13 Jan A (only slightly) less confusing explanation of the Pocket 4K’s ISO settings
The Pocket 4K’s “ISO” settings seem to cause constant confusion. The combination of RAW recording, a dual gain sensor and obtuse official communication from Blackmagic Design don’t help matters. Is there a “best” ISO? Does one ISO have better dynamic range? Which ISO has the best highlight? What is the native ISO? What is the “true” native ISO. What does “native” even mean? Should I over-expose? Should I under-expose?
With much trial and error I think I’ve come to a practical working understanding that I’d like to share. I know that this is far from the last word, and we’ll find ways to argue about the exact terminology and underlying technology until the next camera is released, but hopefully someone will find this information to be useful shorthand.
tldr: Reference the chart at the end of the article.
1. There are two “normal” ISO’s.
This is to say, there are two modes that the camera is tuned for. Low, and High.
A. When recording RAW, only this one gain choice is burnt into the file. (ISOs in-between are metadata exposure “hints”)
B. When recording ProRes, the exposure hints are burnt into the file.
2. There is a third “extended” range.
Every ISO setting above 6400 is a new analog gain setting above and beyond what the two gain circuits the “dual-native” sensor is tuned for. Whether recording RAW or ProRes, this analog gain is burnt into the file. Each step adds a significant amount of noise.
NOTE: For the sake of clarity and brevity, for the rest of this post I will only reference RAW recordings. For similar reasons, “correct” exposure will serve as short-hand for exposing for your subject. Whether that be a grey card at 50% or skin tone at 65% or whatever you deem appropriate to record an image in camera that does not require extraordinary post-manipulation to achieve a natural exposure. (See my post about ETTR.)
3. ISO settings between the two native modes are exposure aids. Use them.
Choosing the correct ISO between 100-1000 or ISO 1250-6400 in camera allows you to make the best exposure decision. All of the camera’s exposure aids will adjust accordingly.
Whatever supposed gains you think you can achieve by sticking to one special ISO in camera and adjusting in post are far outweighed by the problems inherent in having to do so.
Just one practical example: if you record at ISO 400 (because it’s “native” so it must be best, right?) and then have to push it to 800 in post, the only thing you will have really accomplished is maybe not using enough light when shooting and lifting noise when you get back to your computer.
4. It does matter which ISO you choose in camera.
Not because one setting is magically better than another, not because it’s anything other than metadata, but because the ISO you choose in camera will impact choices you make while shooting that are difficult/inferior to change in post. By monitoring the correct exposure, you will know to adjust lighting, adjust your aperture, move your subject, etc. Even if the ISO setting isn’t baked into the shot, these choices are.
5. “Native” ISO is, in empirical terms, meaningless.
The “no ISO is magic” rule applies to ISO 400 and ISO 3200, too.
Does ISO 400/3200 have the least noise? No.
Does ISO 400/3200 have the most dynamic range? No.
Does ISO 400/3200 have the most balanced under/over exposure? No.
“Native ISO” is marketing jargon. Not only does the meaning differ between camera manufacturers, the Pocket 4K’s two “native” ISOs are nothing more than arbitrary settings that someone(s) at Blackmagic thinks is the best all around compromise. (It’s not coincidence 400 and 3200 are near the middle of their respective modes.)
6. Don’t ever make an exposure choice based on dynamic range.
The difference in dynamic range between the two normal ISO ranges is less than one stop. The difference within those ranges is zero. (Remember, ISO 200 vs ISO 400 is just an exposure aid.) Dynamic range only begins to meaningfully drop once you are forced into the extended ISO ranges, and if you have to go that high, then dynamic range is the least of your concerns.
I’m not suggesting that dynamic range isn’t important. I’d take more if I could get it. But the Pocket 4K’s DR is what it is, and any small incremental increase you could gain by trying to game your ISO choice is less than the negative ramifications to your exposure.
7. When straddling the ISO fence, jump to the next bottom.
If your scene is correctly exposed at ISO 1000, consider if you can lower light levels to get it to ISO 1250. The switch to the low end of the high gain circuit will yield much less noise. For the same reason you may also want to go the other direction. For example, if your correct exposure is over 6400, see if you can add light to get it a notch lower.
Put simply, the LOWEST ISO of each range yields a final image with the least noise (because it uses the least possible gain (either in camera or in post).
8. If low noise is your priority, expose for the lowest ISO in a given range. If highlight range is your priority, expose for the highest ISO in a given range.
Notice I didn’t say “pick” the lowest or highest ISO. I said “expose” for it. Changing the ISO and not changing anything “real” about your exposure accomplishes nothing but making you work harder in post.
9. If protecting highlights (highlight recovery) is important, monitor your ISO one stop lower than “correct”.
When you pick an ISO inside the two normal modes, what you are effectively doing is setting the value that the camera considers middle grey. The Pocket 4K is tuned to give more of that range to shadows than it does to highlights. We can debate whether a better design decision would have been to put the middle in the middle – but the net result of the decision that was made is less noisy shadows when correctly exposed.
If you’d prefer more headroom in exchange for more noise, then set the in camera ISO one stop lower than correct. You can then correct the exposure in DaVinci Resolve’s RAW panel and adjust the highlight rolloff to taste. Your shadows will be a bit noisier, but for many, this is preferable to blown highlights.
Underexposing more than a stop starts to become problematic, shifting too much of the middle (likely your subject, the important part) into noisy shadows.