URSA mini 4.6K: ISO

Hello,

I come from DSLR world and I'm a little bit confused about Ursa ISO.

When you change ISO on a dslr the camera works like if you "changed the film". If you step from 200 to 400, for example, the white and black points of the dynamic range booth slide 1 stop. Independently of Jpeg, RAW or H264 format. And after shooting, even in Camera RAW, you can't change ISO value.

On the contrary, it seems that Ursa has only one "true" ISO setting and, even changing ISO, clipping point doesn't variate. Like if Ursa sensor always works at same ISO and then digitally calculate exposure variations.
In fact if you are shooting raw, DaVinci allows you to change ISO value as this was a simple metadata.

This make me think that it would be batter to shoot 800 ISO, even in bright sun, and compensate with ND filters, rather then shooting lower ISO. Am I wrong?

Can someone help me to clarify this?
thank you!

Nobody can help me?
I hope this is not because of my bad english!

Post this in cinematography section.
Video cameras have native (best ISO), so there maybe some point using naive ISO and use ND filters. I'm not 100% sure how it works with BM camera. Some info on the net suggest that you want to stick with native ISO as much as you can.

[Tony Rivera]

Moved to the correct area rather than having a double post.

Alessandro, BMD cameras are ISO invariant. That means the ISO is just a metadata setting in RAW. Highlight clipping cannot be shifted by changing ISO. In ProRes ISO settings just determine how the film log curve is shaped.

Base ISO of URSA Mini 4.6k is ISO 800. I leave the setting there most of the time, but in bright sunlight ISO 400 might give you an easier start for grading. In lowlight situations ISO 1600 can be the best starting point.

Cameras like Canon DSLRs are not ISO invariant. They make use of analog gain depending on the ISO setting.

I think the same can be said for ARRI, RED, Sony, and Canon Cinema Cameras. There is a level where I do miss film with regards to the different speed of a film stock creating choices.

Most of the time I shoot 800 ISO with ND stacked in front of the lens when shooting outdoors. Occasionally I use 400 ISO.

Alessandro, it takes a lot of experimentation to get a feel for ISO changes when shooting ProRes. Further to Robert's remarks, although it may seem contradictory coming from a film background, you may even get the best results shooting ISO 1600 on a very bright scene such as the beach or in snow whereas in a film world, you would shoot ISO 200 or lower. At ISO 1600, more stops above middle grey are allocated to highlights which could predominate the beach or snow scene. At ISO 200 or 400, more stops are allocated to shadows. You learn to choose ISO based on the scene and where you want to see the most or predominant detail. Until you play around with it, shooting ISO 800 is a safe bet.


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Alessandro, this might be an interesting read for you:
http://www.dslrbodies.com/cameras/the-d ... iance.html

For comparison a test shot done with an old Canon 400D (Rebel Ti) with a 2006 DSLR sensor

Lens: Sigma 18-35 F1.8
Settings: Shutter 1/200, F1.8
Shot in RAW: once @ ISO100 and once @ ISO1600

ISO100 out of the box:


ISO100 +5 stops:


ISO1600 +1 stop:

And for comparison the URSA Mini 4.6k:
viewtopic.php?f=2&t=56353

So what is happening here?

Let's have a look how old sensors work:

ISO variant sensors
1. Sensor detects photons and converts them into electrons that collect in a semiconductor "bucket."
2. Charge accumulation - as more light enters, more electrons accumulate into the bucket. The accumulated charge is output as the voltage signal.
3. ISO amplification - voltage signal is then amplified accordingly to the ISO setting before it is fed to the camera circuitry
4. ADC analog digital converter - the amplified analog signal is converted into digital values
5. RAW file is created

The order of signal processing was important because of the high sensor read noise, ADC noise and low dynamic range. ADCs had been in a separate circuitry.

ISO invariant sensors
With further advancements in sensor and ADC technology it was possible to reduce the readout noise significantly, to increase the quantum efficiency and the dynamic range. And ADCs could be integrated directly into the sensor. That made it possible to skip the step of voltage signal amplification, because of the increased signal-to-noise ratio.

1. Sensor detects photons and converts them into electrons that collect in a semiconductor "bucket."
2. Charge accumulation - as more light enters, more electrons accumulate into the bucket. The accumulated charge is output as the voltage signal.
3. ADC analog digital converter - the amplified analog signal is converted into digital values
4. ISO amplification - a meta data value is set accordingly to the ISO setting
5. RAW file is created

Some sensors now even use a hybrid design like the Panasonic Varicam which has two native ISO settings (dual gain).

Thank you very much!
All your answers are very useful.
This is something really new for me and I was very surprised when I realized that highlights clipping limit was the same in ISO200 as in ISO1600.
It's something a little bit astonishing, but now it is quite clear.

In particular thank you Robert, you have been really exhaustive. I will try your curves!

There are some informative videos about how ISO works in ISO invariant sensors in Red's web site.

So what you're saying is the converter's noise floor is no longer an issue and so it's cleaner to boost the signal digitally post converter. But why does​ Arri still use analog gain?

So what you're saying is the converter's noise floor is no longer an issue and so it's cleaner to boost the signal digitally post converter. But why does​ Arri still use analog gain?

I am not aware that ARRI uses analog gain. The are using a dual-gain readout of the sensor but afaik no analog gain stage. The ARRIs act the same like the BMDs.

I saw this graphic when looking into the Cine EI mode of the FS7 and it has helped me visualize how the ISO works for these cameras. https://i1.wp.com/www.xdcam-user.com/wp ... .jpg?w=720

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