The correct ISO for video is not as simple as it seems. Part 1

As people switch from DSLRs and mirrorless cameras to professional camcorders, the question of illiterate exposure is becoming more and more common. People shoot in LOG and RAW simply because they can, without really understanding how these formats work and what they require in terms of exposure. Many of the concepts that have come down to us from brother photographers lose their relevance or become simply incorrect when it comes to shooting video. One such concept is the digital camera ISO.

ISO makes the picture brighter, right? Like yes, but actually no. ISO is an amazingly complex topic that took me a lot of time and effort to understand in depth. And some things I’ve learned are completely counter-intuitive, like using a lower ISO in low light. Wait, what? Yes, we’ll get to that. This article is for those who just know which button on the camera is responsible for what is not enough. If you prefer to understand what is happening “under the hood”, then read on.

In the first part, we will understand:

● Where did ISO come from;

● What is the peculiarity of ISO digital cameras;

● What is gain and how does it relate to ISO;

● What does native ISO really mean and what does it look like.

In the second part, we will completely get into the jungle and analyze how ISO behaves when shooting in LOG and RAW and summarize for lovers of TL; DR.

ISO came to us from film

First of all, let’s dispel some myths. You probably heard about the exposure triangle in the early days of video making that ISO is responsible for the sensitivity of your camera. Now, this is complete nonsense. ISO does not change sensitivity and is not an exposure tool at all. Let’s figure it out.

I won’t bore you with the history, but it’s important to understand that ISO came to us in modern digital cameras from film cinematography. ISO (also known as ASA) is a measure of a film’s sensitivity to light, which is determined by the size of the silver halide grains in the emulsion applied to the film. Larger grains absorb more light, pretty simple arithmetic.

ISO also has an important function when shooting on film. This is the number you need to enter into the exposure meter to get the correct aperture for a neutral exposure because there is no way on a film camera to preview the exposure. You must use a light meter. Well, or train the eye for thirty years.

Thus, at the beginning of the digital era, ISO was used as an easy way to communicate the relative sensitivity of a sensor to people who were used to shooting on film. In terms of exposure, ISO 100 on a digital camera should be equal to ASA 100 film. And that’s where comparisons of film and digital lose much of their usefulness.

ISO does not measure or change camera sensitivity

ISO for film is an exact science. When it comes to numbers, the ISO number you see in your camera’s settings is made up by the manufacturer and is not an accurate indication of your camera’s sensitivity. That’s why, for example, Sony’s ISO 800 and Canon’s ISO 800 are not exactly the same thing, the difference can reach a whole exposure step, depending on the sensor design features. And on some cheaper cameras, the ISO values ​​​​are simply taken from the ceiling.

But it’s not only that. Film analogies were initially misleading. Because you can’t change the sensor in your camera the way you can change a roll of film. You’re stuck with the sensitivity your camera had when it left the factory (we’ll talk about double ISO later). Conversely, you cannot change the ISO of a photographic film, but you can change the ISO in a digital camera.

Then what does ISO mean in a digital camera? If you can’t change the sensitivity of your camera’s sensor, what does changing the ISO do? Well, it reflects the change in gain. Increasing ISO amplifies the signal received from the sensor. And the difference between ISO and amplification is not only in semantics, it’s not about terms. Understanding how amplification works has some pretty serious implications for how we should exposure when shooting with digital cameras. And the exposition is the main thing in the camera business.

ISO actually stands for Gain

To understand amplification, let’s quickly review how a digital image sensor works. It consists of photosensitive diodes arranged in a grid of pixels. Any light falling on the sensor is registered as a signal. The more light, the stronger the signal. But if there is not enough light, the signal is drowned in noise. After all, the sensor produces some stable level of noise as a side effect of its normal operation. You’ve probably heard of the signal-to-noise ratio. So, if most of your signal is above the noise level, you have high signal/noise, the image is clean. And vice versa, if the signal is mired in noise, the image is irrevocably damaged.

And that’s where the reinforcement comes in. You can increase the voltage applied to the sensor to increase the signal level. This is literally the same as turning the gain knob on a guitar amp. Gain is measured in decibels, and every 6 decibels of gain equals one exposure stop or doubling the ISO. Some professional camcorders, like my Sony FX6, can be switched to show gain instead of ISO, but even if yours can’t, it’s best to start thinking about ISO in terms of gain.

