Changing ISO in Post Production: Is It Accurate?

Changing ISO in Post Production: Is It Accurate?

August 12, 2015

When working with RAW-recorded video in post-production is changing ISO in software the same as doing it in-camera? Click and find out.


Fixing It In Post—RAW Thought Experiments: Part 2

Note: This Insight assumes you’ve watched Part 1 of this series.

While I was setting up to figure out if the RAW Exposure slider in DaVinci Resolve 11 accurately reflects what happens when we manually adjust the iris at the camera, it was natural to set up the same experiment with differing ISOs. In other words, if the DP sets the wrong ISO on-set, are our RAW controls accurate representations of what would have happened if he set them properly in-camera? Or are our contrls merely cheating it?

Let’s start by defining ISO.

ISO is a measure of sensitivity to light

Over on Nikon’s website they introduce ISO in part:

With today’s digital cameras you can sometimes [set your ISO] as low as 50 or as high as 204,800. The number chosen has two important qualities associated with it. First, it sets the amount of light needed for a good exposure. The lower the number, the more light required . . .

The other important quality tied to ISO is the amount of noise in the image . . . In digital cameras, raising the ISO means a similar decrease in quality, with an increase in what’s called “noise.” It’s the digital equivalent of grain and results in a sort of “chunky” look to the image.

I’m asking: Can in-camera ISO changes can be accurately ‘fixed in post’? If so, how accurate are we?

In this case, I’m using DaVinci Resolve 11. And as we did in Part 1, I’m specifically testing the RAW controls for CinemaDNG as recorded by the Blackmagic Pocket Camera. While we can’t definitively extrapolate our results to other cameras, it’ll at least get us in the ballpark of what we might expect for other digital cameras.

Summarizing my thoughts and conclusions I have about this Insight:

  • CinemaDNG ISO on Resolve: ISO and Exposure are precisely the same.
  • DaVinci Resolve 12: I’ve tested this in Resolve 12, which adds an ISO option. Flipping ISO up or down is precisely the same as moving up or down whole number increments in the Exposure box. Nothing I show in this Insight has changed in Resolve 12, other than the convenient pull-down for quick ISO changes.
  • It’s very difficult to replicate Exposure and ISO changes on Log-recorded ProRes versus using the RAW exposure / ISO controls on DNG files. In other words, recording RAW does allow you to replicate in-camera options. As a test, here’s a Custom Curve I wrote to mimic a +3 ISO change on a ProRes (not RAW) BMDFilm-recorded file (in Resolve 12)… there’s no writing a curve like this based on a guess or intuition—RAW has its benefits:
Resolve 12 Custom Curve to Mimic a +3 Exposure Change for CinemaDNG RAW
Resolve 12 Custom Curve to Mimic a +3 Exposure Change for CinemaDNG RAW
  • FilmConvert 2.0 was an unexpected hero but remember – after watching this Insight – when we compare the FilmConvert exposure change of a ProRes file with a real ISO change recorded to ProRes? The difference may seem small on a grayscale—but on some images those two different curves will generate VERY different final images. FilmConvert gets close—but sometimes close isn’t close enough.
  • Not familiar with FilmConvert? We have Insights on an earlier version of the OpenFx plug-in FilmConvert.

– pat

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Homepage Forums Changing ISO in Post Production: Is It Accurate?

  • Very interesting. Seems we dont need the ISO control if its eactly 1 stop w CinemaDNG but Ill take it. Great insight! Looking forward to Pt 3. Very curious how much time in the raw tab benefits my overall grade.

  • Patrick Inhofer

    Thanks Jason!

  • Remco Hekker

    Hi Patrick, I LOVE these insights! I’ve been wondering about this myself from time to time.

    Have you found that the noise in the raw footage increases the same with the exposure slider as it does in real life?

    In my own experiments with the R12 ISO box it does look as if noise decreases/increases the same as in camera. I would love to hear what you think about it.

    I work a lot with the BMPCC and in my own test I have found that (when shooting raw and film) the camera sensor clips the highlights when shooting at 200 and 400 ISO. (That’s because 800 is the native ISO, I suppose) 800 ISO is the first setting in which the camera captures its full dynamic range.

  • johnmendez

    Hey Patrick, Curious as to why you didn’t use the log controls for ProRes log? Would the result have been the same as with the primary controls?

  • Patrick Inhofer

    It didn’t occur to me because the Log controls highly restrict each of the tonal range controls. ISO influences the entire range of the image. The same problems I had with the normal Primary wheels I would have had with the LOG wheels… just different.

    In the end, primary and log controls aren’t designed to emulate in-camera settings the way Camera RAW controls are supposed to.

  • Patrick Inhofer

    Interesting. Nice to know for the DPs out there. Thanks for sharing!

  • johnmendez

    Gotcha. Was hoping to settle on a ProRes workflow, but guess I’ll have to consider RAW to protect my exposures.

  • Drew Lahat

    Hi! I was curious if the grayscale ramps you were using were live camera footage (shooting a chart, or a wall)? Also, I suppose that in “here’s a Custom Curve I wrote to mimic a +3 ISO change” you meant “a +3 stops change” (or EV)?

  • Patrick Inhofer

    RE: Grayscale – It was a grayscale ramp generated in Resolve and then I shot it live in-camera on the FSI display.

    RE: ISO vs Stops – ISO is the parameter you change with the Camera Raw settings in Resolve to mimic the 3 Stop change I made on the lens.

  • jim Robinson

    This is an old insight – Does or did Resolve move the stops as it does in camera with ISO? With ISO raising the ISO keeps middle grey static and place more stops above middle grey and less below? Which is why things like clouds etc. have more detail at 800 ISO than at 100 ISO. And the same thing where shadows show more detail at 100 over 800 ISO at the same exposure. If you look at the BMD ISO chart, it shows the different ISO and the amount of stops above and the amount below.
    So wondering if in Resolve changing the ISO is the same as changing exposure slider if the middle grey point is changing?

    • Yea, that’s a good operational question. Been thinking about testing that out myself. Trying to get a good practical understanding of ISO and dual native sensors.

  • Pat Inhofer

    Jim – I haven’t tested on the latest cameras in R17. I’ll put it on my agenda.

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