Alister Chapman on Native ISO

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robedge

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Alister Chapman on Native ISO

PostWed Mar 25, 2020 2:49 pm

ProAV uploaded this today. Alister Chapman has a knack for clear explanations. Good discussion of native ISO, dual native ISO and gamma. One thing that I’d like to see on my Blackmagic Pocket 4K is the option to address sensitivity in terms of gain rather than ISO.



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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 5:16 pm

Nice to see an expert get it right. Not always the case. When the BMPCC4K was first introduced, some folks liked their results shooting at the low end of the ISO 3200 range. I tried that too, but generally decided to shoot ISO 400 or 3200 to maximum the respective dynamic range.

I’ve also been surprised how well the BRAW image can be worked in post when it’s sitting in, for example the lower half of the camera histogram. So it doesn’t need to look like an ideal or ETTR image to get good results in your deliverables.
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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 5:33 pm

None of the BMD Cinema cameras have variable gain settings. The Pocket 4K has two gain settings, low and high (400/3200). Gain is a signal amplifier circuit (hardware) has the side effect of adding more noise as you crank it up, then you need noise reduction to eliminate the noise, and the plastic looking IQ is the result. Variable gain is a feature in Broadcast type cameras, and the DSLRs, not the path BMD has chosen.
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rick.lang

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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 5:39 pm

Thanks, Denny. But we do see a small loss in dynamic range switching to the ISO 3200 range and we see a small increase in noise.

I don’t have your many years of broadcast experience, but I suspect the BMPCC4K is seeing less of a loss of dynamic range and not as much increased noise as a true broadcast camera would. Do you think that’s correct?
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John Griffin

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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 5:51 pm

rick.lang wrote:Thanks, Denny. But we do see a small loss in dynamic range switching to the ISO 3200 range and we see a small increase in noise.

that's because it's a higher gain than the 400 ISO range - there is no free lunch!
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Denny Smith

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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 5:56 pm

Correct Rick any added gain circuit (including the dual ISO/gain sensors like BMD uses) will increase noise and reduce dynamic range a little when you go to the high setting. However, the noise increase is less with the sensor dedicated dual gain, than an external gain amp making the same increase. The Ursa Broadcast the Micro Studio demonstrates this (I know not the same sensor, but an exampe of external gain circuit over sensor built in one.

ISO invariant sensors like the Pocket 4K and Ursa Mini Cinema Cameras do not use a gain/ISO amplifier, so dynamic range is not lost, but underexposing these sensors will increase video noise also, so it is a kind of trade off which way to go. I think the dual gain sensors is the best option available right now. Both the Pocket 4K and the Nikon Z6 Cameras demonstrate the two approach’s quite nicely and can get nice results in their high gain settings, as long as the sensor is not underexposed (none of these cameras can shoot in the dark, they need some light :!:)
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John Griffin

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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 6:01 pm

Denny Smith wrote:I think the dual gain sensors is the best option available right now. Both the Pocket 4K and the Nikon Z6 Cameras demonstrate the two approach’s quite nicely and can get nice results in their high gain settings, as long as the sensor is not underexposed (none of these cameras can shoot in the dark, they need some light :!:)
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Re: Alister Chapman on Native ISO

PostWed Mar 25, 2020 7:40 pm

robedge wrote:One thing that I’d like to see on my Blackmagic Pocket 4K is the option to address sensitivity in terms of gain rather than ISO.


Rob - it is quite easy to work with ISO - +6dB gain equals +1 stop equals doubling the ISO.
So if the base ISO is 400 then this is your 0 dB gain.
ISO 100 = -12dB
ISO 200 = -6dB
ISO 400 = 0dB
ISO 800 = +6dB
ISO 1600 = +12dB
ISO 3200 = +18dB
ISO 6400 = +24dB
ISO 12800 = +30dB
ISO 25600 = +36dB
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John Richard

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Re: Alister Chapman on Native ISO

PostThu Mar 26, 2020 12:52 pm

Original Ursa Mini Pro (Gen 1) has only a single native ISO of 800, correct?
And if you have a low light problem, 3200 ISO is the go too?
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Re: Alister Chapman on Native ISO

PostThu Mar 26, 2020 2:13 pm

John, that may be the practice, but I wouldn’t do that on my original URSA Mini 4.6K. I found even ISO 1600 may be too noisy and only shoot my camera in ISO 800. I feel you can boost it two stops safely in post where you can control the results better than just raising the ISO in camera two stops. People may test each approach and see which they prefer.

If you’re often in lowlight situations, the BMPCC4K and BMPCC6K with their dual sensor 400/3200 may be a very good alternative if you can manage the decrease in dynamic range.
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Re: Alister Chapman on Native ISO

PostFri Mar 27, 2020 2:30 pm

When you shoot BRAW with the Pocket 6K, there are two base settings; 400 and 3200. Isn’t it true that all the other choices are just exposure indices, in other words not actual changes to the sensor gain?
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Re: Alister Chapman on Native ISO

PostFri Mar 27, 2020 2:46 pm

Tom Roper wrote:When you shoot BRAW with the Pocket 6K, there are two base settings; 400 and 3200. Isn’t it true that all the other choices are just exposure indices, in other words not actual changes to the sensor gain?


Yes, until you go above 6400, where analog gain is applied (this is true of the BMPCC 4K, anyway). All that matters is the ISO you expose for. The actual camera ISO setting, in either of the two circuits, doesn't matter, apart from the ease of seeing the screen.

In my view, the difference is also much less consequential shooting Prores than people think -- the so-called "tone curve" of any given ISO value can easily be altered in post, to the point where nobody can tell the difference between shots at 100 v. 1000 -- but that's another debate.
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Re: Alister Chapman on Native ISO

PostFri Mar 27, 2020 2:55 pm

It’s a good point about where “real” analog gain occurs and what actually happens to the image as a result.

Though you can correlate gain and iso, they are not the same thing.

If we’re being thorough 6400 should read 3200+6db.

Good Luck
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Tom Roper

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Re: Alister Chapman on Native ISO

PostFri Mar 27, 2020 4:29 pm

Thank you John and Howard.
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John Paines

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Re: Alister Chapman on Native ISO

PostFri Mar 27, 2020 4:57 pm

In the end, it comes down to a choice: if you expose middle-grey for iso 1000, you'll get lots of highlight headroom, but also lots of noise. If you expose for 100, you'll get a nice clean image, but at the expense of highlight headroom.

Iso 400 splits the difference, between noise and highlight headroom.
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John Griffin

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Re: Alister Chapman on Native ISO

PostSat Mar 28, 2020 7:45 am

‘ISO’ really should be abandoned as a concept for video. It’s a legacy term from film and was based around a mid tone measurement on a standard film response curve. Once you start to use log or flat or BM’s ISO settings it all goes out of the window as they all change the shape of the response curve and lift it in the midrange as a method for tonal compression. This is why the base ISO of log profiles is always higher than other standard profiles and not that the sensitivity of the sensor has changed. Gain is a different concept altogether and comes directly from electronic circuits that amplify signals (audio and video) and describes the changing of all the values up or down and not just the mid range. Like a volume control in audio ‘gain’ raises all the frequency’s equally so the tonal balance is maintained. Applying a log profile or a BM ‘ISO’ is to audio like raising the ‘midrange’ while keeping the ‘treble’ and ‘bass’ the same.
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