Don't forget, you have a T5, which has an ASP-C sensor. So, your 800MM (400MM with 2X), is actually a 1280MM lens.
This first-class sensor features many of the same technologies used by professional Canon cameras to maximize each pixel's light-gathering efficiency and has center pixels that aid in the EOS Rebel T5i's accurate AF performance. This APS-C size sensor creates an effective 1.6x field of view (compared to 35mm format).
EF 100-400mm f/4.5-5.6L IS II USM Full Frame 100-400mm ASP-C 160-640mm f/4.5-5.6 Telephoto Zoom