A single video clip would be limited by three things:
1) The card is formated with a FAT32 filesystem so that it can be commonly read by any PC, Mac, Linux, etc. computer. But this filesystem has a 4GB filesize limit. No single file can be larger than this. You'd have to stop and start a new recording so that it records to a second file.
2) If the card doesn't have 4GB of free space left on it... it will (of course) stop sooner.
3) Even if you set a very low video resolution so that it wont fill 4GB in 30 minutes, it will still stop at 30 minutes. This is because in some countries, "video" cameras are tariffed at a different rate than photographic cameras. By international treaty, photographic cameras are allowed to record video clips... and wont be tariffed as "video" cameras as long as those clips cannot exceed 30 minutes. That's the purpose of the 30 minute cap.
Autofocus:
As for auto-focus during video... the 70D (and I think now the 7D II) have special technology on the sensor that allows continuous auto-focus while shooting video. This feature is available in some Canon models that have "Dual Pixel CMOS AF".
For the other cameras, the sensor has to use a different method to focus and it involves some "guessing". This is "contrast detection AF". Imagine taking a photo of a barcode... a white background with solid black stripes. In theory, if focused accurately, every pixels should be either "black" or "white". And at the edge of a stripe you should have a "black" pixel adjacent to a "white" pixel (that's an extremely high contrast difference between adjacent pixels.) Now imagine you deliberately de-focus the camera. Now you have a "black" pixel followed by several pixels which are progressively lighter and lighter shades of gray... until you ultimately arrive at the "white" pixel. That's a "low contrast" difference between adjacent pixels and it's low becuase the image is blurry.
That means if you can maximize contrast difference between adjacent pixels in some area of the frame, you have nailed focus.
The "problem" with this system is that if it's out of focus, it has no idea what it should do to improve focus. It can try moving focus outward, re-sample the image, and see if things improved. If not, it can run focus inward and try again. Once it figures out which direction is better, it can then start fine-tuning focus until it finally nails the focus. But this system causes something called "focus hunt" where you visually see the camera is trying to focus while you are recording video -- not good.
Pro videographers pre-determine their significant focus points. E.g. if they want to go from subject 1 to subject 2 in a scene, imagine putting a piece of masking tape around the focus ring and using a sharpie market to mark the focus position for subject 1 and another mark for subject 2. When THEY move focus, they go directly from position to position and nobody watching the video sees any "hunt".
So to avoid the "hunt", the camera does not auto-focus continuously while recording video. You can press the focus button on the back (It's usually the asterisk button (*) but I don't have a T5) and this will ask the camera to auto-focus at that moment (but you'll see it do this.)
If you need true continuous AF during video then you'd either want to use a true video camera OR you'd want a model that supports the feature like the 70D.
Making backgrounds blurry:
This is determined by the "depth of field" which is derived by a combination of (1) the focal length of the lens, (2) the current aperture setting , (3) the subject distance and (4) the physical size of the camera sensor.
(1) Longer focal length lenses will reduce the overall depth of field. E.g. a 200mm lens will have a shallower depth of field than a 20mm lens. This means longer lenses can generate a stronger amount of background blur than wide lenses (and very wide lenses generate almost no blur whatsoever.)
(2) Using low f-stop values (aperture value) creates a narrower depth of field. f-stops such as f/2.8, f/2, f/1.4 work well... high f-stops like f/11, f/16, f/22 generate very broad depth of field which means they have very little which would be blurred/out-of-focus.
(3) The depth of field is naturally broader if the subject distance is far. Very close subjects get very narrow depth of field. In other words, if you focus on a subject which is actually very close... while the background is very far away, the "narrow" depth of field on your close subject will mean that the background is well outside of focus and will be very blurry.
(4) Lastly (and this isn't anything you can change), the sensor size technically also effects the depth of field. Your camera, have an APS-C size sensor, is rather large and will have no problem producing the blur you want... given the right low-focal ratio and long focal length lens. I only mention this because smaller sensors (e.g. those on point & shoot cameras or camera-phones) have to have their crop-factor multiplied by the focal ratio to determine an effective focal ratio. When the crop factor is huge (very tiny sensors), this effectively means that even with an extremely low "real" focal ratio, the "effective" focal ratio is so high that the camera can't generate the background blur. Don't worry about this - I only mention it because one of the reasons people like the look of DSLR images is because of that background blur that point & shoot cameras and phones cannot generate and the reason they cannot generate the effect is because their sensors are too small.
When you put all of this together, you want:
(a) a long lens (e.g. 85mm... 100mm... or higher) with (b) a low focal ratio (e.g. f/2.8 or lower... a telephoto lens such as a 55-250mm f/3.5-5.6 reaches f/5.6 as it's lowest possible focal ratio when it's merely about halfway through the range. f/5.6 isn't very low so you wont get a strong background blur.) And (c) you'd want your subject to be somewhat close to the camera and you'd wan the background farther away. You're not going to see this effect (or certainly not much of it) if you put your subject right up against a background such as a brick wall... pull the subjects forward and away from the background.
The "quality" of the blur (not just the strength) is described as it's "bokeh". Some lenses produce a much more beautiful quality than others. More aperture blades is a significant factor as well as how "rounded" the opening in the lens is due to the shape of the blades. But other optical qualities influence the final look as well. Typically the more expensive the glass... the better the look.
Tim Campbell
5D III, 5D IV, 60Da
