Here's a little trick to control the exposure on HV20, while keeping the camera's own exposure locked at f1.8, 1/50 shutter, 0dB gain.
First of all, here's how to lock the exposure to the above: Put on a lens cap, zoom all the way out and lock the exposure. Then set it manually to -7 (50i) or -11 (25p). This is for PAL models, similar steps should work for NTSC. This works only in Cine Mode, which prevents the gain setting from exceeding 9dB.
And now to the trick itself - here's how i do it:
I have not one, but two LINEAR polarization filters on my HV20. In addition to getting the expected effect of a polarizing filter, these can be used to control the exposure (which is the main reason i bought them). When the two filters are at 90 degree angle from each other, they will not let any light through, all other angles will let some, but not all of the light through.
Think of it as a variable ND filter.
Here's a test clip, locked exposure - all the changes in the image were made by rotating the filters, showing both exposure and polarizing effects.
The whole set of filters i have on this clip is
43mm Skylight (to protect the camera lens)
Step up rings from 43 to 52 mm
Two 52 mm linear polarizing filters
52 mm UV filter (to protect the pol filters)
...this was my first test, it seems i need to get rid of the Skylight and/or the UV filter, the setup is too long and shows at the edges of the image. This wasn't visible on the LCD.
BTW: this will NOT work with spherical polarizing filters, they NEED to be linear.
BTW2: linear polarizing filters do NOT seem to cause any problems with the autofocus, which is nice.