This is really useful! Understand this and it will help you understand a lot more about gamma curves, log curves and raw. Even if you don’t shoot raw, understanding this can be very helpful in working out differences in how we see the world, the way the world really is and how a video camera see’s the world.
So first of all what is “Display Referenced”? As the name of the term implies this is all about how an image is displayed. The vast majority of gamma curves are display referenced. Most cameras are setup based on what the pictures look like on a monitor or TV, this is display referenced. It’s all about producing a picture that looks nice when it is displayed. Most cameras and monitors produce pictures that look nice by mimicking the way or own visual system works, that’s why the pictures look good.
If you’ve never used a grey card it really is worth getting one as well as a black and white card. One of the most commonly available grey cards is the Kodak 18% grey card. Look at the image of the Kodak Grey Card Plus shown here. You can see a white bar at the top, a grey middle and a black bar at the bottom.
What do you see? If your monitor is correctly calibrated the grey patch should look like it’s half way between white and black. But this “middle” grey is also known as 18% grey because it only actually reflects 18% of the light falling on it. A white card will reflect 90% of the light falling on it. If we assume black is black then you would think that a card reflecting only 18% of the light falling on it would look closer to black than white, but it doesn’t, it looks half way between the two. This is because our own visual system is tuned to shadows and the mid range and tends to ignore highlights and brighter parts of the scenes we are looking at. As a result we perceive shadows and dark objects as brighter than they actually are. Maybe this is because in the past the things that used to want to eat us lurked in the shadows, or simply because faces are more important to us than the sky and clouds.
To compensate for this, right now your monitor is only using 18% of it’s brightness range to show shades and hues that appear to be half way between black and white. This is part of the gamma process that makes images on screens look natural and this is “display referenced”
When we expose a video camera using a display referenced gamma curve (Rec-709 is display referenced) and a grey card, we would normally set the exposure level of the grey card at around 40-45%. It’s not normally 50% because a white card will reflect 90% of the light falling on it and half way between black and the white card will be about 45%.
We do this for a couple of reasons. In older analog recording and broadcasting systems the signal is nosier when closer to black, if we recorded 18% grey at 18% it would be possibly be very noisy. Most scenes contain lots of shadows and objects less bright than white, so recording these at a higher level provides a less noisy picture and allows us to use more bandwidth for those all important shadow areas. When the recording is then displayed on a TV or monitor the levels are then adjusted by the monitors gamma curve so that the brightness levels are such that mid-tones appear as just that, mid tones.
So that middle grey recorded at 45% is getting reduced back down so that the display outputs 18% of its available brightness range and thus to us humans it appears to be half way between black and white.
So are you still with me? All the above is “Display Referenced”, it’s all about how it looks.
So what is “Scene Referenced”?
Think about our middle grey grey card again. It reflects only 18% of the light that falls on it, yet appears to be half way between black and white. How do we know this? Well because someone has used a light meter to measure it. A light meter is a device that captures photons of light and from that produces an electrical signal to drive a meter. What is a video camera? Every pixel in a video camera is a microscopic light meter that turns electrons of light into and electrical signal. So a video camera is in effect a very sophisticated light meter.
If we remove the cameras gamma curve and just record the data coming off the sensor we are recording a measurement of the true light coming from the scene just as it is. Sony’s F5, F55 and F65 cameras record the raw sensor data with no gamma curve, this is linear raw data, so it’s a true representation of the actual light levels in the scene. This is “Scene Referred”. It’s not about how the picture looks, but recording the actual light levels in the scene. So a camera shooting “Scene Referred” will record the light coming off an 18% grey card at 18%.
If we do nothing else to that scene referred image and then show it on a monitor with a conventional gamma curve, that 18% grey level would be taken down in level by the gamma curve and as a result look almost totally black (remember in Display referenced we record middle grey at 45% and then the gamma curve corrects the monitor output down to provide correct brightness so that we perceive it to be half way between black and white).
This means that we cannot simply take a scene referenced shot and show it on a display referenced monitor. To get from Scene Referenced to Display Referenced we have to add a gamma curve to the Scene Referenced footage. When your working with linear raw this is normally done on the fly in the editing or grading software, so it’s very rare to actually see the scene referenced footage as it really is. The big advantage of using scene referenced material is that because we have recorded the scene as it actually is, any grading we do will not have to deal with the distortions that a gamma curve adds. Grading correction behave in a much more natural and realistic manner. The down side is that as we don’t have a gamma curve to help shift our recording levels into a more manageable range we need to use a lot more data to record the scene accurately.
The Academy ACES workflow is based around using scene referenced material rather than display referenced. One of the ideas behind this is that scene referenced cameras from different manufacturers should all look the same. There is no artistic interpretation of the scene via a gamma curve. A scene referenced camera should be “measuring” and recording the scene how it actually is so it shouldn’t matter who makes it, they should all be recording the same thing. Of course in reality life is not that simple. Differences in the color filters, pixel design etc means that there are differences, but by using scene referred you eliminate the gamma curve and as a result a grade you apply to one camera will look very similar when applied to another, making it easier to mix multiple cameras within your workflow.