So it is the final day of shooting a short film. Everything was carefully lit, our rented Sony FS7 was standing by to record us trashing the set (which we had one try at) and everything was just about perfect to shoot... Except that it wasn't perfect, because it turns out that you need to light in a whole different way for high speed filming. I of course knew this (I didn't at all) and had planned an alternate lighting plot for the scene (nope), and so everything turned out fine (It did, but mostly thanks to the internet).
I had lit everything with lights smaller than 1k, largely Because I had a lot of them, and also because circuit power restrictions mean that you can only draw so much power without constantly tripping fuses. Now as it happens, because of the 50hz AC electrical cycle, the filament in these smaller lights has enough time to cool in between cycles, which is normally invisible when shooting at 24/25fps, but once you get over about 100 frames a second (we were shooting 150) the heating and cooling is visible as horrifying banding across the image. The banding - something I've only ever seen when you shoot off speed with LED or fluro lighting - was unexpected and completely bewildering because one of the lovely things about incandescent light is that you can shoot off speed without problems*.
*there are some problems
Fortunately I shot test footage, and saw the banding, and freaked out, and checked the internet which in true internet style provided me with the solution:
Bigger lights. 1k and above, but really the bigger the better. It turns out that the filiment in the larger lights retains enough residual heat between cycles that it doesn't visibly cool (and dim), therefore providing a constant light. I was using 1k and below to light everything, and so with the close of the final day of shooting looming, I raced to strip out all the small lights and replace them with my 1k and 2k lights. It all worked out ok in the end - maybe a bit less pretty and controlled, but we did ok for a last minute re-lighting scramble.
Long story short, if ever you want to shoot in slow motion under tungston lighting, use big lights.
I'm busy working on my blog posts. Watch this space!