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| p.1 #11 · Lens decision. Show me your 300mm f/4 IS + 2 X TC images. |
You are at the most common conundrum that many wildlife photographers face; a lightweight 500mm solution. Unfortunately there is no good solution at the moment. I certainly would not recommend a 1.4 TC on either the 100-400 or 400 f5.6. Sure, they will AF with your 1D's, but you will not be happy with the AF performance or the IQ.
I have no idea why Canon or Sigma does not make a 500 f5.6.
You may want to wait a year or so, because I believe there will be a better 7D coming. As well, AF and IQ are improving on mirrorless cameras. e.g. the Olympus E-M5 weighs only 15 ounces and the Olympus 75-300 lens gives you a 35mm equivalent lens of 150-600mm and only weighs another 15 ounces. These smaller systems will be serious contenders to DSLR's in the near future as they are improved upon, simply because of their size and weight.
By the way, do not consider the 400 f4 DO, as you will also find that too heavy, and over-priced.