I found a great app on the app store that actually let's you set some of the settings that you would normally get with a driver on windows. As you know on the mac you have no such driver for the logitech c920. But at least this way you can change a number of the settings. It's a way bit pricy at £5.49 for what it does and the people that make it must know that but hey, that's what you pay for i guess.
Logitech C920 HD Pro Webcam for Windows, Mac, and Chrome OS. The compact and powerful Logitech M337 Bluetooth Mouse lets you create on virtually any computing device, whenever and wherever you choose. Its programmable navigation button puts smart control in the palm of your hand. Logitech webcam software for Windows: – Pan.
What's really useful is that you can manually control things which means none of that 'auto-tracking' that the logitech cameras normally do. Fix it manually and your set. And thank you logitech for putting a tripod mount on this fantastic webcam that runs amazing with the onboard h264 encoding.