As more cameras are released with dual native ISO capabilities, it's important to understand how and why it works.
It may not be as straightforward as you think.
Blackmagic Design has been dropping camera after camera with the features that we all want—features that are useful for every filmmaker. In my opinion, one of the most crucial tools a filmmaker can have is the ability to shoot in just about any environment with extreme levels of control of the image regardless of light levels.
With their last few releases in the Pocket Cinema Camera line, Blackmagic has included dual native (or dual gain) ISO as a part of the feature set. For my purposes, this has cemented that camera line into my gear load-out as my ultimate documentary and/or guerilla filmmaking cinema camera. It unlocks a bunch of possibilities that just a few short years ago would have never been available to the average filmmaker.
However, as with most ground-breaking developments in camera technology, it's not just a magic button for seeing in the dark. There are things you need to know to get the most out of dual native ISO.
To get the highest amount of dynamic range with the cleanest image possible, there is quite a bit more to it than just cranking that ISO slider as high as it'll go.
How Does It Work?
I'll be honest with you, you should probably watch the below video wherein John P. Hess of Filmmaker IQ gives a mic-drop level definitive explanation of the entire concept. It's worth a watch-through (or four) in my opinion.
Let's first dive into how a digital sensor works. When a sensor is created, the actual sensitivity of the sensor is more-or-less burnt into the piece of hardware itself. The hardware sensor is a tiny little square with thousands of little "photosites" that gather light and convert them into energy. This energy is then perceived and converted by a thing called an ADC (Analog to Digital Converter). That's the part of the signal chain that converts physical light into what are essentially the zeros and ones that make up your image.
In some pretty over-simplified terms, ISO is essentially like changing the "Gain" knob on a guitar amp. The signal coming out of the guitar is always at the same level, but the amp is amplifying that signal, and the more you turn that signal up, the more distorted or noisy it gets. In your camera, the signal is literally being put through some digital or analog amplification when you're changing the ISO.
When a camera has dual native ISO, it means that there are two amplifiers that the signal can travel through. Two amplifiers where the signal in one is boosted much more than the other.
For this reason, dual native ISO is almost a misnomer, as it's really more of a dual gain system. The native ISO of the sensor itself is always the same (like the pickups on a guitar, to bring back that same analogy).
How to Get the Most Out of It
Every camera is different when it comes to dynamic range and how ISO choice will affect it.
Obviously, when recording RAW, most of these choices are stored in the metadata and can be more of a post-production decision. However, an important note is that when you make the change from one level of the ISO spectrum (the lower) to the other (the higher), you will be stuck with that range of ISO settings in post (i.e. 100-1200 or 1250-25600). This decision will be burnt into the RAW files at the time of recording. So make sure you're on the best "native" ISO for the situation.
However, with dual native (gain) ISO systems, it's important to know what is going on with your footage when you change the settings, whether it be in production or post-production. When you change your ISO, you're changing what your camera is supposed to see as "middle-gray."
In the case of the Blackmagic cameras (which have a dual native ISO that engages at ISO1250), there are a few somewhat counter-intuitive things about how you need to expose your images. Namely, looking at the chart above, pay close attention to the ISO 1000 representation. You'll see that this particular ISO has the highest possible amount of latitude above middle gray.
This means that if you have an image with particularly bright areas, you will want to consider using that ISO.
"Wait, in a bright place I want a higher ISO?" Yes. As I said, it's a bit counter-intuitive.
As you can see in the above image from John's video, the ISO set at 1000 has tons more dynamic range than the shot at ISO 125.
This single tip learned from this video has absolutely changed the way I shoot with my BMPCC. When I roll up to a bright daylight scene, I'm always racking my ISO all the way up to 1000 (thanks to ND filters for keeping me from a totally blown out image).
I am always truly amazed at how much more dynamic range you get with this method.
Alternatively, if you're shooting in a dimly lit environment (particularly one that includes a lot of somewhat textured darker areas below middle gray), you want to shoot with the lowest ISO possible for the cleanest image. This is unless you are on the higher end of the lower native ISO—i.e., don't shoot at ISO1000 when you're in the dark. Go ahead and engage the higher gain ISO setting.
If you can, however, shoot at the lower side of the lower gain setting.
Confused? Well just have a second look at that image above. It explains it pretty well. The middle image (at ISO1000) is the noisiest by far.
So with the BMPCC line, if you're in a bright spot, maybe try ISO1000 (without going over to 1250), and if you're in a dark spot, shoot as low as you can on either end of the "gain" spectrum. It really does help to keep this in mind. I've been able to retain some really bright highlights and clean crisp shadows this way.
Have any other tips? Share them in the comments.