When the signal is amplified, the picture on the monitor becomes brighter, but there is one catch. And such a huge snag – the noise level rises proportionally. If you double the signal, you double the noise too. This is very important to understand, and if you take just one thought from this article, then here it is: it is impossible to increase exposure using ISO. Your camera does not become more sensitive. Image brightness is not the same as exposure. Increasing the gain doesn’t give you more light.

So if you’re shooting a scene in low key where the lion’s share of the information is located in the shadows, increasing the ISO actually kills the image quality. Your stage is already in contact with the noise floor, and now you raise it – it’s bad. On the contrary, you need to lower the noise level by lowering the ISO value below the original one, which means applying a negative gain.

Negative gain is the exact opposite of increasing gain. The signal gets weaker, you lose dynamic range in the highlights, but the noise floor goes down. Thus, the dynamic range is now distributed towards the shadows – this is where your image lives if you are shooting a dark scene. Of course, you will need to compensate for the loss of exposure with aperture, shutter speed, or light. But remember: lower ISO produces clean shadows. I know that this is confusing and completely contrary to the usual understanding of ISO, but just believe it, or better, check it out for yourself on the tests.

By the way, what gives the ARRI Alexa its legendary 14 stops (actual) dynamic range is the use of bi-amping. The sensor is read simultaneously at zero gain (to preserve color reproduction in the highlights) and negative gain (for the sake of low noise in the shadows). The two datasets are then merged into a single high dynamic range image. A similar DGO (dual gain output) technology is implemented in the Canon C300 mk.III and Canon C70 cameras.

Native ISO = 0 dB gain, always

You have probably heard from every iron about the native ISO. Like, you need to shoot in native ISO – there is the cleanest image and the highest dynamic range of your camera, right? Yes, but why? What is a native ISO? Every digital camera has it. Native ISO means only one thing – 0 dB gain on the sensor. This is the voltage level at which the sensor is calibrated by the manufacturer to capture the highest dynamic range it is capable of. Different cameras may have different native ISO values, but they are all 0 dB gain.

Some sensors are equipped with two sets of gain circuits, which gives us the ability to switch between low and high values ​​without increasing the gain and therefore without additional noise. It’s called dual native ISO and it’s a killer feature. It’s like having two sensors in one camera. By the way, this is one of the main reasons why Sony Venice is gaining popularity among top opera posts and stepping on the heels of ARRI Alexa a little bit.

Test your camera!  Acceptable ISO ranges may vary for your camera and/or shooting.
Test your camera! Acceptable ISO ranges may vary for your camera and/or shooting.

If you start thinking of ISO as a gain, your life becomes much easier because you always know that 0 dB gain means native ISO. This does not matter which image profile or dual ISO mode you are in. You don’t have to do arithmetic on the shoot or memorize sets of ISO values. You always know how far from native ISO your gain is. For example, for my camera at low base ISO 6 dB of gain is quite acceptable, 12 dB is the edge, and if I need a boost of 3 or more stops, I’d better switch to a higher ISO base level and lower the exposure with an ND filter.

ISO change doesn’t always mean gain change

You may have noticed that different image profiles have different native ISOs. For example, I have native ISO 800 on FX6. But this is only for S-Log3. If I’m shooting S-Cinetone (what you see is what you get profile), the original ISO is 320. Why 320? As usual with ISO numbers, they don’t mean what you think. Changing the native ISO from 800 to 320 does not mean that the camera has applied negative gain. It doesn’t mean that S-Cinetone is 1.3 stops darker than LOG. The sensitivity has not changed, it is still 0 dB. ISO 320 is just a number that you would need to enter into a light meter to get the correct gray card exposure for that particular picture profile. LOG is a flat profile, but S-Cinetone is not, so the neutral gray color falls on different values ​​on the waveform. If you want to equalize the exposure of S-Log3 with S-Cinetone according to the exposure meter, then you need to use the appropriate ISO values. But the camera sensitivity has not changed. The sensor still has 0 dB of gain.

Notice how much easier it is to find the native ISO in Gain mode.
Notice how much easier it is to find the native ISO in Gain mode.

I realize that this all sounds very confusing, and that’s the problem. ISO is a misleading number. And I am convinced that ISO is not the right setting for shooting video. What you really want to know is gain. Because 0 dB is 0 dB, regardless of the image profile, dual ISO, or even the camera. Whatever you’re shooting on, if you have zero gain, you know you’re in native ISO.

Thank you for reading the SHAGRAL Video Production Blog! In the second part of the article, we will analyze how ISO behaves when shooting in LOG and RAW and summarize. My name is Grigory Shakhanov, and every Friday I post here my observations on industry trends and share the secrets of commercial video production. Subscribe to receive new posts!

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