Thanks for this. Yes, I know about optical vs digital zoom. The Logitech webcam software for my previous model had maybe 8-step 'zoom', but the same software for the 920 does not have that (at 1080p, but does at 720p) and the Logitech website baldly states they know it doesn't work and aren't going to fix it.
Actually, the picture really doesn't look any better; trimming in post-production doesn't really work for me and, to cap it all, I was burglarised last night, and the webcam went along with the laptop and a few other bits and bobs.
So that's solved that problem, in a way.