Foot-candles

[Jacob Steiner]

Hi guys,

I was talking to a DIT at school, and we were talking about the foot-candles system and how he believes its still necessary to shoot proper images. I stated that it's not necessary because our waveform monitors tell us how the cameras need to be exposed and the light meters are pretty much complimentary to the cameras themselves in the sense of setting contrast ratios with incident readings and measuring every inch of the frame with the 1-degree spot. He basically stated that it's a method to expose multiple cameras on set evenly, but I feel it's just as archaic as film itself. To me, light is just light in the way that it falls off in a logarithmic fashion and never changes; the texture, quality and CRI are whole different matter, but at the end of the day, light is just light, and I believe that foot-candles is no longer necessary to shoot a film. But, I'm new to the game, so what are your thoughts on the matter? Am I a wrong, stupid idiot of a person, or are we on the same page?

Regard,
Jacob Steiner

 

[Karim D. Ghantous]

I am not an expert, but I do have some time to kill. From what I have gathered, it's not strictly necessary to understand foot-candles. I think most of us think in stops. Foot-candles help explain why you want a minimum amount of light in a scene. This podcast episode of The Wandering DP explains it fairly simply:

http://wanderingdp.com/podcast/the-wandering-dp-podcast-episode-46-footcandles/

Here I am quoting John Schwartzman yet again:

Nothing else was going to capture the 18,000 footcandles in the highlights of the sky, and the 20 footcandles in the shadows of the jungle, in the same shot.

http://motion.kodak.com/kodakgcg/us/en/motion/blog/blog_post?contentid=4294991961

Note that he could have expressed the lighting ratio in stops. But stops don't measure light, strictly speaking.

In Europe they use nits. Quite frankly I don't think that you need to bring in modern metrics for photography. For example, I think focus pullers are happy to measure focal length in mm but distance in feet. Note that film stock width is measured in mm but length is measured in feet as well. Funny old world!

 

[Martin Stevens]

Using a light meter to read foot candles on set or location can be very useful if you are
lighting the scene BEFORE a camera of a waveform monitor etc. arrives.

I use a light meter to measure and set the output of my light fixtures even when a camera is already on set.

 

[Anton Shavlik]

Light is (typically) linear. If you have one light source and expose for f4, scrim so that the light is half as bright and open to f2.8, it should look identical. That is, the key will look the same, the fall off, the bounce, etc.

I think it's good to have at least one footcandle value in your head, like 14 foot candles is a good exposure for f2.0 400iso 1/48. I try to remember whatever is the darkest exposure I would ever go with my camera / lenses. With that in your head with the fact that light is linear in the most absolutely mundane boring way you can plan lighting way more effectively.

Like a fresnel will not tell you it's a good exposure for f2 at 5 feet from the bulb, but it can specify its foot candles at that point.

Hope that all made sense.

edit: I think I only really think about foot candles when I know, due to the set or whatever, that I need my key light to be 15 feet away, and I wonder if I can get by with a 1k vs a 2k etc.

 

[Adrian Jebef]

Foot-candles and light meters are still relevant when shooting digitally but they are imperfect. These tools are best used when pre-visualizing the scene you are lighting. See Ansel Adams "The Negative" and the Zone System. A waveform or better yet a digital spot meter as in pixel analysis will give you the exact code value your camera is recording. A Leader is a great through pricey tool. The Odyssey 7 has excellent digital exposure tools for a very reasonable price-point. A waveform or decent false color read-out is much more precise than a light meter when actually measuring your exposure and contrast ratios. That being said not all digital cameras output a proper representation of the recorded image. Red cams and the Sony F55 can be misleading at times. The Alexa and F65 line up just about perfectly.

 

[Satsuki Murashige]

Footcandles are still useful as an objective measurement of light quantity. As you increase the number of variables (f/stops, ISO, filters, OLPFs) that can affect exposure, it is often helpful to have at least one known constant in the equation.

Also, there are times when it may be more efficient to use a meter instead of a waveform monitor. For example, when pre-lighting a set using multiple hard units at a distance and needing to match them in intensity. In order to do the same thing with a waveform you need the camera and lens in the right place, stand-ins on their marks, and for a lot of the crew to halt work temporarily. So using footcandles to rough-in lighting can save you a lot of time.

 

[Marc Wielage]

There's some good answers above. I would add that you need good key-to-fill ratio regardless of whether shooting on film or digital. Without that, you're going to risk blowing out the highlights or losing detail in the shadows. A light meter can help you avoid that problem. In effect, a waveform monitor (and a histogram) are kind of like light meters, but not exactly. Understanding how many footcandles are being used to light different parts of the set will allow you to light more evenly and run into fewer problems.

There are many excellent books on the subject that go into this in great detail, including the American Cinematographer Guide, David Mullen's book Cinematography, and the digital theory is covered in David Stump's Digital Cinematography. Just because the tools change doesn't mean the methods used to create the results are all that different.

The greatest advice I can give you is: when in doubt, shoot tests. Almost all the exposure-related disasters I've seen happened because people blithely went into a project assuming if they could see it on a monitor on set, everything would be fine. The reality is that monitors will lie to you, particularly about noise and sharpness, and things can go south very quickly if the lighting and exposure isn't nailed down.

 

[Sabovic Adis]

Don't You ever dismiss measuring unit until You've found a better one! Not for Your life!!!
If You do You'll be always "guilty" and "one to blame" to Your customer, just because You'll be standing "naked" in front of him.

And, light is never just light. Light is always "some quantity of light" (brightness) and "some quality of light" (color), unless we're talking Poetry here. :biggrin:
Darkness's different - with darkness You can go without light-meter. :rofl:
:thumbsup: