Your camera only has an f/1.6 maximum aperture at maximum wide-angle. As soon as you start to zoom towards telephoto (even a bit) your lens loses speed such that at full tele it's only got an f/3.5 maximum.

So put it another way. You're filming in the gloom using f/1.6 and +6dB say. If you zoom to telephoto you only have f/3.5 as a max apererture, so the gain has to go up to +18dB (two stops) to compensate. If you don't allow the gain to increase you'll greatly under-expose your shot.

tom.

Thanks Tom. Very good info. I'll play with the cam with these things in mind.