Autofocus performance depends upon the camera, the lens and how the user sets them up.
The 19-point AF system in the T6i is a simplified version of what was introduced on the original 7D and was very high performance in that camera. On the plus side, all 19 AF points are the "better" dual axis/cross type. On the negative side, the T6i only have three "AF patterns" as you show (7D has five), but the good news is that you have the two most important patterns for BIF (single point and 9-point zone). On the negative side, the T6i doesn't have dual image processors along with a separate chip dedicated to running the AF system, as can be found on the 7D-series and 1D/1DX-series cameras and is much of the reason for those cameras' high performance AF. T6i uses a single processor for images, AF and everything else. It is a Digic 6, two generations newer than the dual Digic 4 that the original 7D used. No doubt the newer processor is a lot more powerful, partly making up for it having to handle all camera functions.
So, there's both good and bad stuff, when it comes to the camera's AF and fast, challenging subjects like BIF. You can increase your odds of getting a good percentage of in-focus shots with your camera settings.
- Set camera to its maximum frame rate for continuous shooting.... I think the T6i's top speed is 5 frames per second. Use this to take short bursts... perhaps 3 to 6 images. Odds are that some of them will be in good focus.
- Set camera to AI Servo focus mode (
not AI Focus mode).... This is designed to work with moving subjects. When you press the button, focus is acquired and then will track and continually update as long as you hold pressure on the button. You don't get "focus confirmation" with AI Servo. That only works in One Shot mode, which is for stationary subjects. (Hint: If your camera has an audible "beep" for focus confirmation, enable it. Then you will be alerted immediately any time you start shooting if the camera is in One Shot mode.... fine if you're subjects aren't moving, but a problem if they are. The "beep" simply alerts you about how the camera is set and can help insure you don't use the wrong focus mode. Of course, it also confirms when One Shot has achieved and "locked" focus.)
- Optionally, set up Back Button Focusing (BBF) on the camera... This separates the focusing from the shutter release button. Many experienced sports and wildlife photographers prefer BBF because it puts them more in full control of when and where the camera focuses and allows them to use AI Servo as their default focus mode, with both moving and stationary subjects. There are a number of advantages to this, but in a nutshell it just means being "ready for anything", moving or not, without having to pause to reset your camera.
Lenses also influence autofocus performance in a couple ways. I don't use your lens, so cannot say much about it, specifically. One key factor is the type of focus drive motor used in the lens. There are basically three types:
- Micro motor (also called DC motor or piezo motor, I think) is the slowest and noisiest type of focus drive. Canon uses it on their most entry-level lens.
- Stepper motor (such as Canon STM) is smoother, faster and quieter than micro motor. It's generally preferred for video.
- Ultrasonic motor (such as Canon USM) is the fastest, though it's typically not quite as smooth and quiet as stepper motor. Canon offers EF-S 18-135mm lenses that use all three types of AF drive and they state the USM version is 2X to 4X faster focusing than the STM version. They don't compare either to the micro motor version, though it's a good bet that the STM is about 2X faster, so the USM would e 4X to 8X faster. USM type focusing is generally the best type for action photography, but may not be ideal for video. (Note: Some recent Canon lenses are using a new "Nano USM" focus drive that blends the best of both... is fast like USM, while also being smooth and quiet like STM.)
Your lens uses something called "HLD" focus drive.... which Tamron says means "high-low focus drive". I have no idea what that means or how it fits into the different types of focusing drive motors described above.
In addition, a lens' maximum aperture is a consideration. Much of this is just plain common sense.... the larger the aperture (smaller the number), the more light that's let into the camera for its AF system to work with. But also some Canon cameras have one or more individual AF points that offer increased performance when used with f/2.8 and faster lenses. Important... this
does not have anything to do with the lens aperture the photographer chooses to use. With modern lenses, when you select a stopped down aperture (or let the camera select one with an auto exposure mode), the lens doesn't actually stop down until the very instant of exposure. It remains "wide open" during focusing.... so the max aperture marked on the lens is what needs to be considered. Keep in mind that each stop larger effectively doubles the amount of light admitted into the camera. For example, an f/2 lens allows in twice as much light as an f/2.8 lens. And f/2.8 allows in double the light that f/4 does.... Likewise f/4 lets in 2X the light that f/5.6 does.... etc.
