70-200f/4 IS L + 1.4tc or 70-300 f/4-5.6 IS L?

True that the 300/2.8L IS is one of the best telephotos made. However, at over $7200, compared to OPs questioned lenses ($1350 + $500 for the 70-200/4L IS and 1.4 extender or $1600 for the 70-300L IS) is not a realistic argument.

To answer the OPs question and keeping within a budget - the 70-300L IS is a great choice for a single lens answer.

Another option might be the 70-200/4L IS and the 300/4L IS (I got mine used, but in like new condition for $900). Of course this is a two lens option that costs a bit more, but one that I find flexible (I also have a 1.4 vII extender for even more options). The 300/4 IS is a very sharp lens and gives you a full stop extra speed vs the 70-300L IS. The 300 prime also takes the 1.4 vII extender very well and keeps AF on my 5D mkII.

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Jim 'Carnac' Carnahan