I am planning to buy an 1.4x extender for my 550D with 100-400L lens to shoot with.
I really don't understand the difference between 1.4x iii and 1.4x ii. Could someone please explain the difference among them? And also will it be a good to go with 550D body and 100-400L lens?
Really, I know how you feel, wanting a little extra reach for as little as you can put out. The bad thing is, the converters really don't work very well at all with that camera, I ought to know, I have one. I have tried the Kenko 1.4 300, the Kenko 2.0, the Canon 1.4II, but I have not tried the Canon 2.0III. These things are designed for the 1D series really, I mean, they will work on other cameras, but to get full focus, you'll need the 1D's. You can get full focusing on your camera using a Kenko brand, but I promise, you will more than likely regret that decision. I tried it all with mine, and my 100-400L too, I got full focus, but it was slow, and the images looked like CRAP!
(I hope that I don't come off sounding like I am a rude know-it-all, or a jerk, I'm just trying to save you some money) I have since upgraded to a 1DIV with a 400-2.8 lens that will handle them all just like I wished it would've with my old cameras and lenses. I still have the T21/550D and the 100-400mm L lens, but there is no way that I would use the extenders on them.
Actually, now I can get acceptable results using two converters at the same time, (1120mm) I use 2 of the 1.4's all of the time, but no way can a T2i do that.
(To be honest, I guess it's all in what you think is acceptable, as to what "good" means to you)
Now, I will say that any lens that is a Canon 2.8 (or faster) or a f/4. 'can' work, but the degradation of the camera alone will hurt the images. Yeah, I know, a lot of people will say they have perfect luck, but you need to see the shots to prove their claims. I'm not calling people out at all, heheh, I just mean that people say theirs works.
Now I do see proof of a 70-200mm L lens working with the Canon 1.4, but it's usually on a 7D, but still, from what I understand, it's not as good as the lenses that they are made for. (From what I read on here, and reviews on Amazon, and the two main sellers here B&H and Adorama)
I honestly know how you feel, and I wish I could tell you different, but you'll more than likely place it in your bag and let it sit, just like I did if you buy one. Mine sat in my bag until recently I decided to try them on my newer camera, it worked really well, but I had to pay a LOT for that too, to be able to do that.
You'll get smeared looking shots, along with slow focusing, even with a Canon 1.4-III.
Also, from what I read, the 100-400mm L is not a good lens (or, at least the best choice) to put an extender on anyway, plus, what I know mine looked like, I wouldn't recommend that anyway. There will be people that won't agree, but I know I have read it many times here, and I know what I see on my monitor.