At AbelCine, we offer a class on video formats that covers a variety of different terms and compression formats. One of the more common questions I'm asked is, "What's the difference between uncompressed video, raw and Log recording?" With so many cameras these days offering different recording options, combined with the popularity of external recorders, it's no wonder there are a lot of questions about this topic. Recently, I was asked if shooting Log was like shooting raw. A short answer is, "Well, yes and no," which I know isn't terribly clear. Raw recording is very different than Log, but they have similar applications. To really answer the question, and to understand the difference between all of these formats, we need a little bit of background. ARRI's ALEXA camera is unique in that it can output raw, uncompressed and record in a Log format, so I'll use that camera as an example throughout this discussion. Let's start with raw, which comes first for many reasons.
RAW LIKE SUSHI
The idea of raw recording for motion pictures wasn't popular until the release of the RED ONE camera a few years ago. RED brought the idea of raw recording to the masses, though they weren't the first. Both ARRI and DALSA had cameras that could output raw sensor data. Raw recorders weren't exactly common on-set gear though, so we have to hand it to RED for getting the raw party started in the motion picture business.
So what is raw anyway? Simply put, it's just sensor data before any image processing. In a single-sensor camera, like the ALEXA, color is produced by filtering each photosite (or pixel) to produce either red, green or blue values. The color pattern of the photosites most often used is the Bayer pattern, invented by Dr. Bryce E. Bayer at Kodak. The raw data in a camera like this represents the value of each photosite. Because each pixel contains only one color value, raw isn't viewable on a monitor in any discernible way. In a video signal that we can see on a monitor, each pixel contains full color and brightness information; video can tell each pixel on a monitor how bright to be and what color. This means that raw isn't video. Raw has to be converted to video for viewing and use. This is usually done through a de-Bayer process, which determines both color and brightness for each finished pixel in your image. The converting of raw information into video can be time-consuming for postproduction, though great tools are available to make it fairly painless. The upside to raw recording is that no typical video processing has been baked in. The sensor outputs exactly what it sees—no white balance, ISO or other color adjustments are there. This, along with a high bit rate, allows for a huge amount of adjustment in post.
Every camera has a "raw" step in the image-capturing process. The sensor information is always gathered before converting to a video output, but not all cameras let you record or output that data. The cameras from RED record exclusively raw data. Sony's new F65 can record in raw or HD video, and the ARRI ALEXA can output raw data over an SDI connection, while recording video internally at the same time. These cameras are converting the raw data into video for monitoring; the ALEXA adds the ability to output the raw data at the same time.
So if the raw data is the real information coming off the sensor, does that mean that it's uncompressed? This is where things get a bit trickier.
HOW UNCOMPRESSED IS IT?
Raw data isn't necessarily uncompressed. In fact, it's usually compressed. The RED cameras shoot in REDCODE, which has compression options from 3:1 to 18:1. Likewise, Sony's F65 has 3:1 and 6:1 compression options in F65RAW mode. The raw data is compressed in much the same ways that traditional video is compressed, and the process does have some effect on image quality. How it shows up in the finished video output can be hard to detect, and the less compressed options in both of these cameras are often considered fairly lossless (limited quality loss). On the other hand, ALEXA outputs uncompressed raw data, which can be recorded externally. This would be the closest thing to a true uncompressed signal.