Most cameras have a light meter that will show on the display to tell you if you are over exposing or under exposing. Adjust the aperture/iris (also called F-stop or T-stop) till the camera's built-in light meter says the scene is properly exposed. Double check the view finder to make sure that you can see detail in the person's face without being to dark or washed out too white. Now set the hand-held light meter to the same F-stop and shutter speed as the camera and point it at your human subject. Again, make sure that the meter is near the camera and that you are not casting shadows. Now on the light meter, adjust the FILM SPEED until it shows you having the correct exposure. This will be the approximate ASA, EI, ISO of the camera.
To give an idea, the Canon GL1 seems to be at about 160 ASA, the Panasonic AG-DVX100 is around 300-400 ASA (depending on gamma and CCD modes) while the AG-HVX200 (the so-called high-def camera) is around 300 ASA. Helpful hints; do this indoors and white ballance the camera properly prior to the experiment. If you do not know how to do this, consult your manual. Also, make sure you are not using a diffusion dome on your light meter if it has one. Also, this is an APROXIMATE of film speed and only under laboratory conditions with a proper test target and a waveform monitor connected to the output of the camera can you detimerine the precise sensitivity of the camera. Happy testing.
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