I've got two cameras...a Panasonic GS400 and a Sony VX2100. I was playing with them both in a fairly lit room (about 60lux) and noticed that the GS400 with an open iris and 18db is not as bright as the VX2100 at 4.8. Why is that? Granted it could have something to do with the ccd size...but I thought that the settings would be a little more objective rather than a subjective subject.

F-stops are a measure of the amount of light that is allowed past the iris in relation to the lens diameter. How that light is then handled by the CCD, DSP etc, determines what sort of image you get.

F-stops are just calculated by the lens aperture and focal length and don't take anything into account for the sensitivity of the CCD's. As we know, the VX-2100 is the low light champ and has bigger CCD's than the GS-400. The GS-400 also has a higher pixel density on the small chips which make it even less sensitive. So in other words, your test would be like comparing two film cameras with different speed films in them.

I have a PDX-10 which has similar (if not identical) CCD's to your GS-400 (1/4.7" 1152x864 IIRC). In side by side tests I found about a 2.5 f-stop difference from my VX-2000.

The F-stop setting is often referred to as the "speed" of the lens. Exposure also depends on the "speed" of the film or CCD and the speed of the shutter (short, fast exposure or long,slow exposure). Reducing the F-number by a factor of the square root of 2 (e.g. from f/5.6 to f/4) increases exposure 1 stop. Doubling the film or CCD speed (e.g. from 125 ISO to 250 ISO) increases the exposure by 1 stop. Doubling the shutter "speed" (e.g. from 1/60 to 1/30) increases the exposure by 1 stop.