Speaker A
Hi, John Hess from FilmmakerIQ.com. In our course video Diving Into Dynamic Range, someone asked what a camera with Dual ISO, like the Blackmagic Pocket Cinema Camera 4K, does with dynamic range.
Explore how Dual ISO technology in the BMPCC 4K affects dynamic range and shooting strategies for filmmakers.
Key Takeaways
- Dual ISO cameras behave like two cameras with different noise and dynamic range characteristics depending on ISO.
- Understanding the native ISOs and dynamic range blocks helps optimize exposure and noise performance.
- Shooting in LOG or RAW is necessary to access the full dynamic range benefits of Dual ISO.
- Use low ISO settings for shadow-rich scenes and higher Dual ISO settings for cleaner shadows in low light.
- Dynamic range decreases at very high ISOs despite noise improvements.
Summary
- Dual ISO technology in cameras like the BMPCC 4K uses two different analog gain circuits to optimize noise performance at low and high ISOs.
- ISO is not the sensor's quantum sensitivity but a gain applied to the sensor's analog signal before and after analog-to-digital conversion.
- The BMPCC 4K has two native ISOs: 400 (low range) and 3200 (high range), each with different dynamic range characteristics.
- Between ISO 100-1000, the camera uses digital gain with a consistent 13.1 stops of dynamic range, shifting middle gray without changing noise.
- From ISO 1250 to 6400, the camera switches to a different analog gain path with 12.3 stops of dynamic range and improved noise performance.
- Above ISO 8000, dynamic range decreases due to increased analog gain and noise.
- Shooting in LOG or RAW is essential to fully utilize the camera's dynamic range capabilities.
- The Zone System helps visualize exposure zones; shooting low ISO is best for shadow-heavy scenes to maximize detail below middle gray.
- Dual ISO allows filmmakers to choose between better shadow detail or highlight retention depending on lighting conditions.
- Practical tests show that jumping from ISO 1000 to 1250 reduces highlight range but significantly improves noise floor in shadows.
Full Transcript — Download SRT & Markdown
Speaker A
Well, I dug around and noticed that the BMPCC 4K’s dual ISO sensor is actually a great demonstration of some of the fundamental principles outlined in my Dynamic Range video.
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If you haven’t watched it yet, you should. You really should, along with our deep dive into the Science of Exposure. Both of those videos provide the background for what we’re going to discuss about today.
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So let’s get right into it. First, let’s recall what ISO is. ISO is not the quantum sensitivity of the sensor itself. The sensor sensitivity is set at the point of manufacturing.
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A certain number of photons hit the silicon sensor and produce a certain number of microvolts of electricity. I get into the mechanics of this on my Science of Digital Sensors video.
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But from this one single sensitivity, we send those microvolts into some microcircuits that amplify and create an analog signal, which is then sent into an Analog Digital Converter and processed.
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We can change the sensitivity of the entire imaging system by changing how much gain we apply to the signal both before and after the ADC.
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ISO is a standard rating for film speed. At any given ISO rating, a certain predetermined amount of light should return middle gray regardless of the camera brand, though there is a lot of wiggle room, but that’s another topic altogether.
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So what’s with this concept of “Dual ISO”? Well, a sensor that has been given a lot of light presents noise differently than a sensor given very little light.
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Some system designs incorporate alternative strategies for noise cancellation when the camera is engaged in high ISOs as opposed to low ISO settings.
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Let’s diagram this out: We start out with the same photosensitive silicon slab which, as I said before, does not change quantum sensitivity.
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When a lower ISO is engaged, the signal is sent through a basic amplifier which applies an analog gain to the signal before it is sent to the Analog Digital Converter to convert to digital.
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Then a digital gain is applied and the signal is recorded onto a codec. When a high ISO is engaged, the signal coming off the sensor instead takes another path through a more powerful amplifier which has a higher gain and additional noise cancellation
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circuits before going to the ADC where again more digital gain is applied and the signal is recorded to media.
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So what that all boils down to in practice is we essentially get a camera that behaves like two different cameras depending on our ISO setting.
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As of this video, I believe Panasonic, Sony, RED, and now Blackmagic are offering cameras with this Dual ISO capability.
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I may have missed a few, but right now I’m just going to focus on the 2018 Blackmagic Pocket Cinema Camera 4K as an example of how this feature will affect your approach to shooting.
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This is the official dynamic range chart from Blackmagic for the BMPCC 4K. Immediately you should identify three regions.
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According to Blackmagic literature, the block on the left has a "native ISO of 400" and the area on the right has a "native ISO of 3200."
