What do we really mean when we talk about exposure?
If you come from a film background you will know that exposure is the measure of how much light is allowed to fall on the film. This is controlled by two things, the shutter speed and the aperture of the lens. How you set these is determined by how sensitive the film stock is to light.
But what about in the video world? Well exposure means exactly the same thing, it’s how much light we allow our video sensor to capture. Controlled by shutter speed and aperture. The amount of light we need to allow to fall on the sensor is dependant on the sensitivity of the sensor, much like film. But with video there is another variable and that is the gamma curve…. or is it????
This is an area where a lot of video camera operators have trouble, especially when you start dealing with more exotic gamma curves such as log. The reason for the problem is down to the fact that most video camera operators are taught or have learnt to expose their footage at specific video levels. For example if you’re shooting for TV it’s quite normal to shoot so that white is around 90%, skin tones are around 70% and middle grey is in the middle, somewhere around the 45% mark. And that’s been the way it’s been done for decades. It’s certainly how I was taught to expose a video camera.
If you have a video camera with different gamma curves try a simple test. Set the camera to its standard TV gamma (rec-709 or similar). Expose the shot so that it looks right, then change the gamma curve without changing the aperture or shutter speed. What happens? Well the pictures will get brighter or darker, there will be brightness differences between the different gamma curves. This isn’t an exposure change, after all you haven’t changed the amount of light falling on the sensor, this is a change in the gamma curve and the values at which it records different brightnesses.
An example of this would be setting a camera to Rec-709 and exposing white at 90% then switching to S-log3 (keeping the same ISO for both) and white would drop down to 61%. The exposure hasn’t changed, just the recording levels.
It’s really important to understand that different gammas are supposed to have different recording levels. Rec-709 has a 6 stop dynamic range (without adding a knee). So between 0% and around 100% we fit 6 stops with white falling at 85-90%. So if we want to record 14 stops where do we fit in the extra 8 stops that S-Log3 offers when we are already using 0 to 100% for 6 stops with 709?? The answer is we shift the range. By putting the 6 stops that 709 can record between around 15% and 68% with white falling at 61% we make room above and below the original 709 range to fit in another 8 stops.
So a difference in image brightness when changing gamma curves does not represent a change in exposure, it represents a change in recording range. The only way to really change the exposure is to change the aperture and shutter speed. It’s really, really important to understand this.
Furthermore your exposure will only ever look visibly correct when the gamma curve of the display device is the same as the capture gamma curve. So if shooting log and viewing on a normal TV or viewfinder that typically has 709 gamma the picture will not look right. So not only are the levels different to those we have become used to with traditional video but the picture looks wrong too.
As more and more exotic (or at least non-standard) gamma curves become common place it’s very important that we learn to think about what exposure really is. It isn’t how bright the image is (although this is related to exposure) it is about letting the appropriate amount of light fall on the sensor. How do we determine the correct amount of light? Well we need to measure it using a waveform scope, zebras etc, BUT you must also know the correct reference levels for the gamma you are using for a white or middle grey target.
You might also like to read this article on understanding log and exposure levels.