How expose a low light scene with a pyrotechnic effect ?

Maeva Merour · May 5, 2018

Hello :)

I wondered how a scene supposed remaining in low light had to be exposed to anticipate effects of pyrotechnics without the effect being (too) clipped? Do you assume it or do you try to decrease the contrast ratio at best ?

So far in SDR, the clipped effect seemed more or less accepted as is but I have the impression that with the HDR and its latitude capabilities, this is no longer the case. Yet an explosion is too bright to not be clipped. It's like filming the sun, no?
I would like to understand :lol:

How do you do it?

Thank you !

Mark Kenfield · May 5, 2018

It's all about balancing your exposure. If you have a low-light scene, and your pyrotechnics are 5, 6 or 7+ stops brighter than the ambient light level, then you're going to have them clip out.

You need to lift your ambient light level (or at least the level of your key light) to bring it within say 4 stops of your pyrotechnics, in order to hold the detail in everything.

Chris Steel · May 5, 2018

The brightest parts of fire are usually the same intensity as north facing blue sky (84000 LUX or 15EV)

It will depend on what material is burned. Magnesium will have a much brighter burn but it will be pure white so being over exposed is probably fine.

If you set the camera so that 15EV is 6 stops over middle grey and light the scene from there. Make it look as bright or dark as you like.

For reference 15EV is about T72 at 24fps 800ISO or 7 1/2 stops over T5.6

Bruce Greene · May 5, 2018

I was faced with this a few years ago and finally decided that to avoid clipping the fire would require way too much lighting. And I was using the practical emergency flashers in the scene, and they were not very bright. So I shot at ISO 800 at f2 and let it clip.

You can see the result here:

45 seconds into the clip.

I think I made the correct choice :)

At NAB last month I saw a bit of HDR demos with fire in the shot. Even in HDR they were clipped, and it wasn't a very good result, in my opinion, for HDR. I think the solution here is to really dial back the brightest HDR highlights when they are not small sources or it's very distracting, clipped or not clipped.