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.