According to Canon Rumors a new Canon 100-400 is going to be released soon. This has me thinking and looking into my archives. Why you ask? Because as a nature and landscape photographer part of my kit includes a 70-200 2.8 IS II. This lens is one of the best lenses Canon makes and it's the sharpest lens I own. So looking through lightroom at my past images where I use the 70-200, approximately 93% of those images were shot at 100mm or greater. More than half of those included a 1.4x teleconverter. I've used it to photograph bears, eagles, deer, elephant seals and other subjects like flowers, bees, and various macro style images. Most of the time I have it maxed out at 200mm. With the 5DIII I can focus at f8. This means I could use a 100-400 with a teleconverter, giving me a range of 140mm-560mm. The 70-200 is a killer lens but I feel a 100-400 would better fit my style and would help more with the wildlife aspect of my portfolio, which is basically what I was using the 70-200 for anyways. There is downside to the 100-400. It's a variable aperture lens, meaning 4.5 is the widest it will go. The 2.8 on the 70-200 is great for low light wildlife photography. If you know anything about wildlife, they like early morning and late evening when the amount of available light is low. The other option is a 200-400 but thats a little out of my price range and would be a pain to lug around on a backpacking trip.
I'll wait to see the price and the MTF Chart, but I do believe Canons latest lens offerings are tops in the business. Nikon, (which uses Sony sensors) and Sony have been producing awesome results with the high megapixel sensors, but Canon has been crushing it with lenses. If the rumored 100-400 is anything like the 70-200, 24-70, and 16-35, I will be selling my beloved 70-200.