Defining ISO for Video Exposure
This article is all about ISO. What it is, how you use it and things to be aware of. When it comes to exposure tools on the camera, ISO is the odd one out. All the other ones control how much light reaches the sensor. But ISO is different, it artificially darkens and brightens the image.
But boosting the ISO can also introduce visible sensor noise. Noise is random moving fuzz, it’s usually more noticeable in the darker areas of the picture.
Cameras have a native ISO. This recommended setting is designed to give an acceptable level of noise in the picture, while also protecting the highlights. And we’ll get onto noise and protecting highlights in a moment.
Some cameras have dual native ISO, which is a big help if you’re shooting in low light. When you switch to the higher setting, the picture is passed through a different circuit, which boosts the signal and filters out noise. As an example, the Blackmagic Pocket 6K has dual ISO, one is 400 and the other is 3200. So Blackmagic is saying these are the magic numbers for this camera.
Let’s imagine you started at 400 ISO and then raised it by 2 stops to 1600. If you start to see noise in the image, raising the ISO another stop to 3200, will reduce the noise. While this is handy, using the higher native ISO can reduce the dynamic range of the picture. The dynamic range is the number of stops between the darkest and brightest image the camera can record.
I’ve shot some video with the aperture closed and the lens cap on. So, no light was reaching the sensor. When we look at the waveform, you can see the sensor noise lurking in the shadows. When I increase the ISO, the noise expands and moves up into the mid-tones. This is where it starts to become visible.
So when we say the native ISO represents an acceptable level of noise, the noise might not be completely crushed in the shadows, but it’s not going to be obviously visible either.
Low light performance is a big selling point for new cameras. And what this really means is when you’re shooting in a dark environment you can crank up the ISO and still get a reasonable picture without too much noise. This probably isn’t because sensors are getting better, but because the algorithm that removes noise keeps improving.
Protecting the Highlights
Cinematographers sometimes say they prefer the native ISO setting, because it protects the highlights. But what does that actually mean?
Where we see highlights in the real world, there are two types, diffused and specular. Diffused highlights are not particularly bright. A good example is a white t-shirt or a white wall. Because the surface has a texture instead of being flat and shiny, the light is reflected in many different directions.
Then there are specular highlights. Think of the sun reflecting off metal, glass or water. In this case the surface is shiny, so all the light is reflected in one direction, making it very bright.
Cameras have always been good at reproducing diffused highlights, but not so good with specular. This is why cameras with a High Dynamic Range exist. They can record these brighter highlights, making images look more natural. But there are limits to what the sensor can handle. If highlights become too bright, then part of the image will clip and you’ll lose detail.
When we talk about protecting the highlights, this means that the ISO is at a level where we can still see a good reproduction of specular highlights in the picture.
So this is the other use for ISO, which I see as its true purpose. Creating a good compromise between sensor noise and protecting the highlights. I’ll show you how this works and then we’ll look at a couple of examples.
ISO and Dynamic Range
In the image below, the grey scale represents the brightness range of the real world that we can see with our eyes. I’ve labelled the different areas of the range, to better see what’s going on. Middle grey is a fixed brightness value. You can see it marked on the image.
Up until quite recently the dynamic range of video cameras was around 6 stops. Which is pretty puny. It would cover the mid-tones, the diffused highlights and a little of the shadows. If you were lucky the range would stretch a little bit further and cover super-whites, which was a small bit of the specular range.
For this example, let’s imagine we’re using a modern camera with 15 stops of dynamic range. When it’s set to the native ISO, there are 8 stops below middle grey and 7 stops above. But even with a high dynamic range, it can’t reproduce what our eyes can see. There are shadows and highlights beyond the capabilities of the camera. So at this setting, I should have an acceptable level of noise in the image. And stops 12 to 15 are protecting some of the specular highlights.
Now let’s imagine I want to open the aperture by 2 stops to decrease the depth of field. So if my aperture was on f/5.6, I need to set it to f/2.8. To compensate, I’m going to lower the ISO by 2 stops. So if we started at 800, we need to quarter it to 200. Doing this not only makes the image darker, it also moves the whole dynamic range. So we now have 10 stops below middle grey and 5 stops above.
This balancing of ISO and aperture is fine for the mid-tones and diffused highlights. In general nothing has changed. We lost 2 stops when we lowered the ISO and gained 2 stops by opening the aperture.
But at the extremes of the brightness range, there’s more going on. Because of the shift in dynamic range, only stops 14 and 15 are protecting specular highlights. So you might see more clipping in the image. Clipping is where detail is lost forever in the highlights. But we also gained 2 extra stops in the shadows, which pushes down sensor noise.
Dynamic Range Examples
When shooting a city exterior in bright sunshine, we’re more likely to encounter specular highlights. You might see the sun reflected off windows and metal. In that situation, you might want to increase ISO and cut the amount of light hitting the sensor. This this will give more protection in the highlights, decreasing the chance of clipping.
Alternatively, you could allow the highlights to clip. Specular highlights are usually small points of light, so it might not so noticeable. Ultimately it’s a creative decision. Whatever works best for the story you’re trying to tell?
Now let’s imagine you’re shooting this scene. The highlights look very bright because they’re contrasting with the rest of the image. However they’re all diffused. The light is reflecting off clothing, skin and the back wall. There aren’t any specular highlights.
If you want to see more detail in the character, you can simply open the aperture. The highlights will over-expose but it’s unlikely they’ll clip. In this situation, there’s no need to change the ISO.
Finding the Dynamic Range
Before I wrap up, I just wanted to say a bit more about the camera’s dynamic range. Because dynamic range is used to market a camera, it can be exaggerated. While a camera might technically have 15 stops, they might not all be usable. The extremes of the range might give you poor quality pictures, so the true dynamic range might be more like 12 stops.
All cameras have their limits so it’s not a deal breaker. But you might want to shoot some tests with your camera to see how far you can push it. I’ve got a free guide that walks you through the process of testing your camera’s dynamic range. To get the guide, sign-up below to receive emails from Indigo Film School.
So, I hope this gives you a new insight into ISO. While it’s useful when you need to shoot in low light, its real purpose is to find a good compromise between sensor noise and protecting the highlights.
Did you find this info useful? Will it change how you work in the future? Let me know in the comments.
If you found this useful, check out the rest of my fantastic Camera Training videos:
- Don’t ETTR, Use a 50% Grey Card
- Exposure Controls for Video
- Balancing Video Exposure – Thinking in Stops
- White Balance Video Like a Pro
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