if i have a canon 400D and i put on a 2x extender and an EF 90-300mm Autofocus F4-5.6 AF Zoom Lens would it have a better zoom than if i used a Canon 300mm f/4 L IS USM EF Lens 300 F4
Using a 2x convertor would give you an equivalent to 270mm - 600 mm f8 - f11 lens. The other 300mm lens is just that, it has no zoom feature but gives you 2 extra stops so will be better in low light or for fast shutter speeds.