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Now, I'm not a big fan of the term "native ISO" because I think it hides some strategic benefits of ISO, but I digress. You'll see more what I mean as we continue on.
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Notice the area ISO 100-1000 is one solid block rectangle. There is no change in 13.1 stops of dynamic range here.
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The camera just slides what is middle gray up and down. This is done by adjusting gain applied after the ADC, or digital gain.
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On stills cameras, this might be called an ISO invariant range. One caveat with the Blackmagic Cinema Pocket Camera: if you record RAW, this digital gain is recorded only as metadata. If you were to start recording and adjust the ISO inside
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this range mid-recording, you would not record those ISO changes. But if you shot in ProRes or DNxHR, you would actually see the image brightness shift.
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The next rectangular area is between ISO 1250 and 6400, this time with 12.3 stops of dynamic range.
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This represents the new path and the additional analog gain applied. Then from ISO 8000, you see this stair stepping down and we get decreasing dynamic range. I believe at this point we’re just cranking up analog gain.
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Let’s see how this plays out in real life. I rented a Blackmagic Pocket Cinema Camera 4K for a few days and ran a bunch of tests.
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Shooting my dynamic range tester, and yes it has gotten a little out of whack since I last used it, but let’s focus on the brightest light on the right.
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At ISO 100 shooting in LOG, the brightest I can expose this to is around IRE 80. If we consider middle gray to be around IRE 50, that gives us 2 stops to work with above middle gray, just as the official Blackmagic dynamic range chart shows.
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Keep in mind, I cannot expose anything higher than IRE 80. The camera literally clips at IRE 80.
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Now watch as I step up the ISO. Notice on the vectorscope everything just shifts upwards, even that right spot light, and the clipping point or max exposure ceiling.
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So by the time I’m hitting ISO 1000, I now have 5 spots of light above IRE 50. Again, my tester needs a tune-up, but you get the idea. Five stops of light match up pretty closely
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to 5.3 on the official Blackmagic chart. But now watch what happens when we make the leap from 1000 to 1250.
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It’s not a shift up. The top two spots are now hitting the ceiling with the third pretty close to clipping. Again, this was illustrated with the dynamic range chart.
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Making the leap from 1000 to 1250 immediately chops off about 3 stops from the top end.
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But then look at the bottom. Check out that big fat noise line from ISO 1000 and compare that to the much tighter noise floor of ISO 1250. We gain about 2.5 stops in the darks.
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Now I’m showing you LOG footage because that’s the way the camera can visually squeeze 13 stops into what you’re seeing now as 8-bit video.
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This will also apply if you’re shooting RAW, but as I said, the ISO is only recorded as metadata except for when you make this jump between the two different ranges of the Dual ISO.
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If you’re not shooting LOG or RAW, then you’re not getting the full dynamic range for the camera, so the rest of what I say may or may not have mileage.
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Technical tests out of the way, let’s talk about how you best approach shooting with Dual ISO.
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The trick is to know the kind of image you are ultimately trying to produce.
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For a good way to visualize this, let’s bring in Ansel Adams’ Zone System. We talk a bit more on the Zone System in my Exposure video.
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Zone V is middle gray. Zones VI-X are above middle gray, and Zones 0-IV are below middle gray. Each zone is one stop higher or lower than the surrounding ones.
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If your scene is largely in the shadows, that means your shot is mostly below Zone V, Zones I-III, dark objects with lots of textures in shadow. You want to shoot as low an ISO as you can to give yourself as much room as possible for data below middle gray.
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I’m not talking underexposing here. You want to expose middle gray for what middle gray is, but open up your exposure as much as you can—that’s shutter speed and aperture. If you’re on a controlled set, add more light, then shoot at the lowest ISO you can.
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If we shot ISO 100, for example, we would have 11.1 stops below middle gray.
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However, the Dual ISO gives us a second useful option. Take a look at this shot which I exposed at f2.8, ISO 400, 1/24 shutter. Unfortunately, there’s not enough light to use any lower ISO here.
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At ISO 400, I’ve got 9.1 stops below middle gray before we get lost in noise.
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Compared to ISO 1000, which only has 7.8 stops below middle gray, you can easily see the noise starting to sneak in.
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But now let’s make that Dual ISO leap to ISO 1250 and immediately you see the noise is much less noticeable. I went from 9.1 stops below middle gray at ISO 400 to 7.8 stops below middle gray at ISO 1000 and then 10 stops below mid.
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That jump from one part of the dual ISO to the other, added more shadow detail even with a lesser exposure.
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Here’s another dark scene example: this time I was able to expose ISO 100 at f2.8 and shutter speed of 1/24th a second.
