Footcandles to expose image on F3

Since I left behind the world of shooting with 35mm adapters where I was working with effective ISO’s from anywhere between 50 and 160 to a bright new world of 800 ISO with the F3, lately I’ve been obsessing over different light fixtures and their output performance combined with what’s exactly needed to expose a “good” image with the F3.

I did some reading of course and really liked the way ARRI was rating the “performance” of their lights. Most manufacturers rate their light output in lux or footcandles at set distances, but since light output is inversely proportional to the square of the distance from the point source ARRI additionally provides a “performance” number for spot, medium and flood and this let’s you do your own math based on distance to calculate out your footcandle power.

I like this, because I can make a quick reference guide comparing “performance” on everything from Kino Diva Lites, Lowel lights, ARRI Tungsten, ARRI HMI, Light Panels… well you get it – practically anything!

The formula is listed on every photometric data page on ARRI’s site. Basically, footcandles = performance / distance X distance. So for example, an ARRI 200w HMI has a medium focus performance number of 20,000 (there are separate performance numbers for spot and flood). To figure out how many footcandles I’d have at 20 feet just divide 20,000 by 20ft x 20ft (or 20ft squared basically). You get 50 footcandles. While this is an exciting way to compare lights and also figure out the approximate illumination you will have at set distances, what I really wanted to know now is how it would translate into shooting with the F3 and its crazy noiseless ISO capabilities?

My tests revealed that I would need only 42 footcandles 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. To do the test I used a 42″x32″ softbox to illuminate a 18% grey card and filled the complete frame using a 85mm lens set to f4.0 on the F3. I moved the light further/closer away from the grey card until the marker function on the F3 read exactly 50%. I then took a meter read using a Sekonic 508 set to footcandle mode and measured out 7.5EV which according to the look up table in the Sekonic manual is 42 footcandles.

For those curious the softbox had a 500w tungsten bulb installed with two diffusion panels and was 134cm from front diffusion panel to grey card. By using knowing the footcandles and distance, I am able to come up with the “performance” of my own softbox and see how it fits in with lights by other manufacturers.

Here’s the quick reference I started putting together on different light “performance” numbers:

Diva Lite 400                  2808
Diva Lite 200                  1404
Arri 1k (med focus)        41,800
Arri 575w HMI (med)    50,000
Arri 200w HMI (med)   20,000
Lowel Rifa88 1000w      5,600
48″x32″ 500w softbox   850

Let me know if you think the test I conducted is accurate or not, I’d be interested to hear what people think.

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