What is Rec.709? Things You Must Know!!
What’s going on everyone! Welcome back to yet another tutorial. Today we are going to be breaking down what rec.709 is. You see my tristack posts with the three different grades (log, rec.709, and final), and the most asked question I get is what is rec.709? So in this video I will define what rec.709 is and put it in practical use.
REC.790 or BT.709 is a color gamut, developed in 1990 by ITU-R (International Telecommunication Union).
Now before I tell you what color gamut is, let’s take a quick detour. This is the visible light spectrum.
Past red is infrared and past violet is ultraviolet. Both of which we cannot see. These are measured in nanometers.
Now let’s move on and check out something we are familiar with and that is the CIE chart, which was created in 1931.
The numbers on the horseshoe are the same as the numbers on the visible light spectrum chart. If you are like me and you want to nerd out about this, check out the book by Blain Brown called Cinematography: Theory and Practice.
Now that you have some context, we are ready to talk about color gamut. Color gamut is a range of colors inside of the visible color spectrum. Now moving back to our chart, we are going to look at it with the REC.709 color gamut.
You can see that this is the range of colors that are visible to the REC.709 or SRGB color space. Anything outside of that will be thrown out. So think of REC.709 as this:
A common language between your camera and your display.
So your camera captures a log footage, then it comes in and gets stretched out and gets converted to a semi-linear conversion and what you say is “presentable.”
Now let’s show you what I mean. We will be using this clip, which was shot on Arri Alexa.
The purpose behind shooting in log is to give yourself the most amount of range in post. If you remember that more old school cameras would shoot in a more standard color profile, something like this.
So when the footage would come in like that, the image from the gate looks pretty good, but you have no range up top or at the bottom to bring it back. That information is just gone and it’s clipped. Whereas with log, you have so much more room. The caveat is that you have to do some legwork to get it to where it needs to be to start grading. But back to when I said that rec.709 is a common language between your camera and your display. What I mean is check out my display. It’s set to rec.709.
This footage was shot in log, so the rec.709 conversion will get this footage looking how it looked on set. To do so, we are going to take our CST and manually plug in that information. Now if your footage was shot on RAW, Resolve is smart enough to do the conversion for you. Otherwise you’ll need to plug in the information. So we are going to make these changes in our CST to get our rec.709.
There. That’s what it looked like on set. The reason we don’t fill out the output settings in the CST is that our timeline settings are set to rec.709 and gamma 2.4.
Now if we add a node prior to this and make changes, that is not a pure rec.709 conversion anymore. You manipulated the image and already started making changes. This isn’t what it looked like on set. As a colorist, it’s always important to have your rec.709 image ready to go because anytime the director or DP goes, can you show me how it looked on set, you can say “yes, no problem.” Sometimes they will be using their own luts and you can bring those in and get it to that custom place.
Now the rumor is that rec.709 is such an old standard that it really is starting to show it’s age in the world of HDR. HDR TV’s are so cheap now, so everyone is noticing the difference. Look at this from Sony.
You guys know that even consumer or prosumer level cameras are capturing around 8-10 stops of dynamic range. When you talk about cameras like Alexa, you are getting about 14.2 stops of dynamic range. So keeping that in mind and seeing that your output is only 5.2 stops, it’s heartbreaking. Now the HDR TV’s are putting out about 10 stops of dynamic range. This jump might seem like only 2x, but no. It’s exponential.
Just think about the brightness limitations on your SDR at 100 nits. Compared to the HDR which is over 1000 nits.
Now let’s compare the CIE charts of rec.709 vs rec.2020.
The TVs aren’t even taking advantage of all that rec.2020 has to offer.
Now here’s the dilemma. Unless you are working with Netflix, or Apple TV plus, or one of those big streaming services, no one is putting out HDR content. So until broadcast accepts HDR (which will be a while because it’s a standard and it needs to be accepted around the world), it’s not going to be possible for everyone to start grading in HDR.
Hopefully this helped untangle some of those concepts that you might have been struggling with! Just remember, work hard, get obsessed, and get possessed.
MORE LIKE THIS
