Canon 300 f4 is V Canon 400 f5.6

Folks , Has anyone tested the 300 F4 L is lens against a 400 F5.6 L. I would be interested in anyones opinion. I own a Canon 1.4 mk2 TC so i could put this on the 300 to get over the 400 mark but i am not sure if its best to do that or just go for the 400 prime. I would be using the lens for sports (motor racing) and Wildlife.

Cheers

If I were going to be doing a lot of my shooting at the 300mm than at 400mm, I would go with the 300 f/4 and buy an 1.4x if I didn't have one already. That way, I can 300mm f/4 and 420mm f/5.6 out of the same lens.

However, if 400mm and maybe a little longer were more of what I needed and wanted the images to be the sharpest it could be, then 400 f/5.6 would be the one to choose.

The 300 is nice with the IS and the fact that you can put the 1.4TC on it and still have AF. However in wildlife, usually you want the longest lens possible. The 400 F5.6 is very sharp and light to boot. I am currently "testing" a 400/5.6 and to date love it. At lower speeds however, you have to tripod/monopod it to stop movement. It would the a very sweet lens with IS.
Remember a 400 with the 1.4TC is a 560. The 300 is only a 420. Toss in the 10D crop factor of 1.6 and all of a sudden the 400 + 1.4 =896
The 300 + 1.4 is *only* 672.
All my experience with the 400 has been in good light. This weekend I plan to try so early morning shooting and some tripod mounted 400+1.4 efforts.
Will be very interesting to see what happens.

Thanks for the info , my one concern is that if i put an 1.4 TC on the 400 i would lose AF. Don't really want to do the tape trick that i have heard mentioned recently. Can you confirm that with the TC on the 400 it does lose AF. I must admit the 400 does appeal to me though. Have seen pictures and they are very good

You will definitely lose AF with the 400 and the 1.4x unless you have the 1D series camera.

Thanks, thought it would. Now the decision. Thanks for your help folks

i have the 400 5.6, and it DOES AF with the Tamron 1.4. The Canon
1.4 is better quality, but will NOT AF. the Tamron has to be the standard
1.4 not the SP, and you need decent light, but you don't need tape.

thanks,
eric1