March 20, 2005

Choosing the Right Lights for Your Set
Using Illumination Tables

back to Example 1: Kodak 7279 film

Example 1: Canon XL-2

The Canon XL-2, successor to the incredibly popular XL-1 and Xl-1s cameras, adds native wide-screen acquisition and increased control over color and exposure curves.  It is highly favored by independent filmmakers because it has an interchangeable lens system and very clean and saturated picture for a camera in its price range.

Canon (as do most manufacturer's of camcorders) do not provide exposure tables for their cameras.  They are considered to be irrelevant to video acquisition and, indeed, exposure indexes can be drastically altered by various menu settings within the camera, including pedestal, setup, gamma curves, and gain.  However, as a cinematographer, I want to know how these cameras respond to light.  With a little extra effort on my part, I am able to test a Canon XL-2's median light response at the default menu settings by pointing it at a medium-grey card and comparing it to foot-candle/f-stop readings on a Minolta Spot-F spot meter.  Exposure indexes are certain to vary with changes in menu settings and white balance, but the following table serves as a base-line reference for the XL-2.

Here is my table of the response characteristics of the Canon XL-2:

From the above table, we can see the specific illumination, in foot-candles, required to expose a medium-grey as 50% video level on the XL-2 at three different gain settings.  I'm really interested in the best color and lowest noise, so I'm going to narrow in on the -3dB gain setting yielding a 250ASA exposure index.

We are also presuming a standard shutter speed of 1/60th second for progressive video acquisition.

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

We can see that, at 250ASA, we can expose the camera with f-stops ranging from 1.4 to 11 using a range of illumination from 10 to 640 foot-candles.  Now, let's isolate a specific, desired f-stop of 2.8.
 

Gain/ASA    Aperture	fl1.4	fl2	fl2.8	fl4	fl5.6	fl8	fl11
-3dB/250	10	20	40	80	160	320	640

The XL-2 requires 40 foot-candles to expose a medium-grey with a 4.0 f-stop.  (Again, all I want is to know how much light I need to expose an 18% grey card at the center of the 500ASA emulsion.)

Going back to our foot-candle relationship chart:

light source table light distance subject illumination

foot-candles 40fc <-->

recording medium table aperture - f4.0 exposure time - 1/60 sec exposure index - 250ASA

Having locked down exposure index, exposure time, and aperture,
we now know we need 10 foot-candles to expose our medium grey.

Now we know how many foot-candles we need to expose our medium.  Let's examine a possible lighting instrument next.

Kino Flo 4Bank The Kino Flo 4Bank fluorescent head delivers a broad, diffuse light that is very flattering to talent.  It is valued for it's small size, and low power consumption and operating temperature.  The 4Bank's beam pattern may be manipulated with a snap-on louver (to limit beam spread) and integrated barn doors on the long side of the head.  The head accepts daylight and tungsten balanced bulbs as well as specialty bulbs for illuminating blue and green screen set pieces.  The tubes may be individually powered providing greater control over exposure. The 4Bank is designed to be used near the subject as a soft key-light. Learn more about the Kino Flo 4Bank. Here's the 4Bank's illumination table:

*with all four tubes turned on
beam angle = 100 degrees at 1/3 f-stop of center beam

from http://www.kinoflo.com/sales_catalog_2004/4bank_system/001_4bs/4bnk_sys.html

The above table provides foot-candle illumination at various distances.  The 4Bank delivers 40 foot-candles of illumination at 8 feet from the subject. Here's the graphical chart:

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

The Kino Flo with all four tubes turned on, would be a nice soft source for the Canon XL-2's -3dB gain at right around 8 feet from the subject.

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 only reduces the amount of light striking the subject, sometimes quite drastically.  Practice and experience will inform you how these creative tools alter the illumination.

back to Choosing a Light - Main Page

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