At AbelCine, we do a lot of camera matching; customers bring in their cameras, and we do our best to create matching scene files. Recently, two cameras were brought in with a simple request attached, reading, "Please match to Rec. 709." This may seem like a normal request to some of you, or it may have you scratching your head. The term Rec. 709 implies a lot of things, and leads to a lot of confusion. We also frequently get the question, "Why does the Rec. 709 mode on one camera not match another?" The answer to this question, and the reason our camera-matching request wasn’t so clear, is the topic of this issue’s Help Desk. Let’s dive into that mysterious term, Rec. 709, and how it applies to cameras, monitors and post.
So, what exactly is Rec. 709? Well, it’s properly called ITU-R Recommendation BT.709. That’s a mouthful, but it explains a lot about where the term comes from. The ITU (International Telecommunication Union) is a specialized section of the United Nations that’s responsible for issues around information and communication technologies. The ITU coordinates the global use of the radio spectrum, works to improve telecomm infrastructure and assists in the development of worldwide technical standards. The ITU-R (ITU Radiocommunication Sector) is one division of the ITU that helps implement these standards. BT.709 is a recommendation of the ITU-R, which specifies standards for high-definition TV. This includes global standards for HD, such as resolutions, frame rates, color gamut, gamma and white point. Remember NTSC and PAL? Well, in HD, they’re both replaced by this BT.709 recommendation. Why, then, do we call it Rec. 709? Well, it’s a heck of a lot better than that long name.
Okay, so Rec. 709 is a bunch of standards that define HD, but what they’re really defining is how HD is seen on televisions. They’re not defining exactly how a camera produces video. Most cameras out there conform to the standards of Rec. 709. You can pretty much assume that a camera conforms unless it says otherwise. But modern cameras bend, or completely break, these standards to make a better-looking image. To understand how and why that is, we have to look under the hood of Rec. 709 a bit further.
Part of the Rec. 709 specification is a standard gamma correction that’s to be applied to the video. It specifies that a power function of 0.45 be applied to the linear input of the sensor. A Rec. 709 display is specified to have an inverse power function applied to it of 1/0.45 (or 2.2). Applying both power functions, 0.45 at the camera and the inverse 1/0.45 at the monitor, means we get results we expect on the screen. Our old CRT monitors naturally had this gamma, so applying the inverse at the camera was required. Rec. 709 is a current specification, although not many of us are looking at CRT monitors today. Rec. 709 still uses this gamma standard because having a standard across all displays is important, and the 0.45 power function not only worked out for old TVs, but also maximized perceptual information stored in video.
So, Rec. 709 does specify gamma, but that doesn’t mean that cameras actually conform to it. The standard gamma correction in video would only allow for around 5 stops of dynamic range—not so good. Camera manufacturers have found ways to improve this range by adjusting the gamma curve. Some of the first tools used to alter a gamma curve were knee point and slope adjustments. This adjusted the gamma curve above a specified brightness level, the knee point. Other cameras introduced hyper-gamma or film-like gamma modes, which stretched the gamma curve even further to get more dynamic range.