Tag Archives: grain

cameras, cinematography, PXW-FS5, Technology

ISO Confusion Once Again!

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I’m going to keep bringing this up until people start to take note and understand that with an electronic camera ISO is NOT sensitivity.

With an electronic camera ISO is a guide to the required shutter speed and aperture needed to get the correct exposure. This is different to sensitivity. The ISO rating of a video camera and it’s sensitivity are closely related, but they are not quite the same thing. Because different gamma curves require different exposures the ISO rating for each gamma curve will be different even though the gain and actual sensitivity of the camera may be exactly the same.

Lets take the  Sony PXW-FS5 as an example.

If you shoot using the standard camera settings you should expose white at 90%, middle grey will be around 42% and skin tones typically around 70%. At 0dB gain the camera the camera will display an ISO equivalent rating of 1000 ISO. So let’s say you are using a light meter. You set it to 1000 ISO and it tells you you need an aperture of f5.6 to get the right exposure.

Now you change to S-Log2. If you do nothing else your white card will now be at around 75% and middle grey will be around 40%. At 0dB gain the camera will show an equivalent ISO of 3200 ISO.

But hang on – The camera is still at 0dB gain, so there is no change in sensitivity. .But the camera is over exposed, S-Log2 is supposed to be exposed with white at 59% and middle grey at 32%.

So we go to our light meter and change the ISO on the light meter from 1000 ISO to 3200 ISO. Because the light meter now “thinks” the camera is more sensitive by almost 2 stops it will tell us to close the aperture by nearly 2 stops. So we go to the camera and stop down to f10 and bingo, the image is exposed correctly.

But here’s the important thing – The camera hasn’t become any more sensitive. We haven’t replaced the sensor with a different, more sensitive one (as you would do with a film camera where you actually change the film stock). We are still at 0dB gain (even though the camera tells us this is the equivalent to a higher ISO).

The only reason that ISO number changes is so that if we were using an external light meter we would get the recommended exposure levels for the gamma curve we are using. In this example closing the aperture increase the highlight range that the camera would be able to cope with and this helps us get that full 14 stop range from the camera, although closing the aperture means less light on the sensor so the pictures end up a little noisier as a result – That is unless you choose to rate the camera at a different ISO by over exposing the log a bit.

ISO is useful, but you need to understand that it isn’t really sensitivity. After all we can’t change the sensors on our video cameras and that would be the only way to truly change the sensitivity. Any “sensitivity” change is really nothing more than a gain or amplification change. Useful but not the same as changing the actual sensitivity. Gain will make a dark picture brighter but it won’t allow you to see something that the sensor can’t detect.

It is much easier to understand dB gain with an electronic camera as it actually tells you exactly what the camera is doing and it is actually my recommendation that people use gain rather than ISO for all of the above reasons.  The use of ISO on electronic cameras is very badly understood, in part because it’s a largely meaningless term because it doesn’t tell us how sensitive the sensor is, how much gain we are using or how much noise we are adding. Give any experienced camera operator a camera and ask them how noisy will it be a 18dB gain and they will have a pretty good idea of what the pictures will look like. Give them the same camera and ask them how noisy will it be at 8000 ISO and they won’t have a clue.

The problem is ISO is trendy and fashionable as that’s what “cinematographers” use. But lets be honest with ourselves – we are using electronic video cameras, whether that’s a Red, Alexa or FS5 so really we should be using the correct terminology for an electronic camera which is gain. It would eliminate an aweful lot of confusion and tell us how much noise and grain our pictures will have. It’s noise and grain will levels will determine how good a clip looks and how much we can grade it, so we need to clearly understand how much gain is being added in camera and dB gian tells us this. ISO does not.

Side Note: Modern film stocks will often have 2 ratings, the ISO or actual measured sensitivity of the film stock plus the EI or Exposure Index which is the recommended setting for the light meter to get the best exposure. In some respects the ISO rating of a video camera is closer to the EI rating of a film stock. Perhaps we should stop calling it ISO and use the term EI instead, this would be me appropriate and signify that it is a reference for best exposure rather than true sensitivity.

UPDATE: A comment on facebook was why not display both ISO and Gain side by side. This is an obvious solution really. Why do camera manufacturers force us to choose either ISO or gain? Why can’t we use a hybrid of the 2? I see no technical reason why cameras can’t show both the gain and ISO at the same time – Problem solved.

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What is ISO and how does it compare to gain?

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With more and more people using 35mm size sensors, more of the old traditional filming styles and techniques are trickling down from the high end to lower and lower production levels. This is a good thing as it often involves slowing down the pace of the shoot and more time being taken over each shot. One of the key things with film is that you can’t see the actual exposure on a monitor as you can with a video camera. A good video assist system will help, but at the end of the day exposure for film is set by using a light meter to measure the light levels within the scene and then you calculate the optimum exposure using the films ISO rating.
So what exactly is an ISO rating?

Well it is a measure of sensitivity. It tells you how sensitive the film is to light, or in the case of a digital stills or video camera how sensitive the sensor is to light. Every time you double the ISO number you are looking at doubling the sensitivity. So ISO 200 is twice as sensitive as ISO 100. ISO 1600 is twice as sensitive as ISO 800 etc.
Now one very important thing to remember is that ISO is a measure of sensitivity ONLY. It does not tell you how noisy the pictures are or how much grain there is.  So you could have two cameras rated at 800 ISO but one may have a lot more noise than the other. It’s important to remember this because if you are trying, for example, to shoot in low light you may have a choice of two cameras. Both rated with a native sensitivity of 800 ISO but one has twice as much noise as the other. This would mean that you could use gain (or an increased ISO) on the less noisy camera and get greater sensitivity, but with a final picture that is no more noisy than the noisier camera.
How does this relate to video cameras?

Well most video camera don’t have an ISO rating, although if you search online you can often find someone that has worked out an equivalent ISO rating. The EX1 is rated around 360 ISO. The sensitivity of a video camera is adjusted by adding or reducing electronic gain, for example +3db, +9db etc. Every 6db of gain you add, doubles the sensitivity of the camera. So taking an EX1 (360 ISO) if you add 6db of gain you double the sensitivity and you double the ISO to 720 ISO, but you also double the amount of noise.
Now lets compare two cameras. The already mentioned EX1 rated at approx 360 ISO and the PMW-350 rated at approx 600 ISO. As you can see from the numbers the 350 is already almost twice as sensitive as the EX1 at 0db gain. But when you also look at the noise figures for the cameras, EX1 at 54db and 350 at 59db we can see that the 350 has almost half as much noise as the EX1. In practice what this means is that if we add +6db gain to the 350 we add +6db of noise so that brings the noise level 53db, very close to the EX1. So for the same amount of noise the 350 is between 3 and 4 times as sensitive as the EX1.
Does your head hurt yet?
There is also a good correlation between sensitivity and iris setting or f-stop. Each f stop represents a doubling or halving of the amount of light going through the lens. So 1 f-stop is equal to 6db of gain, which is equal to a doubling (or halving) of the ISO. You may also hear another term in film circles and that is the T-stop. A T stop is a measured f-stop, it includes not only the light restriction created by the iris but also any losses in the lens. Each element in a lens will lead to a reduction in light and T stops take this into account.

So there you go. The key thing to take away is that ISO (and even the 0db gain setting on a video camera) tells you nothing about the amount of noise in the image. Ultimately it is the noise in the image that determines how much light you need in order to get a decent picture, not the ISO number.