2x EF Extender III or ???

[vmk]

I Have 70-200 f/2.8L II USM, and i need little longer zoom .

Is it worth buying 2x EF Extender III for 70-200 f/2.8L II USM (or) would it be better to go for 100-400mm f/4.5-5.6L IS USM Autofocus

Body: 5D Mark III

Please provide your valuable suggestions.

Thank you

[THX723]

Seeing as you already have the 70-200 f/2.8L II covered, it would be cost effective to go with the 2x EF Extender III.

During one of my visits to a Formula One race, I shot back-to-back using the 100-400 and the 70-200/2x combo. I found the two configurations to be comparable in both IQ and AF performance. I will say however I much prefer carrying the 100-400, for it's lightness and compactness (1380g), over the prominently protruding 70-200 + 2x (1815g combined!).

[vmk]

How amount image quality in terms of Sharpness / noise?

[THX723]

As mentioned above, image quality (IQ) was a toss up; neither showing a perceptible advantage or disadvantage during my real-world use; they were both impressively sharp for the occasion. I was not able to readily discern which lens took which shots during post-process.

There would be no difference in terms of noise, as that's solely the function of your camera and what ISO-sensitivity you're operating at. Both configurations also have a comparable max aperture of f/5.6 (the 100-400 is negligibly ahead at f/4.5 on the wide-end).

Here's a quick summary for you ...

EF 70-200 f/2.8L II + 2x EF Extender III
Pros:
Flexibility to break down from f=140-400mm to f=70-200mm
2X Extender costs ~3x less than EF 100-400L

Cons:
Massive. Physically longer and heavier
As a combo, starts off less wide at f=140mm

EF 100-400 f/4.5-5.6L
Pros:
Compact. Physically lighter and much shorter (when collapsed).

Cons:
Stuck with f=100-400mm
~3x more expensive than the 2X Extender

[neuroanatomist]

In my experience, the 100-400 @ 400mm has a very slight sharpness edge.  The 70-200 II + 2xIII has very slightly better bokeh (but honestly, neither lens delivers especially well there).  The 100-400 will AF very slightly faster.

One other consideration - the 70-200 + TC is weather sealed, the 100-400 is not fully sealed (lacks a mount gasket).

In your place, I'd get the 2xIII.

[Halfrack]

Do you already have the 1.4x III tele?  If not, the only thing you need to keep in mind is that the 2x tele is a special beast, and it is fairly limited in where you can use it - f2.8 lenses (or 135/2 or 200/2).  There isn't as much of a market for them on the used side, so make sure you really want it first.

[vmk]

I haven't had any extenders before, this will be the first one

Will go for 2xIII ...

[vmk]

Incase if i ever wanted 400mm lens, what would be the best one to pick?
My price range would be around 3000$

[dslrdummy]

I have the 70-200 and 2x iii extender and recommend the combination. The only issue for me with only one body has been that if I want to swap between 70-200 and 140-400 in the field (which I usually do) taking the extender off and on can be a bit of a hassle. With a second body I would definitely think about swapping the extender for the 100-400. 

[tphillips63]

I just went through this myself and purchased the 2x EF Extender III.
I was considering the EF 400 f/5.6 not the 100-400 but same issues basically and am glad that I got the converter.

It is a lot heavier combo than I thought but for occasional use it is a great option and I'll probably get the 1.4x before I get another longer lens.

You could always see if you can borrow one or go to a nice local shop and try it out, maybe rent with rental amount to purchase if you decide to do that but from all the advice of others a lot more experienced with these I went with the converter first.

[Halfrack]

I haven't had any extenders before, this will be the first one

Will go for 2xIII ...

Honestly, the 1.4xIII would be a better start. 

There isn't another way to 400mm that is between $1500 (100-400 / 300f4 +1.4x / 400f5.6) and $3k.  There is lots of hope for a 300f4 IS 2, and a new 100-400 but both with have a price point much higher than current.

[kentnish]

I've been using a 300 2.8 + 2x III extender on a 5D Mark III.  It's not the sharpest thing ever, but it gets the job done and it's nowhere as terribad as the ver2.