Canon 70-200 f/2.8 L IS at 200mm
Canon 70-200 f/2.8 L IS + 1.4x at 280mm
Canon 300mm f/4 L IS
Canon 300mm f/4 L IS +1.4x at 420mm
Zoom and usage of 1.4 teleconverter supposed to decrease quality
You can notice what 280mm image (which is zoom + 1.4x converter) is essentially the same as that of 300mm prime L lens .
