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.
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.