I am an athletics photographer, and I have a dilemma. First off, I use the 5D Mark II body. Currently, I own the 100-400mm f/4.5-5.6 USM II, and I LOVE it... except for indoor photos. Before I purchased this lens, I used the 70-200mm f/2.8 for indoor volleyball and the photos turned out great, but I needed the 400mm for other outdoor sports.

You most likely need both lenses. 70-200/2.8 is great for indoors where distances are not as great as they are outdoors and the artificial lights are dim. It's darker indoors than what you think. 100-400/4.5-5.6 is great for outdoors in daylight where light is plentiful and distances may be large.

But recently I went to another indoor game, and the quality with the 400mm is just terrible.

The reason is that Canon 100-400 is a slow lens. It's mainly useful during the daylight hours outdoors. Indoors, you can't reasonably shoot fast action with the 100-400. Your choices to gather light are:

• Bump up the ISO on your current body. This results in grainy images, and there is a limit: 5D Mark II cannot go over ISO 6400 unless you use ISO expansion (hint: don't!). The recommended option is to shoot RAW and use Canon Digital Photo Professional on a computer to apply good noise reduction to the RAW images, but it can't do miracles. A 5D Mark II is a dated body and newer bodies would have less noise with high ISO values.
• Slow down the shutter speed. But, this results in motion blur.
• Purchase a new body so that you can bump up the ISO without having excessive noise/graininess. Unfortunately, a 5D Mark IV costs a lot.
• Use a faster aperture lens such as 70-200/2.8.
• Use a flash. Seriously, don't do this for indoors sports!

A 5D Mark II body goes up to ISO 6400; a 5D Mark IV body goes up to ISO 32000. This does not necessarily mean 5D Mark IV can shoot at 1/4 of the shutter speed with no additional noise; the 5D Mark IV images at ISO 32000 are probably more noisy than 5D Mark II images at ISO 6400.

I'm considering swapping out my 400mm for the 200 with a 2x extender, so I can get those indoor shots

Don't. The image quality degradation of a 2x extender may be severe. As far as I can see, there are only two reasons why you might want to have 70-200/2.8 + 2x extender instead of 70-200 + 100-400:

1. The weight. 70-200/2.8 + 2x extender is lighterweight than 70-200/2.8 + 100-400
2. The price. A 70-200/2.8 is an expensive lens and so is 100-400.

Of these, I don't see (1) as a problem. Typically when you shoot something, you have an idea of what you're going to shoot so you can decide what gear to bring with you. I wouldn't carry both 70-200/2.8 and 100-400 at the same time ever.

So the only problem is (2). Something to consider:

• Canon 100-400 costs 2200 EUR
• Canon 70-200/2.8 IS costs 2100 EUR
• Canon 2x extender costs 500 EUR
• Tamron 100-400 costs 800 EUR
• Tamron 70-200/2.8 G2 costs 1500 EUR

Let's say you can recoup 70% of the price when selling your used gear. For your Canon 100-400 you can get 1540 EUR. Now, what to buy with it:

• If you buy Canon 70-200/2.8 IS + 2x extender, you have to pay 2600 EUR, meaning you need to somehow get 1060 EUR from somewhere.
• If you buy Tamron 100-400 + 70-200/2.8 G2, you have to pay 2300 EUR, meaning you need to somehow get 760 EUR from somewhere.

I would pick the Tamron two-lens combo instead of the fast Canon telezoom + 2x extender.

I see third-party lenses as a better option here. However, there are some things to consider:

• Autofocus may be less accurate on 3rd party lenses.
• Autofocus may be slower on 3rd party lenses.
• 3rd party lenses may not be compatible with Canon teleconverters.
• 3rd party lenses may require firmware update to work on newer bodies, should you upgrade your body.
• 3rd party lenses after manufacturer's support ends may not receive any firmware updates anymore, so you may not have the ability to purchase a new body anymore after a long amount of time.
• 3rd party lenses do not have in-camera lens corrections.
• 3rd party lenses do not have lens corrections in Canon's Digital Photo Professional; especially you cannot use Digital Lens Optimizer with them.