Videographers who transition between shooting in Log and RAW video might find that it isn’t always smooth, especially when it comes to color grading.
According to Syrp Lab, the difference between shooting in Log versus Raw has to do with approach, and to better understand these approaches, it’s important to understand how a camera saves image data.
As light passes through the lens and lands on the camera’s image sensor, the light is converted into an electrical charge. The brighter the light, the greater the electrical charge. This charge is then transferred to the processor which processes it into digital data, which is then saved and written to storage.
But there’s a problem: The electrical signal is “analog” so as the electricity flows up and down, so goes the value of the signal strength and a processor has to assign a specific value and break it down into steps. It can be thought of akin to going down a slide or descending a flight of stairs: both go down, but very differently.
The value assigned to each pixel at that moment in time is then rounded up or down by the camera, which is bit depth. Syrp describes an 8-bit file as having 256 possible values while a 12-bit file will have over 4,000 possible values. Therefore, the greater the bit depth, the more information, and the larger the file.
When saving video as a RAW, the bit depth will preserve all the possible data values and those values will be available. During color grading, the image remains smooth because of all the data values it has available to it. This is much like a slide: smooth and consistent.
In Log, there is a certain amount of data latitude to grade with, especially in higher bit rates, however, there is a point where the image begins to break. So, it’s best to make gradual changes and not extreme ones due to Log throwing out some of the repeatable values of an image in order to keep the file size small.
“Once you get into the grade and you realize, hey this shot could do with being a little different, you start to stretch your values along that curve, and things… break,” says Chase Madsen of Film Science. “Think of it as that staircase again. When you need to change the shape of the staircase, some of the stairs become too big to climb up.”
Even a 12-bit image is mapped to an 8-bit curve. Therefore, when changes are made, that data is not available to adjust, and as such, image banding occurs. Add in file compression, noise reduction, and image sharpening, and the banding can become very noticeable, and the image becomes unusable.
This is where saving images in RAW comes in handy for color grading. It keeps all the available image data values for that bit rate, and therefore offers far greater latitude. But when using Log, the grade can only do so much before it begins to break.
Shooting in the highest bitrate possible will preserve more color values and data. Even shooting in 10-bit over 8-bit will provide four times the amount data, so videographers should shoot in the highest bit rate they can if they plan to color grade the footage.
If Log is the only option, or the preferred one in order to keep storage requirements down, videographers need to make sure they are properly lighting a scene and using the highest possible bitrate to give themselves the most wiggle room in post. RAW is going to be the best bet though, even if it means that it requires much larger storage capacity in the camera and in the edit bay.
More examples and a greater description of the principles explained above can be found on Syrp Lab’s website.
Image credits: Photos by Syrp Lab