I have a Canon XL2 and was shooting some footage over the weekend which I thought would be in 16:9 ratio, after all, that was what the format selector switch was set to.
After down loading the footage it's clear it has captured in 4:3. So I ran a little test, shooting more footage in the 4:3 ratio then clicking the selector round to 16:9.
On down loading this footage, I can clearly see both formats recorded as per the switch selector.
So why would it capture 4:3 when I have my camera always set on 16:9?