Comparison of 300 f.4 and other similar range lenses

Hi Everyone.

I was wondering what people thought about the 300mm f.4L and how it measures up to the 400mm 5.6L and 100-400mmL and maybe the Sigma 50/150 - 500 mm. I am into bird photography and was wondering how it fares in regards to general I.Q, autofocus speed, low light performance....anythin​g basically. Any thoughts would be appreciated.

I'm very happy with my 300 F4. Can't comment on any of the others you mentioned, I've never used them.

I've tested the 300 F4, 400 f/5.6 and 150-500.
Today I have the 150-500 and the 400 f/5.6.

IMO, it's pretty much like this:

300 f/4 and 400 f/5.6 are both similarly sharp.
If you need 400, the 400 is sharper than the 300 with a 1.4x tele @ 420.
If you don't need 400, the 300 is just as good, and you get IS.

the 150-500 is great for flexibility: it has a big range and great OS. Image quality is a bit lower, but you have to pixel peep. If you don't crop and don't pixel peep, it can be actually difficult to tell photos apart.

150-500 is 500mm only if focused at infinity. If you focus very close, the 400 will give you more magnification than the 500 @ 500mm. This is something not many people talk about.
From 6 meters up, the 500 magnifies more.

I can handhold my 400L at 1/250, but many people need monopods since it lacks IS.

So, I'd resume it to this:

Get the 400 if you need the absolutely best IQ at 400mm and can live without IS.
If you can't live without IS, and can sacrifice a tiny bit of IQ (very tiny), the 300 f/4 plus a 1.4x tele is the way to go.
If you need lots of flexibility and can sacrifice a bit more IQ for that, get the 150-500, save some bucks and be happy.

I use both the 400mm f/5.6L and 300mm f/4L IS lenses but, if I were forced to choose only one, I would opt for the 300mm f/4L IS. Because:

It is a stop faster, it has IS, and because I can use it with a 1.4x TC. The 300mm + 1.4x TC is not "quite" as sharp as the 400L and it doesn't focus "quite" as fast but, few lenses are as sharp or focus as fast as the 400L. The 300L + 1.4x TC produces very satisfactory results. Additionally, the 300mm f/4L IS lens also focuses a lot closer that can the 400L...

BTW: I usually use either of these lenses in combination with my 70-200mm f/4L IS on a pair of cameras...

If I did not have the 70-200mm or only wanted to shoot with a single camera, I would opt for the Canon 100-400mm f/4.5-5.6L IS or the new 70-300mm f/4-5.6L IS lens...

I cannot speak for third party brand lenses because I have not used any of them...

I had the sigma 150-500 and I loved it. I was happy with it. I didn't know any better.

I got a good deal on a 300 f4L is on craigslist ul date code 1997 I think. Did not intend to replaced the sigma with this lens. I liked it so much I didn't use the sigma anymore so I let it go. The 300L killed the sigma in sharpness and bokeh it does a semi macro with the 1.5m focusing distance.

Just picked up a 400 f5.6L uv date code. This lens rocks and don't miss the IS so far. Feels very similar from a reach stand point to the sigma at least to me. This lens also is sharper then the sigma buy a notable amount. Much lighter too. Now I think I will keep them both they are my first and only L lenses but I doubt will be my last. I don't feel thet have to much over lap at all. Unless i find a 300/400 f2.8 at a great price.

I have both the primes and could not imagine living without either one. Both are very sharp and easy to use, although the IS on the 300mm certainly is helpful. For the most part, I agree with Alex and RPCrowe. I've tested my 300mm f4 with the 1.4 extender against the 400mm, and the 400mm prime definetly wins that comparison. I would say if you're buying this for birding, the 400mm 5.6 would make more sense. I think you'll be pleased with either of these great lenses.