Hi Ron,
Just to clarify.... Most of the time when we refer to a camera being in horizontal orientation that's the same as "landscape" orientation, or sort of the way the camera naturally falls to hand, or the way it would sit atop a tripod without any adjustments to the tripod... in other words with the longer axis of the image area runs horizontally.
Vertical orientation is often refered to as "portrait" orientation, i.e. with the camera tipped up on end and the longer axis of the image area running vertically.
However there is little reason your camera should focus much differently in horizontal/landscape vs vertical/portrait orientation (some Canon models do allow different auto focus settings depending upon orientation, but the T3/1100D is not one of them). In the viewfinder-based, 9-point AF system, there is some difference in the various points. The center one is cross-type, dual axis, will work with detail running both vertical and horizontally.... The top-center and bottom center AF points (viewing them with the camera in landscape orientation) are horizontal, single axis points, which work best with detail that runs vertically. And the other 6 points are vertical, single axis points that work best with detial that's running horizontal.
However....
Live View uses a completely separate focusing method than what you see through the viewfinder. The image sensor itself is used for LV focusing. It's a contrast detection mehod and slower than the usual phase detection done with the sensors seen in the viewfinder.
Auto focus requires adequate light levels, as well as reasonable subject contrast. A plain wall with no detail (or a plain blue sky, etc.) will give the AF system nothing to lock onto.
You might try switching back to focusing with the viewfinder based system instead of Live View. If you do that, the center point is more sensitive than the others and you mayt have to use only that one in lower light conditions. Or, if there just isn't enough detail to focus upon, you might need to focus manually (Important: if you do, be sure to first turn off AF on the lens.... manually focusing it with AF still turned on can damage the mechanism in the lens. Some lenses do not require this, but unless yours is a "USM" or Ultrasonic Motor lens, you need to turn it off before manual focusing.)
Depending upon the main dial selection, you might be able to choose among the three auto focus modes: One Shot, AI Focus and AI Servo. One Shot is for use with stationary subjects and gives you focus confirmation (note: Live View focus mode only uses this type of focusing). AI Focus really isn't a focus mode at all, set to this the camera is supposed to decide for you whether or not the subject is moving, then switch to the correct focus mode. