March 20, 2005

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Example 1: Kodak 7279 Film

This scenario starts with the assumption that we will be shooting with Kodak 7279 16mm emulsion rated at an exposure index of 500ASA and designed for use with tungsten light sources.  Kodak 7279 is highly favored by independent filmmakers because it delivers fine detail and minimal grain structure while responding well to smaller, less expensive light instruments on the set.

We are also presuming a standard exposure time of 1/48th second.

So, we have locked down two variables in our simplified lighting scenario:
exposure index and exposure time.

Here is an illumination table for all of Kodak's film stocks:

From the above table, we can see the specific illumination, in foot-candles, required to expose various film-stocks rated at exposure indexes from 12 to 2000ASA.  Let us narrow in on Kodak 7279, rated at 500ASA.

ASA/Aperture fl1.4 fl2 fl2.8 fl4 fl5.6 fl8 fl11 500 5 10 20 40 80 160 320

Kodak 7279 requires 40 foot-candles to properly expose a medium-grey object (the 18% grey card) with an 4.0 f-stop.  (I'm not making any creative choices, at this point, about how much contrast I want in the scene or how bright or moody I want to light it.  All I want is to know how much light I need to expose a medium-grey object at the center of the 500ASA emulsion.)

Remember the foot-candle relationship between recording medium and light source:

light source table light distance subject illumination

foot-candles 40fc <-->

recording medium table aperture - f4.0 exposure time - 1/48 sec exposure index - 500ASA

Having locked down exposure index, exposure time, and aperture,
we now know we need 40 foot-candles to expose our medium grey.

We are now ready to research the illumination tables of light instruments in order to discover which light sources will properly expose our 18% grey card.  Let's examine one possible lighting instrument next.

Mole-Richardson 1,000 Watt Baby Solarspot (#407) The Baby is a 1kW fresnel fixture.  It's beam intensity and diameter are modified with a sliding light and reflector focus assembly.  It can also accept a variety of globes ranging from 500watts to 1000watts.  As such, it can deliver a wide range of foot-candle intensities depending on distance from the subject and focus setting. Learn more about the Mole Baby. Here is the Baby's illumination table for a 1000 watt bulb:

*Diameter is where intensity is approximately 50% of beam center.
from http://extranet.mole.com/store/images/backpage/407-L.gif

The above table provides foot-candle output at various distances and for the extremes of flood and spot focus settings.  Note how the foot-candle output drops drastically when the instrument is fully flooded rather than spotted.  Here's another way to represent the above table:

the foot-candle table in graphic form also depicting beam angles & diameters

The Baby, at 20 feet from the subject and fully flooded, will deliver 45 foot-candles of illumination to the subject with a beam diameter of about 17 feet.  So, we may assume the 40 foot-candle sweet spot needed by our 500ASA Kodak film is somewhere around 22 feet.

At full spot and 20 feet, the Baby delivers much more illumination than necessary to expose a medium-grey object on the Kodak 7279 properly (and in a spot only 4.5 feet in diameter.)  We could move the Baby to approximately 50 feet (not noted on the chart) to hit the 40 foot-candle sweet spot.

The Baby provides a wide variety of light distances (from 20' to 50') and beam diameters (from 3' to 43') that will illuminate a subject at 40 foot-candles.  With the addition of lower-wattage bulbs and/or light-reducing media like scrims and gels, the light can provide a proper exposure at closer distances as well.

Once we compare the Baby's range of abilities with the size of the set and subject we wish to illuminate, we may find it is an ideal light source for our chosen medium of 500ASA Kodak 7279 film emulsion.

Working in the Real World

These comparisons of illumination tables are useful for light sources directly illuminating a subject.  In real-life scenarios, light sources are diffused, bounced, gelled, cut, scrimmed, netted, and all manner of manipulated.  Every manipulation reduces the amount of light striking the subject, sometimes quite drastically.  Practice and experience will inform you how these creative tools alter the illumination.

forward to Example 2: Canon XL-2

Take a look at the following light instruments.  Each page includes illumination tables to assist you in choosing the right light for your project.

click on a light to learn more about the instrument