Author Topic: Parametric ISO  (Read 453 times)

ShutterRecoil

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Parametric ISO
« on: August 15, 2022, 06:13:16 PM »
Hi,

I am trying to shoot a photo with long exposure 30 seconds at daylight.
The model uses the light source to draw something up in the air.
Exposure finishes the shot with rear-flash.

Flash power is finite and if background is very distant and bright (e.g. mountings) then
catching background at the moment of the rear flash shines, my camera should increase ISO, IMHO.

That's why I would like to see the parametric ISO feature.
Use case:
Shutter speed: 15 seconds
Flash mode: rear
ISO(t): if t < 14.9s then 100 else 8000


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names_are_hard

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Re: Parametric ISO
« Reply #1 on: August 16, 2022, 03:01:22 AM »
Are you saying you want different portions of the same exposure to be captured at different ISO?  If so, this is not possible - the sensor accumulates light / charge with nothing like ISO being involved.  Only when the sensor is read out are amps used to select ISO - and at that point all the light for the exposure has already been collected.

tupp

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Re: Parametric ISO
« Reply #2 on: August 16, 2022, 11:53:39 AM »
I am trying to shoot a photo with long exposure 30 seconds at daylight.
The model uses the light source to draw something up in the air.
Exposure finishes the shot with rear-flash.

Flash power is finite and if background is very distant and bright (e.g. mountings) then
catching background at the moment of the rear flash shines, my camera should increase ISO, IMHO.
It sounds like you are trying to balance the exposure for three different elements in a single frame:
  • the flash exposure for the foreground and for the model;
  • the exposure of the model's handheld light source;
  • and the exposure of the background.
If so, why does the ISO/gain need to change during the exposure?  Just set the desired exposure for the background, based on a 30-second shutter speed.  Then, set the flash to give the proper exposure for the foreground.  Then, adjust the output of the model's handheld light source so that its brightness balances with the foreground and background.

That's why I would like to see the parametric ISO feature.
Use case:
Shutter speed: 15 seconds
Flash mode: rear
ISO(t): if t < 14.9s then 100 else 8000
There could be a few ways to "effectively" change the ISO gain during exposure.  Not sure if "parametric" is the proper term for such a method.

The simplest way to increase the "gain" during such a long exposure would be to set the camera to the higher gain/ISO and then simply hold an ND filter in front of the lens for the first half of the exposure, and then remove the ND filter for the last half of the exposure.

Another possible method would be to use the Dual ISO feature during a 30-second video with a 360-degree shutter speed (i.e. 1/24th of a second at 24fps).  Then, "exposure blend" all of the video frames into a single still photograph, but use the lower ISO scan lines from frames of the first 15 seconds and use the higher ISO scan lines from the frames of the last 15 seconds.