What sets the 70D apart from other DSLRs is the dual-pixel auto-focus system.
A typically DSLR has dedicate AF sensors and these are usually in the floor of the camera... not on the sensor. When the reflex mirror the down position, it bounces the optical path up into the viewfinder -- but it does have some semi-transparent areas through which some light actually bounces down into the AF sensors which use prism to split light into two phases. The camera is able to detect focused distance to an object based on the phase. This is a very fast focus system BUT... it requires that a mirror is bouncing light into the dedicated sensors.
When you shoot video, this is a problem for most cameras because the reflex mirror has to be out of the way so the sensor can capture the video... thus the really fast and accurate focus system cannot be used. Instead the cameras use "contrast" detect auto-focus. I always use an anology of a barcode pattern. Image a white background with black stripes. If the barcode is focused then every point in the image is either a "black" pixel or a "white" pixel and at the edge of a stripe you've got a black pixel next to a white pixel (very high contrast). But if it's out of focus, the black stripes in the barcode will have soft fuzzy edges... part white, part black -- and appearing as some shade of gray. If you were to inspect each pixel in a row, you'd have black pixels next to dark gray pixels which are in turn next to medium gray, then light gray, and ultimately white. In other words the contrast between each adjacent pixel isn't very much. When the contrast ix very strong the camera knows the image is focused. This is a very accurate focus system... but also very slow. The phase detect system can instantly tell which direction and how far it needs to move focus to snap a subject to tack sharp fcous. But the contrast system has to "hunt" for focus through trial and error. It's a terrible system if you need to focus in a hurry (and the reason why people with point & shoot cameras complain that they press a shutter button and there's a long delay before the camera takes the image.)
All this was true... until the 70D came along.
Canon has had a few cameras where they've toyed with improved ways to do on-sensor auto-focus that would be faster than the contrast-detect system. The 70D has their best system to date and it's actually pretty good. Their new dual-pixel CMOS AF system is actually pretty fast and accurate. No other DSLR has anything like it.
The advice of using a real video camera for video is VERY good advice. Video cameras are designed for this. But the 70D has a continuous AF system while shooting video that rivals what you can do with a video camera -- and of course you get the ability to change lenses. Canon's Cinema EOS gear is fantastic... but very expensive.
If you're buying a DSLR with the primary goal of shooting video and can't afford a $5000 cinema camera then I'd go with the 70D. The T3i is not going to have the focus performance unless you can pre-plan the focus and focus manually -- not very practical for action sports because you don't get to pre-plan focus.
You'll want fast focusing lenses (fast motors). The USM focus motors (UltraSonic Motors) are the fastest. The STM motors (Stepper Motors) are also pretty quick (and very quiet). The standard motors (neither USM nor STM) are pretty slow -- so avoid those for action sports.
Tim Campbell
5D III, 5D IV, 60Da