Lens type can be a factor. For example, wide angle lenses usually only need to move their focusing elements a short distance to go all the way from closest focus to infinity, so might be quite quick focusing, so may not see a very significant difference in focus speed with high performance focus drive system.
Telephoto lenses are more likely to need to move internal elements quite a ways to focus from one end of their range to the other. For this reason, some telephoto lenses have a "focus limiter" that the photographer can use to restrict the lens to working only within part of that range, for better performance. I don't know if your lens has a focus limiter, but you can check and, if it does, learn to use it. (Note: Some specialty lenses... such as macro and ultra large aperture portrait lenses... are designed to focus more slowly. This is because of the very shallow depth of field possible with these lenses, which demands high accuracy. They deliberately use "long throw" focus to prioritize focus precision over speed.)
Your particular lens is an ultra-wide ranging zoom (actually the widest ranging that anyone makes, presently) and it includes a fairly powerful telephoto end. I would expect that makes for challenging focusing. But, with no personal experience using it, I really can't say how well it's focus drive performs or if it's fully up to tracking fast moving subjects.
What you asked about are the choice of AF patterns. My general recommendation is to "always use as few as possible", in order to be in most full control of exactly where the camera and lens focus. However, that's not always practical with all subjects... including BIF. It can be challenging to keep a single point of a fast moving subject. For that reason, there are times the 9-point zone pattern might be preferred. I'd use that when the subjects are well separated from a distant background and/or the background is very plain... such as a blue sky or distant clouds. The problem with a multi-point pattern like the 9-point zone is when they get "distracted" and re-focus on something behind the subject or on an obstruction that's closer. Experiment with 9-point zone to see how well it works and to learn when it gets distracted. With the 19-point AF system you can position that zone freely, since all the points are the same (except when using an f/2.8 or faster lens).
One concern with multi-point is when shooting a larger bird that's relatively close... leaving it to the camera to choose which AF point to use. It will usually focus on the closest wing, which can leave the bird's head and body out of focus. In those cases, it would be better to use a single point.
It also can help to select a somewhat smaller lens aperture, such as f/5.6 or f/8, for more depth of field. This doesn't effect focus performance and can make minor focus errors less worrisome.
Finally, don't beat yourself up too much. I can assure you that there will be a lot of missed focus shots with BIF. We all have them, but we trash them and never speak of them again! It's inevitable there will be some, since all gear and users have their limitations. Practice with your gear and familiarity with your subjects can help, but won't guarantee perfection.
I was out shooting some landscapes with my 5D Mark II one afternoon when I spotted a hawk hunting nearby. It was ignoring me (and some other people in the area) and concentrating on getting lunch. Great opportunity for some BIF! But the camera I had in hand has one of the worst AF systems for any sort of action photography.... Still, I put the longest lens I had (300mm) on it and spent the next 45 minutes taking 100 or 200 shots to get a few "keepers"!
P.S. Exposure mode is entirely up to you... won't effect focus in any way. I used Aperture Priority (Av) for the above and use it often, even for sports (which is prob. 90% of what I shoot). I just keep an eye on my shutter speeds. The aperture I choose determines the "look" of my images... the blur of the background and how well the subject is separated from it, for example. Shutter priority (Tv) is good to insure you freeze movement, but so long as I keep my ISO high enough I'll always get plenty fast shutter speed.
In fact, for the hawk shot above I had ISO 800 and aperture set f/5.6 to shoot the bird perched in the shade of a tree, when it suddenly took off right past me. Because I was in Av mode I ended up with 1/8000 shutter speed! Waaaaayyy faster than was necessary!
I
have been using Auto ISO a lot more often recently. But I
only use it with M...
Never with any of the other auto exposure modes. It's essentially "dueling AE modes" if you use Auto ISO with Av, Tv or P modes. No telling what might happen and no useful purpose that I can think of for combining Auto ISO with any of those other AE modes.
When I need to do so M with Auto ISO allows me to control
both aperture and shutter speed. That's useful!
Last but not least, when shooting birds against a blue sky or even blue sky mixed with some billowy cumulus, you will probably need to dial in some + (plus) Exposure Compensation. Blue sky (and white clouds) on a sunny day will cause a camera to want to under-expose images in any of the auto exposure modes (Av, Tv, P or even M w/Auto ISO). It's especially true when shooting the shaded underside of the bird... note how fortunate I was, that the hawk in the above shot posed so that it was evenly lit across it's whole wingspan!