ISO index

On set, you constantly work with three variables: aperture, shutter speed, and the ISO index. The ISO index determines how sensitive your sensor is to incoming light. If you double the ISO from 400 to 800, you gain a full stop of exposure – meaning you can either stop down the aperture or shorten the exposure time. Sounds practical? It is. But the price is noise, and you'll notice that immediately in the grade post.

In practice, it works like this: You're in a classroom, daylight only from the front, and the director wants a tight 2.8 shot with a moving camera. Underexposed at ISO 200, properly exposed at ISO 3200 – but now you can already see the grain in the viewfinder. With modern digital cameras (Red, Arri Alexa, Sony), the noise at 1600–3200 is still manageable, especially if you shoot Log footage and can specifically de-noise it in the edit. But at 6400 and above, it gets critical: color separation breaks down, fine textures blur, and the colorist will curse later during grading.

Hence the trick: Your camera's native ISO is your friend. Every model has one or more ISO values at which the sensor works optimally – typically ISO 800 for the Alexa Classic, often 200–800 for modern variants. Other ISO settings are digitally boosted, which inevitably means information loss. If you have to go beyond native, it's better to work with an ND filter and open the aperture than to push the ISO. A 0.6 ND (two stops reduction) costs you less image quality than two stops of ISO push.

Practical scenario: Night shot, cars driving, you need 1/50s for motion blur, aperture 4.0, but at ISO 400 it's dark. Instead of going to ISO 1600, you mount a 0.9 ND on your lens, set the ISO to 800, and increase the aperture to 2.8 – the result is cleaner, more contrasty, with less electronic noise. On set, you should always ask yourself: Can I add light or use a filter? That's cheaper and better than digital error compensation through ISO. Sensor sensitivity isn't the problem – incorrect usage is.