Canon 400mm F5.6

I have heard that the 400m F5.6 Canon lens is really good for birds in flight. Is the autofocus quicker than the 300mm F4 IS which I have. Note I often use the 300mm with a 1.4 Canon TC.:S3:

Hi Mat. I've got 400mm f/5.6. I don't know if AF is faster than 300mm f/4 as I've never owned one but it is faster than my previous 100-400. It's really sharp as well and I never had to adjust the aperture. These days you can get it second hand for good money. If I'll ever manage to buy 1D4 and 600mm f/4, I'll certainly keep my 400mm f/5.6 for BIF shots.

Hi Mat, I have had both the 300mm & 400mm, I sold the 300mm, the 400mm is faster focusing & much faster than the 300+1.4 TC, & pin sharp wide open.

Mat, I used to have the 300mm non-IS lens before the 400. great lens( so is the IS version) and pretty fast AF without TC. But it does slow down a bit with TC.

If you are comparing AF speed of 300+1.4x with 400f5.6, the latter wins easilly. dont even think about that. There are other things to think about though...like IS, minimum focusing distance and versatility.

Thanks all for your comments. Something for me to think about....

I have the 300 f4 IS (and the 300 f4 non IS) and the 400 5.6. I use the 400 for BIF. It will focus very fast and accurately. Nice sharp lens. The long minimum focus distance (MFD) and the lack of IS make it problematic for general photography. The 300 IS lens doesn't focus nearly as fast but has a very close MFD and IS so that lens works well for butterflies, dragonflies and perched birds. It takes the 1.4 TC very well. So, for BIF, get the 400. For close focus, the 300 is great. The 100-400 is a very good all-rounder but is heavy and doesn't focus as fast as the 400.

Now I don't own the 400 prime yet but I did have the 300 + 1.4 and found the AF quite slow for birds in flight. I was testing the 400 f5.6 the other day and found that it felt terrific on a 7D, very well balanced, and much lighter than I would have thought...very fast-focussing as well...if I do buy the lens I'll be sure to give you a report. I can't decide whether to buy it new or used...there's a $265 price difference, 9 mos. less time on the warranty, and the used version does not include the tripod collar...hmm...now that I write this all I think the decision should be easy...

I bought both the 300 f4 IS + 1.4 TC and the 400 5.6. I'm keeping both. I use the 300+ for butterflies, dragonflies and perched birds. The 400 is my BIF lens. I use the 300 a lot more than the 400 but when doing BIF, the 300 just can't keep up.
My advice, buy both!