I need some input here to help make a major decision by having gathered as much info as I can. I am in a position right now to purchase either a Canon EF 400mm f/2.8L IS USM or the Canon EF 500mm f/4.0L IS USM. I already have an EF 100-400 f/4.5 - 5.6 L IS USM and find f/5.6 at 400mm a problem many times shooting at dusk or dawn, when a good majority of my shooting ops. take place ( my passion is wildlife photography). What i'm seeking is , personal expirience and / or technical knowledge. I am leaning towards the 400 f/2.8 because I can really appreciate the large aperture light gathering aspect and capturing many previously missed photo ops. On the other hand, I like the added reach I would gain with the EF 500mm. My main questions are. #1 If I went the 400 f/2.8 route and added a 1.4 x TC I would get a 560mm f/4 lens when needed but at what loss of IQ? How much worse would a 2x TC be and what would the max aperture be with the 2x #2 If I went with the 500 f/4. Would f/4 be enough of a light gatherer for what I want, knowing that my EF 100-400's main sore point with me is its f/5.6 aperture. And would the IQ be better than the 400 f/2.8 with a 1.4x TC? Please discuss. I an hoping a good discussion here might bring up points I haven't even thought of.