadows are the noisiest parts of the image, so if you can li your image up out of the shadow area, you can take advantage of the cleaner upper range of the sensor and gamma curve; later, you can drop the footage's levels back down to where proper exposure would dictate it should be. Furthermore, those who shoot raw footage frequently embrace ETTR because raw sensor data is stored linearly, not logarithmically; that means that the vast majority of available "bits" are allocated to the brightest stops, and the darkest stops receive the fewest "bits" (i.e., the least number of shades of gray that they can represent). is is indeed a concern with raw footage, but is not nearly as much of a concern when using a logarithmic gamma; a LOG gamma redistributes the bits so that they are more equally dispersed along the entire dynamic range of the sensor. As such, the value of li ing the shadows and dark tones up is less important when recording with a logarithmic curve than it would be with raw footage. e general idea behind ETTR is to expose the image as bright as you possibly can, so long as none of the video information "clips" o the top. Regardless of how dark an image should look in the nal footage, the idea is that if you expose it in the top part of the exposure range, you'll get the cleanest, lowest-noise images, and you can always push it back down to proper exposure in post. ETTR proponents use the histogram as an exposure tool to accomplish this, because the histogram plots out all the exposure levels in a given scene, and if there is any unused space on the right edge of the histogram, that means that you have room to brighten up the exposure (which will shi the histogram's graph over to the right within its frame; hence Expose To e Right). In theory, it sounds great; on still photographs, it can work great. On VLOG-L, it doesn't quite work that easily, because of the maximum 80 IRE brightness imposed by the VLOG-L curve. It means that you can never truly get the image "to the right." You can still use the ETTR technique, but you would want to use the zebras set at no higher than 80 IRE to warn you when the image was clipping. If you set your zebras at 80 IRE, you are free to expose up until the zebras appear. Anywhere that the zebras are displayed, you've clipped the image and would need to back o your exposure. Do be aware though that at higher exposure levels, even though the luminance may not have clipped yet, an individual color channel may begin clipping before the zebras display. As such, you might want to back o a little more (by setting the zebras no higher than 75 IRE), to leave a little room to minimize any clipping of chroma channels. Now, as a general technique there's nothing wrong with ETTR, it does work and it is a reasonable choice. However, it is not necessarily the best choice, because ETTR is designed to preserve the highlights with no consideration of what happens to the midtones. It can result in retaining a lot of detail and in making the least-noisy footage, but it will mandate extensive post-production correction 5