Can anyone explain how spot meter on the PDX10 works?

If you have an over exposed subject and spot meter it, the exposure will go down to adjust. If you have an under exposed subject, the exposure will open up.

My question is, how does it know HOW much to open up or to go down?

Is there a set Sony calculation that the camera like and is that changable? (like say, over all 50% exposure at that point on the LCD screen?) Does the camera guess?

What does it use as the baseline/reference point.

I noticed that Sony is put spot focus and meter on their consumer cameras now.

I don't know for sure, but in still photography light metering (spot or otherwise) is calibrated around a shade of grey (or gray if you prefer) with an 18% reflectance level - the accepted mid-point. The origins of this probably lie with Kodak or Ansel Adams.

I imagine that DV cameras will adopt a similar principle, if not identical.

Instead of adjusting the exposure based on the full picture average (average content kept to 18% or 50IRE) the cam selects a small (spot) central area in the image and adjusts the local average again to 18%.

Can we get this straight Ronald? You're not talking about the spotlight and backlight exposure buttons are you? If so Pat and Andre are barking up the wrong tree. If they're right and I'm wrong it's me who's barking mad.

tom.

I think he's talking about the "spot meter", as he says. In this mode you use the LCD touch panel and point to the object of interest. So Andre and Pat seem to be on the right track, based on my own very limited knowledge of such things...

yep, spot meter.

so the reference point is 18% grey you guys guess?

Yes, if you shoot a uniform white (or gray) surface, it will be exposure controlled to get 50 IRE (18%). Video however, due to the electronic signal it generates, has more degrees of freedom to process signals which contain non uniform gray scenes: some concepts are more "peak detectors" in order to avoid washed out highlights. In these cases the 18% is not kept for all image contents.