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Shooting at ISO1000 using f2.8 and shutter of 1/200 - roughly the same exposure but the higher ISO obviously results in more noise in the shadows.
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Then if we take one step up from ISO1000 to ISO1250 shooting same f-stop but 1/250th - the noise floor plunges away.
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It’s does literally feel like we're shooting with a brand new camera. The rule applies - In a dark scene shoot the lowest ISO possible.
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But with dual ISO you’ve got that second much higher baseline to start from. But even if you have a well lit set, reserve the lower ISOs, the ones starting at 100 and 1250 for scenes that have a lot of shadow in them.
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The other side of the rule - in scenes where there is a lot of light and the most important things are going to be above middle gray - that’s zones VII-X - shoot the highest ISO given your tolerance for digital noise.
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Again, I did not say over expose, expose properly but use the highest ISO. And of course Dual ISO puts a caveat in this situation as well.
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The best demonstration of this is shooting backlit clouds. Clouds by their very nature are going to be a stop or so above middle gray or Zone VI or VII - but back lit clouds are going to go much higher.
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Here’s a shot of some backlit clouds at ISO 1000 1/1000 shutter speed at f/6.2.
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Now compare that with this second shot: ISO 125 1/125 shutter speed at f/6.2. We dropped down 3 stops in ISO and made up with a 3 stops in shutter speed but all the detail in our clouds is completely washed away.
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You can see this in the Blackmagic chart - With ISO 1000 we have 5.3 stops above middle gray to work with - but with ISO 125 we’re limited to only 2.3 stops - that’s not a lot.
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Now watch what happens when we step up from ISO 1000 to ISO 1250… the same loss of detail occurs because ISO 1250 also only has 2.3 stops above middle gray.
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So in a bright situation focusing on highlights, shoot the highest ISO that gives you tolerable noise, but when shooting dual ISO make sure you don’t cross over into that next tiered ISO.
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Now you probably won’t be shooting such extremes highlights in everyday normal production and it’s often perfectly acceptable to let hotspots clip lose detail.
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Again let’s look at the Ansel Adams Zone System: 3 stops above middle gray is Zone VIII - which Adams identifies as textured snow.
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With low ISOs on this BMPCC4K textured snow run the risk of being untextured pure white.
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But if you aren’t shooting snow - or in my case since I live in Southern California I have to shoot backlit clouds - then you could potentially get away with clipping the highs in every day shooting.
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Most of the time you will be shooting scenes that are 3 or so stops above and below middle gray.
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Shots like this lake shot contain both highlight and shadow detail but nothing too extreme in either direction.
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Something like this I may prefer to go with as little noise as possible shooting so ISO 100 and live with only 2 stops above middle gray.
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After all the only thing that is above middle gray is the blue sky which doesn’t have much detail anyway.
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But if you’re finding yourself clipping when shooting these outdoor shots - say you’re shooting into the sun, start pushing the ISO higher to give yourself more room in the highlights at least up until you have to make that switch to the other part of the Dual ISO.
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I hope with our exposure and dynamic range videos that I’ve given you a perspective on how camera exposure works as a strategy.
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There is no one size fits all approach - the more you think about how you want the final result to look or at least the more you think about how the camera can best capture the data you’re interested in, the more fine tuned you can get your exposure practices
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and the better your final image. But it takes practice, understanding the basic principles and, most importantly, getting an intimate feel for how your camera works.
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The numbers I stated here are only for the Blackmagic Pocket Cinema Camera 4K. But every camera has it's own personality even if they are very much similar.
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Now one last word on Dual ISO. It’s tempting to look at this strategy and think that all cameras should utilize this technique.
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But Dual ISO isn’t the only strategy out there and it certainly won’t be the last.
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Every technique has a tradeoff in terms of cost and engineering. As neat as dual ISO is in some ways - it can also tangle people up who don't understand it which is why I felt it was important to put this video together to explain this interesting
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quirk in the way the Blackmagic Pocket Cinema Camera handles dynamic rant. If you liked this video throw us a like and subscribe and consider becoming a patron on Patreon - every little bit does indeed help.
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Thanks to our A list patrons you guys rock. Don’t forget to check out our merch store - new designs coming very soon I promise.
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If you didn’t like this video I can’t think of a better insult than to buy a pallet of premium cotton Ts for a sacrificial bonfire to your pagan gods who reside in the shadows and spew digital noise for all of us to capture.
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Until next time my friends - go out there and make a great exposure. I’m John Hess and I’ll see you at Filmmaker IQ.com
Topics:Dual ISODynamic RangeBMPCC 4KBlackmagic Pocket Cinema CameraISONoise PerformanceLOG FootageRAW VideoFilmmakingExposure
