I have recently gone FF from 30D, and while I find my 24-70L wide enough, now 70mm is not long enough. I shoot primarily my family, our active 2yr old son running around.
I know I will get a 70-200 2.8 IS first. This lens has been described to be somewhat legendary, especially for a FF body. I think it will serve me well as my walk-around lens, as well as for indoor portrait. That's $1600 (after $100 off w/WPPI code @B&H). That leaves me $1050.
Now, I used to have a 70-300mm IS, which I was pretty happy with its reach on my old 30D (480mm equiv). I didn't use my telephoto "a lot", but only occassionally when going to sport events, and going somewhere w/lots of animals and flowers. So, with $1050 left, I am tempted to get something to go beyond 200mm.
My first thought was getting the 1.4x extender. That gets 70-200 2.8 to 98-280... close to 300, but nowhere near what I had before (480mm). However still not a bad option for $280.
Then I thought of 100-400mm f/4-5.6 L... although 400mm is nice, given that I already have 70-200 covered, it feels redundant. Also, it's an old design that may be due for a refresh, and the pumping dust "issue" just isn't so appetizing (personally!).
Then I started looking at the telphoto primes. 300mm f/4 IS looks good, and so does 400mm f/5.6. I would probably pick the 300mm f/4 IS, cuz of its IS and may be I will be okay with 300mm. But then, if I could have 280mm f/4 at fraction of the price (w/1.4x extender), not sure if 300mm f/4 IS makes any more sense. $1150 looks kinda expensive... plus these aren't weather sealed. Thus, 70-200 2.8 w/1.4x still wins, so far.
Then I started looking at Sigma/Tokina/Tamron telezoom alternatives. They seem "okay", but compared to Canon 100-400mm, the sharpness just isn't there. I want my tele to have at least 100-400mm sharp.
So I guess we're back to 70-200 2.8 w/1.4 extender. If I waited until April (sometime), Canon may offer enough rebates on both 70-200 and 300 f/4 (double rebates?), and I may be able to save $500... only then it seems justifiable for me to grab a 300mm, instead of 1.4 extender.
I know I'm rambling a little here - I would appreciate any other ideas and suggestions! Thanks.
(btw my 1DsMk2 can autofocus at f/8, so I could probably even live with - in theory - a f/4 lens w/2x teleconverter, provided again sharpness matches 100-400!)