@EWTrout wrote:
Thanks TTMartin & ebiggs1. It appears to be operator error. I was using the One Shot AF mode incorectly. All seems well now.
Let me guess... You were using One Shot, but your subjects were moving.
I see (well, actually "hear") people doing this all the time!
One Shot is for stationary subjects. AI Servo is for moving subjects. (AI Focus isn't really a focus mode at all... I recommend trying to avoid it if possible.)
One Shot starts, achieves focus, stops and locks, then gives you "focus confirmation". Because focus has locked, if the distance to the subject changes at all (i.e., if the subject moves, you move, or you both move), focus will be off.
Focus confirmation is only possible in One Shot mode and includes an audible "beep", if you have it enabled. I photograph sports events, frequently notice other photographers' cameras "beeping", and know for certain that most of their shots will miss focus. (And they will likely blame the camera!)
If there is any movement while you are in One Shot mode, you have to lift pressure off the button fully, then reapply it to make the lens refocus. With even moderately fast moving subjects, you can go crazy trying to do this, and still will likely miss focus in a lot of shots!
Instead, switch to AI Servo, which is continuous focus. So long as you maintain half-press on the shutter release (or the Back Button pressed), the camera will continuously update and maintain focus. It's important to keep your AF point right where you want the camera to focus. I recommend using a single AF point, not All Points and letting the camera suto select, which is another way to get a lot of mis-focused shots.
The above link to tutorial videos is a good starting point. If you are shooting a lot of action/sports, you also might want to try Back Button Focus, which Rudy Winston explains in this article.
Auto focus performance actually involves three important elements: the camera, the lens and the photographer themselves.
Camera AF systems vary... I am not familiar with the T3's, but most Canon have an extra sensitive AF point at the center. It's often best to limit to using only that sensor. This can lead to overly centered photos... one easy solution is to simply not frame the image too tightly, so that you can crop a little later, in post-processing. There are times and places to use multiple AF points... but most of the time it's best to select one yourself. That puts you most in control of where the camera focuses.
Lenses with bigger apertures (f1.4, f1.8, f2, f2.8) deliver more light for the camera's AF system to work with, but unfortunately also tend to be bigger, heavier and more expensive. Also, there are different AF drive systems in lenses. Less expensive lenses generally use a micro motor (piezo motor) that's slower, noisier and may not be as accurate or consistent, but is usually plenty good enough for many types of photography. Canon USM or ultrasonic drive lenses are faster, more accurate and consistent, as well as quieter... but also tend to be the most expensive. (Sigma makes HSM lenses and Tamron USD, which are both similar to Canon USM.). Canon now also offers STM or stepper motor lenses, which aren't quite as fast as USM, but are faster than micro motor and are smoother, quieter than both the other types. Generally speaking, USM are most ideal for sports/action and STM are particularly good for videography. STM lenses cost a little more than micro motor lenses... though usually not a great deal more.
The photographer themself also is a big part of AF performance. Learning your camera, practicing with it, and experience with the type of subject you're photographing all can make a big difference. So, study those tutorial videos and your camera manual, maybe get one of the guide books available for it, then get out and shoot a lot. There's no substitute for practice!
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
