misleading base ISO for bmcc6k?

I've just read an interesting exchange on YouTube regarding base ISO settings for the Blackmagic cinema 6k. here is what was being said:

"Not sure. The only ISO’s that affect overall exposure (the whitest whites and the blackest blacks) are 100 and 1250.

So if you use ISO 400, you’ve basically got an overall exposure equivalent of ISO 100 with a two stop boost to gamma in camera.

To be clear ISO 400 doesn’t give you two additional stops of overall exposure vs ISO 100. So you cant change other aspects of the exposure triangle without affecting the overall exposure.

The only reason I can think to use ISO 400 etc. is that the gamma boost will help see more shadow details when monitoring in camera.

BlackMagic marking materials basically say this in a way, albeit they explain it as how different ISO’s distribute stops of dynamic range in the shadows and highlights.

Sorry for the long explanation!

All of the above is why people think the BMCC6K has poor low light. They are using ISO 3200 not realising the exposure is ISO 1250 and they actually need more light in the scene and/or wider aperture to get a good overall exposure.

So their footage will have around 2 stops of boosted shadows. If they take it into post and boost the shadows again by another couple of stops they could be left with around 4/5 stops of shadow recovery.

This is a lot for any modern camera and will result in a lot of noise and potentially banding.

I have never had a problem with noise from thE BMCC6K when exposing based on 100/1250 and then carrying out moderate post adjustments (up to around 3.5 stops of shadow recovery)."

video in reference was this:

would be great to hear thoughts on this as I have a night time shoot coming up and plan to do some testing too but I know sometimes there are technical gotchas that may only show up in certain scenarios.

Thank you

This video looks like cinema without even trying.

I don't have any valuable input here, looking forward to some experts chiming in. What he's saying makes sense to me. However, I have also realized that in bright situations is better to protect the highlights and in lowlight situations it's better to protect the shadows. Simple, but seems counterintuitive when you are hearing all this ETTR stuff on YouTube.

Protecting the highlights is a myth.

If you shoot something that is 6 stops above middle gray and blown out, it's not going to suddenly not blown out at ISO 400.

The sensor of the Pocket 6K has the lowest noise at ISO 100 and ISO 1250.

The only good thing about ISO 400 is that it matched right at the native ISO which means ISO will be right at middle gray.

Protecting the highlights is a myth.

If you shoot something that is 6 stops above middle gray and blown out, it's not going to suddenly not blown out at ISO 400.

I think this is what is meant by protecting highlights. I guess you were saying that you can never ignore highlights, whether in a bright or dark setting?

So are we really saying that we get a cleaner image at 100/1250? Really? This is kind of massive for the 100’s of people shooting at 400/3200 believing these two base ISO’s will provide the cleanest image.

Exposing for 400 and 3200 is regarded by the manufacturer as the best compromise between noise and dynamic range, for each of the two circuits (1250+ for low light scenes). That's all it means.

For braw, when exposing for middle-grey, you can set iso 1000 and get the illusion of "protecting" the highlights (i.e., adding stops). All you're really doing is increasing the brightness of the screen so you can stop down the lens accordingly (and "add" stops). You'll also get plenty of noise when you do it, because the shot is under-exposed. It's no different than leaving the camera at 400 and under-exposing the shot at the same f-stop you used for 1000. You'll capture exactly the same image at the same exposure.

For any value between 100 and 1000, the camera clips at the same f-stop. And as long as the f-stop remains constant, it doesn't matter whether the camera is set to 100 or 1000. All that changes is the screen brightness. You capture exactly the same image. It works the same way on the higher circuit, for low-light scenes.

This matter has been discussed many times before. The technical term is "ISO invariant". Look it up....

Protecting the highlights is a myth.

If you shoot something that is 6 stops above middle gray and blown out, it's not going to suddenly not blown out at ISO 400.

I think this is what is meant by protecting highlights. I guess you were saying that you can never ignore highlights, whether in a bright or dark setting?

If it's blown out at ISO 100, it will be blown out at ISO 400. People think If the sensor has a dynamic range of 6 stops, moving the ISO down to 400 from 100 ISO, it's suddenly going to add to more stops of dynamic range. Clipping is still clipping.
If you want to protect highlights? Use an ND Filter.

So are we really saying that we get a cleaner image at 100/1250? Really? This is kind of massive for the 100’s of people shooting at 400/3200 believing these two base ISO’s will provide the cleanest image.

The noise floor is lowest at 100/1250, but the middle gray point is exactly at 0 at ISO 400 / 3200.

A image at 100 with proper lighting will produce a cleaner image than one exposed for 400, since the noise floor is lower.

Hi John. Totally agree that what you say is true, but I think it may mislead less experienced people.

I think most less experienced people are coming from still cameras where one controls exposure with shutter speed as well as F stop and ISO. So just talking about F stop will probably puzzle them.

The main thing I think you’re saying is that how much light coming in is the main arbiter of noise. So if you reduce the light - either by closing down the F stop or increasing shutter speed - you’ll get more noise. Ditto with putting ND in front of the lens.

So if you raise the ISO and then close down the F stop you may get what looks like the right exposure but you WILL get more noise than if you didn’t close down the F stop. Upping the ISO doesn’t get you better noise control.

One thing I’m not absolutely sure about is the crossover from the lower to upper “native ISO.” I THINK that shooting at 1000 ISO is more noisy that shooting at 1250 ISO. This because 1000 is the top (and thus most noisy) of the lower range, while 1250 is the bottom (and thus least noisy) of the upper range. Do I have that right?

Yes, on the BMPCC 6K exposing for 1000 is much noisier than exposing for 1250, when the camera is set to those values. 1250 is the first value of the upper circuit, optimized for low light. You pay in reduced dynamic range using 1250+, but it's usually a good trade, because it's much cleaner than an underexposed shot on the lower circuit, at the same f-stop.