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Using Magic Lantern => General Help Q&A => Topic started by: Tyronetheterrible on November 24, 2018, 08:18:01 AM

Title: Dual-ISO for video for a low-lit scene
Post by: Tyronetheterrible on November 24, 2018, 08:18:01 AM
Suppose one were to shoot video with Dual-ISO in a dimly lit area and capture as much information and dynamic range as possible.

Let's say the lighting conditions are so low, that in accordance with exposing to the right for the lower number, you end up setting it to your maximum usable ISO (Let's go with 6400 for example, as that appears to be the upper usable limit for the 6D before significant degradation). On the viewfinder it looks fine enough aesthetically/artistically speaking, however, on the histogram the highlights barely reach the middle.
 
What would the second ISO value need to be then? Since you've already maxed out your first ISO trying to expose to the right?

Or would this simply be a case where dual ISO is not ideal in this scenario, and is best just to stick to 6400 as a single ISO?
Title: Re: Dual-ISO for video for a low-lit scene
Post by: Kharak on November 24, 2018, 05:33:20 PM
I think this is the case where Dual-ISO is not usable.

If your highlights dont reach halfway on the histogram, there is no need for a recovery ISO of darkness. Also you would be degrading the image by interpolating 6400 iso with say 200 ISO (where iso 200 only captures noise.

Dual-ISO is ideally made for High Dynamic Range scenes, specifically where the highlights/shadows exceed the histogram. Like shooting indoors and there is a window and you wanna keep everything within range of clipping.

Does your scene have any specular highlight? Lightbulps? That you want the highlights be retained of?
Title: Re: Dual-ISO for video for a low-lit scene
Post by: IDA_ML on November 24, 2018, 06:31:05 PM
Kharak is asolutely right.  DualISO makes sense only if you have a high-contrast scene whose dynamic range goes beyound the dynamic range of your sensor.  No matter how dimly lit your scene is, always expose to the right to achieve maximum signal-to-noise ratio in the darkest areas of your scene.  When you expose to the right, make sure you do not blow up the highlights.  Once you blow them up, they are gone.  There is no way that you can recover detail in there.  I always use zebras that are quite useful to protect them.  ETTR gives you the lower Dual ISO number.  The higher one, you chose to be no more that 3 stops higher than the lower number (e. g. 100/800, 200/1600, 400/3200, 200/400, etc.).  Anything higher than that, (say 200/6400), will produce ugly artefacts.  That is why ML warns you to use less aggressive settings. 

Every phenomenon has its limits governed by the laws of physics.  Dual ISO is no exception.  It can increase the dynamic range of your 5D3 by up to 3 stops (a total of 14 stops) with very pleasing results.  If you need 17 stops, you better buy a SONY.
Title: Re: Dual-ISO for video for a low-lit scene
Post by: Tyronetheterrible on November 24, 2018, 07:52:31 PM
@Kharak
@IDA_ML

This makes perfect sense. The scene I am shooting takes place in the interior of a car at night, very minimal diffused light, and no specular highlights or lightbulbs that I need to retain the highlights of.

Thank you so much, this was a massive help!
Title: Re: Dual-ISO for video for a low-lit scene
Post by: ibrahim on January 14, 2019, 05:37:23 AM
Could you guys explain how autoETTR and dual iso is used in both the nightly build and in the experimental (and which build). I have had hard time to use these in the right manner along with the ordinary ISO. 
I want to shoot in 24p and continuous. I use both 5d3 1.1.3 and 1.2.3.
 
Title: Re: Dual-ISO for video for a low-lit scene
Post by: Walter Schulz on January 14, 2019, 06:45:57 PM
Do you have browsed the links in Camera Help -> ETTR.mo section (https://wiki.magiclantern.fm/camera_help?do=#ettrmo)?
Title: Re: Dual-ISO for video for a low-lit scene
Post by: ibrahim on January 15, 2019, 10:28:34 AM
Thanks Walker. I'll study the content surrounding autoETTR and dual iso via the provided link.