The T1i doesn't have continuous auto-focus during video... it never did.
The issue back when the T1i was a new body is that cameras have two major auto-focus systems. When shooting still shots through the veiwfinder (not live-view) the camera has special phase-detect AF sensors on the floor of the camera. The reflx mirror has a semi-trasparent area with a second smaller mirror hiding behind it. This bounces light down into the AF sensors. These include a beam-spliter (like a prism) and light is projected onto the sensors.
The slick thing about these phase-detect AF sensors is that when you focus, literally every AF sensor takes a measurement at the same time and ... when something isn't in focus it's split-beam light will not be "in phase" on the sensor. But the really impressive part is that in just a single reading the camera can determine exactly how far out of focus a subject is and if it needs to focus closer or farther to attain shapr focus. So the camera basically takes one sample and immediately knows precisely how much to change focus and in what direction.
The result is that the lens snaps right into perfect focus very quickly.
When doing "live view" the reflex mirror is flipped out of the way to use the camera's image sensor. This means it's not possible to send light through that phase-detect focus system. So the camera has a second focus system for this situation.
This alternate focus system uses something called "contrast detect" AF. The easiest was to explain it is to imagine taking a close-up photo of a bar-code. A typically bar code is white with black stripes.
If that bar code is in perfect focus then every pixel in the image should be either a "black" pixel or a "white" pixel. You'd have nice sharp edges with black pixels (for each of the bars) immediately next to the "white" pixels representing the background. That means it's a very high-contrast difference along the edge of those stripes. But if the lens was out of focus then you'd get a blurry edge... in other words you have a "black" pixel next to a very dark gray pixel next to a medium gray pixel next to a light gray pixel and eventually a "white" pixel. So the transition from black to white isn't instant... it's gradual. That's a low contrast change from pixel to pixel.
The "contrast" system toys with focus while looking for "edges" of contrast in your image. When it thinks it found "edges" of contrast, it adjusts focus to try to maximimize the contrast along those edges. To do this it has to sample contrast and look for edges, adjust focus and then re-evaluate, adjust focus again and re-evaluate and it tries to work out which direction improves focus and eventually finds optimal focus. But it DOES take many "guesses". This is often called "focus hunt" and in your video you'd see the camera quickly focus in and out as it works and settles on optimum focus.
Years later Canon introduced models which have some ability to do focus using technology similar to phase-detect auto-focus... but be able to do this directly on the camera sensor. The Canon EOS Rebel SL1 had a feature called "Hybrid CMOS AF".
The big advanced came with the 70D which had Dual-Pixel CMOS AF... and that technology has gone through a few iterations/generations of improvement.
All the Rebels after the SL1 kept getting versions of Hybrid CMOS AF through the T6i... but the new T7i finally now has Dual-Pixel CMOS AF (the better system) which is the same system found on mid-range (70D and above) and pro series cameras.
You can still force AF during video. During video you have to press the button with the asterisk (*) on it and this will force the camera to perform a contrast-detect AF focus... but you will notice this in the video because it does require several iterations to work out best focus.
The Dual-Pixel CMOS AF system works much more like the phase-detect system (except it can do this on-sensor). As a result it actually can continuously track focus in live-video while your subject moves around and it will keep focus on your subject. It is very good, fast, accurate, and you don't see "focus hunt".
If you're video work is planned (not ad-hoc video) -- e.g. if you're making a film -- then you can wrap a piece of masking tape around the focus ring of your camera. Pre-focus (before shooting) on each focus position and mark that focus position on the tape. As you film you rotate the focus ring from point to point so that the resulting video has focus that moves perfectly from one sujbect to another with smooth focus transition and no guessing.
If you think you'll be doing a lot of video and want continuous AF, then I'd suggest upgrading to the new T7i, 77D, or 80D cameras (the 80D is basically the ideal camera for video right now.)
Tim Campbell
5D III, 5D IV, 60Da
