I might consider getting the EF 50mm f1/.4 USM.
I'm imagining you will use this to capture images of students performing moves -- these are "in motion".
When capturing motion, you can either (a) use a faster shutter speed to "freeze" action, or (b) use a slightly slower shutter speed to allow the image to imply a sense of motion blur (and sometimes that's actually a more compelling shot.)
Indoor photography with faster shutter speeds is tricky because usually the lighting is poor (fortunately the 6D has a very high ISO range.) But it helps to have a lens that naturally collects significantly more light.
Your 24-105mm lens has a maximum focal ratio of f/4. The ratio describes the number of times the diameter of the clear opening in the lens (the aperture) can be divided into the focal length. This means that at, say, 100mm... the aperture opening in the lens is able to dilate to 25mm wide and since 25mm divides into 100mm exactly four times, it's an "f/4" lens (the aperture blades can constrict smaller... the value printed on the lens always describe the ratio using the largest possible aperture size.)
A 50mm f/1.4 lens collects quite literally EIGHT times more light. That means it can use a shutter speed eight times faster than the 24-105mm... or it can use a lower ISO setting (to reduce "noise" in the image.)
I'm also considering a focal length based on subject framing. You can use the "dimensional field of view" calculator at this website: http://www.tawbaware.com/maxlyons/calc.htm
So imagine you want to capture a student performing a kick in which you believe you'll need a frame at least 10' high. You can use the calculator to determine what focal length you'd need from a given distance to capture that. If I plug in a 50mm lens, assume I'll be 20' away from the student, then I can plug in those two values. There is a third value which is the "focal length multiplier" (also known as the "crop factor"). Your 6D is a "full frame" camera which means the crop factor (or multiplier) is always exactly "1" (if you had a camera with an APS-C size sensor.. a smaller sensor... then your crop factor would be 1.6).
If I enter "50" for the focal length of the lens, "1" for the focal length mulitplier, and "20" for my distance from subject, and click the "compute" button, it tells me that the physical dimensions of the frame area from that distnace is 9' 7.2" high (in the vertical direction) and 14' 4.8" wide in the horizontal direction. That's a comfortable framing (close enough to 10') to frame the subject and perhaps also their target.
I could try other focal lengths too... I could plug in 85mm as the lens focal length, but then I have to try changing the distances to get a similar framing and I discover that at 85mm I'd really need to be about 35 feet away (instead of 20' away.)
You can imagine how going to higher focal lengths will get to a point where you're running out of space in your 50' room because if your subject is in the MIDDLE of the room, you really only have about 25' and you're back is against the wall trying to get that shot. Hence... I'm thinking 50mm is probably a good focal length (again, this is all predicated on the assumption that you want to capture a vertical height of about 10'.)
One bit of caution... when you use a lens with a lower focal ratio, the depth of field (an imaginary range of distances relative to your intented focus distance at which subjects will appear to be in acceptable focus) becomes narrower.
With a 50mm lens using f/1.4 and a lens focused to a 20' distance, the "depth of field" is just over 4' thick... distances of 18.1' through 22.3' will seem to be in reasonably acceptable focus.
Compare that to f/4 (using the same 50mm focal length and 20' distance) and the depth of field is 12.7' thick... distance of 15.5' through 28.2' are in acceptable focus.
You can see that while the f/1.4 focal ratio collects eight times more light, it comes at a trade-off in depth of field (btw, many people really LIKE that blurred background and go out of their way to achieve the effect.).
If your subjects distances are changing, then you'll want to use "AI Servo" focus mode (instead of the camera's default "One Shot" focus mode) which causes the camera to continuously re-evaluate and tweak focus as your subject distance changes (in "One Shot" mode it will only focus until it acheives a lock... and then stops focusing even if your subject moves and the distance is no longer valid.)
Flash: You didn't ask about flash, but I thought I'd offer this because it's something I'd probably consider doing in this situation.
This gets a bit tricky because of the "max flash sync" speed (on the 6D that speed is 1/180th second shutter speed.) That's not fast enough to "freeze" action if that action is at fairly high speed.
To camera uses two "curtains" (doors) which slide across the sensor. When take a photo, one opens to begin the exposure... a moment later the second one closes. The doors have a maximum physical speed that they can move to either open or close. On your camera, a shutter speed of 1/180th second or slower allows one door to open COMPLETELY, then fire the flash (exposing the entire sensor to light) and then close the second door. Any speed faster than 1/180th requires that the second door already start closing before the first door has finished opening. So to create high speed photos, the first door starts opening but the second door starts following -- really exposing just a "slit" of light that sweeps across the sensor. This allows your camera to use shutter speeds which are faster than the doors can actually move. If a flash fires when there is only a slit exposed, then only the part of the sensor exposed by that slit will get the benefit of light.
This brings up issues like want a flash that supports "high speed sync" mode with enough power for the multiple-light bursts necessary when using high-speed-sync. This means the flash pulses rapidly as the shutter operates so that every part of the sensor benefits from flash. But since the flash has to perform multiple bursts, so single burst can fire at "full" power because the flash has to reserve enogh power for all the needed bursts. Having an especially powerful flash (like the Canon 600EX-RT) is helpful.
You can also use "second curtain sync" mode if you want to imply motion... normally when you use a flash, the first door opens, then the flash fires... and the camera wait out the amount of time needed for your exposure time... and THEN closes the second door. But if something is in motion, it means you'll get a brightly lit subject (by flash) and then the ambient light continues to provide a small amount of light on your subject which is continuing to move. That dimmer light on the moving subject creates a ghosted trail... but the trail is "in front" of your subject rather than following "behind" your subject -- so it looks a bit strange.
"Second curtain" mode causes the camera to defer the firing of the flash until just before the second door closes. This puts that ghosted trail "behind" your subject.
BTW, this second option (implying motion by using a flash on "second curtain sync" mode while using a deliberately slow shutter speed) also helps the camera do a much better job capturing the background exposure of the room (we call this technique "dragging the shutter" because we deliberately use a slower shutter speed than is necessary when using a flash.)
Tim Campbell
5D III, 5D IV, 60Da
