This comes up in many of my workshops. It seems like a very simple question and the correct answer is really very simple, but many cameramen, especially those from a TV and video background actually get this a little wrong.
The word “expose” means to lay open, reveal or un-mask. In film terms it’s obvious what it means, it is opening the shutter and aperture/iris to let the correct amount of light fall on the film stock. In the video world it means exactly the same thing. It is how much light we allow to fall on the sensor.
Exposure is controlled by the speed of the shutter (how long we let the light in) and the aperture of the lens (the volume of light we let in).
So why do video people get a bit confused about exposure? Well it’s the down to the way we measure it with video cameras.
In the film world you would use a light meter to measure the intensity of the light in a scene and then perform a calculation to determine the correct amount of light we need to allow to fall on the film based on the sensitivity (ISO) of the film stock. But in the video world it is common practice to look at a monitor and asses the exposure by looking at, or measuring, how bright the picture is using a waveform meter, zebras or histogram etc.
What are we measuring when we look at a video picture or measure a video signal? We are not measuring how much light is falling on the sensor, we are measuring how bright the picture is on the screen or what the recording levels of the video signal are. Most of the time there is a direct relationship between on screen brightness and exposure, but it is important to make a clear distinction between the two as variations in brightness are not always due to changes in exposure.
It’s important because something like changing a cameras gamma curve will alter the brightness of the on screen image. This isn’t an exposure change, this is a change in the recording levels used by that particular gamma curve that in turn result in a change in the brightness levels you see on the screen. This is why if you take a camera such as the FS7 or F5/F55 and correctly expose the camera using Rec709 as the gamma curve you will find middle grey at 42% and white at 90%. Then switch to a Cinegamma or Hypergamma without adjusting the shutter speed or aperture and you will find middle grey at and white at much lower, perhaps the very same white target as low as 70%.
In both cases the exposure is correct, but the on screen brightness greatly different. The difference in on screen brightness comes from the different recording levels used by 709 and Hypergammas/Cinegammas. In order to be able to record a greater dynamic range than the 6 stops offered by 709, we need to compress the original 6 stop 709 range into a much smaller range to make room for the extra stops of dynamic range that the Hypergammas or Cinegammas can record.
So as you can see, exposure should really be the absolute measurement of the amount of light falling on the sensor. Brightness is related to exposure, but just how bright the picture should be depends on many factors of which exposure is just one. Once you realise that brightness and exposure are not always the same thing it becomes easier to understand how Cinegamma, Hypergamma, log and raw recording works. Levels are just levels and it doesn’t really matter whether something is recorded at 90%, 70% or 61%. Provided you have enough data (and this is where 10bit or better recording really helps) you have the same amount of picture information at both levels and you can easily shift from one level to the other without degrading the image in any way in post production.
Of course we do want to have our video levels in the finished production at the right levels to match the levels that the TV, monitor or display device is expecting. But when shooting, especially with non standard gammas such as Hypergamma or log it’s perfectly normal to have levels that are different to what we would see with plain vanilla 709 and these typically lower levels should not be considered too dark or under exposed, because they are not. Dark does not necessarily mean under exposed, nor does it mean a noisy image. How much noise there is depends on the signal to noise ratio which is dependant on the amount of light that we let on to the sensor. I’ll be explaining that in my next article.