Is it safe to say that most video cameras are industry rated at 320 ISO? I've heard this about a few cameras including the XL1 and even the CineAlta.

Also why aren't ISO ratings marked on cameras instead of decibels?

ISO (International Standards Organization) in this context, is a rating of films sensitivity to light. ISO insures that films labeled 400 ISO are the same in light sensitivity. Imagine if each manufacture of film had their own numbering system.

Video is not film and electronic signal are measured in decibels, volts, signal to noise ratio etc., but not ISO. Film responds in a very linear fashion except at the extremes in timed exposure (over a second or two and over 1/10,000 of a second), while video typically does not. Applying various camera settings effects the image, such as sharpness, white balance, cine gamma etc. So, it would be very difficult to apply a film standard to video in this regard. Many people attempt to correlate the two mediums using handheld meters and exposing scenes under similar lighting conditions. At best the 320 IS0 rating is an approximation and under many conditions is inaccurate.

Jeff- you are dead on. The "ISO" on a video camera is variable-- it chages as you change the settings of the camera.

Mike- if you want to be able to use an exposure meter with your dv camera, there is a way that will get you *ROUGH* readings. You may want a second opinion as I am a little shaky on this, but here it goes:

Set your camera shutter priority so that it will automate the exposure (make auto gain is off). Evenly light an 18% Grey card and fill your frame with it. Take a reading with your meter (I've used a digital incident meter for this, but you can use anything). Make sure the meter's shutter speed/ frame rate is set at 24FPS for 24p or 30FPS for 30p/60i. Once youve taken the reading, cycle through the ISO settings until you see the F stop that matches what your camera is displaying. Which ever ISO setting you landed on is the approximate F- stop for your camera. I found my Xl-1 to be roughly ISO 250. Of course, changing the shutter speed, frame rate, gain setting, etc. will mean that the ISO has changed. I find using a meter completely impractical while actually shooting, but it can be quite useful on location scouts and for pre-lighting as long as you determine you camera settings ahead of time.