footcandles to expose image on F3 - test results

[Dennis Hingsberg]

I wanted to know how some of my existing low power lighting would match up to the amazing low light sensitivity of the F3, and also since most film lighting manufacturers rate light output lux or footcandles I wanted a good way at determining what lighting I would need to buy/rent for future projects.

My test revealed 42 footcandles are needed to expose middle grey to 50 IRE @ f4.0 on the F3 using the following camera settings: 0db/800ISO, 1/48th shutter (or 180 degree), 709 Rec mode, no ND filters engaged.

I also wrote about the usefulness of ARRI's "performance" numbers used to calculate light output at any given distance. More information on that is in my crazy blog: Footcandles to expose image on F3 | hingsberg's blog

Let me know if you thing my non-scientific test was done properly, or if I missed something here.

[Dave Sperling]

Dennis,
The issue is not whether you are being scientific, but what basic parameters have you chosen for your camera setup, and how will the related 'speed' of the camera change as you change your shoot settings. Will you regularly be shooting at 24p, for instance, and always keeping the shutter at 1/48th? And more importantly is this regularly your gamma of choice for filming? I tend to throw the exposure numbers out the window since I will often use different gamma settings for different shoots, depending what I perceive to be the clients needs, and as I adjust the gammas I also need to adjust my exposure settings.
Also, when you say 85mm lens at f/4, is that an f/4, or a T/4? -- If it's a Cine lens it may actually be a T/4 -- if a stills lens at F/4, the true transmission may be closer to T/4.2 or T/4.5.

[Dennis Hingsberg]

Since I have in fact provided the parameters I used on the camera to reach my results, it would then be quite easy to calculate new values based on different shutter speeds or even gamma selections.

For example if you double your shutter speed from 1/48 to 1/96 then you will need 84 footcandles to maintain exposure at f4. Or if you keep your light at 42 fc, then you would lower your aperture from f4 to f2.8.

As for cine gammas, same thing. If in Rec709 middle grey is 50 IRE and in Cine 1 it is 40 IRE then you know you can stop down to f5.6, or lower your light to 21 fc for put your middle grey at 40 IRE. With SLOG, middle grey goes to 35 IRE, and so on and so on.

I put this quick refrence guide together showing all the different gamma curves which show middle grey points and where the highlights are clipped: http://www.starcentral.ca/Sony%20PMW.pdf

Overall my goal was to establish the required amount of light for the F3 and put it in relative perspective to the various lighting equipment that exists and the specs manufacturers use to rate the output power of their equipment.

I hope some people find this as insightful as I did. (I just hope I did it right)

[Brian Lai]

Dennis I'm late at catching your post but am surprised how few people responded. I can only guess that our fellow cinematographers are not interested in quantifying the amount of light in its barest form (foot candles) needed for any given exposure, because it is a measurement of incident light, vs. measurements of reflective light readings in ire's on a wave form monitor, being the platform they may have grown to judge exposure with. I'm old school film BG and lived with 100 FC @ 100 ASA @ T 2.8 & 1/50 shutter renders a gray card correctly at mid printer lights, as one can validate on the back of the old Spectra meters when foot candles was a common unit for quantifying light on a controlled set. I find your test very relevant and am about to conduct a similar test with my new S Log upgrade, trying to find an optimal ISO setting. Gamma settings doesn't come into play with S Log as I also see Dave's point in different Gamma settings in Rec709 will affect your outcomes like using different film stocks with different ISO's.