Updated: 04/13/2008

Determining the EI / ISO / ASA*

Sensitivity of a Video Camera 

Although video camera manufacturers talk about the lux or foot-candle sensitivity of their cameras, few provide any EI equivalents, which is what Directors of Photography in film and motion pictures have long relied on in evaluating the light in scenes.

Light sensitivity is important when there is a need to consider depth of field and selective focus prospects in a particular setting. "Crossover" film cameramen may also need to know if a particular video camera will be able to get pictures in the same light that their motion picture camera did when it was loaded with 500 speed film.

Here is a procedure for determining the EI sensitivity of a video camera. For this you will need a waveform monitor, a standard video chip chart, and a incident light meter.

1. Focus the camera on an evenly illuminated chip chart and observe the waveform.

2. Open the camera's iris until the white chip is at 100 (or the crossover chip equals 55 IRE units).

3. Take readings from the surface of the chip chart. Alter the EI setting on the meter until it agrees with the f-stop on the camera. The EI speed you end up with is the speed (EI sensitivity) of your video camera.

If you want to find out what the speed indexes are at different dB gain settings on the camera -- which would be equivalent to push processing the film to various degrees -- just repeat the process with each gain boost setting.

When you get done, you can make a chart for yourself showing all of your video gain settings and the equivalent EI speeds.

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*All three indexes -- EI, ISO, and ASA -- are used for measuring film speed or sensitivity, and there are both historic and minor technical differences between them. For the purpose of this discussion, however, we can consider them equal.