Oscillating the ISO on Alexa

[Joao Papa]

I'm a director, but most of the times I will also be the dop on my films. When I am not, though, I have the chance to work with other great dops and learn a lot with them and their methods.

 

I've recently shot a commercial with a ASC dop and other with a ABC dop, and they both did change the ISO a lot during the same film. When asked why, the answer was simply that this way they would change the dynamic range according to the scene.

Even though that makes sense, I have always been doing the same just changing the exposure and balancing my fill lights, usually working with ISO 800 during the whole process.

 

I was wondering how do you guys approach that, and why.

Edited June 26, 2016 by Joao Papa

[David Mullen ASC]

With the Alexa, the dynamic range doesn't change when you change ISO ratings, just the balance between high and shadow information (and the overall noise changes). However, if outdoors with a lot of bright highlights and you use a very low ISO rating, you will sometimes shorten the dynamic range because you've sacrificed some highlight information for shadow information that you may not be taking advantage of if all your shadow information falls within just a few stops below medium grey. So you've given up some detail at the high end without gaining anything more in the low end.

 

That said, it is not uncommon for cinematographers (like me) to adjust ISO ratings on the Alexa in a more conservative range - for example, 400 ISO in day exteriors, 800 ISO for most interior work, and 1000 to 16000 ISO for low-light work (for practical reasons.) An ISO change by less than a stop isn't going to change the look much, but it can help you out now and then, like when you've lit the set to f/2.8 and suddenly you have to put an f/3.5 zoom on the camera. Or maybe you've added enough light to shoot at 48 fps wide-open on the lens and then the director wants to go to 60 fps.

[Joao Papa]

Actually that was what they said, but I got lost writing it properly since english is not my first language.

 

They wanted to change the balance, not the dynamic range.

 

With the Alexa, the dynamic range doesn't change when you change ISO ratings, just the balance between high and shadow information (and the overall noise changes).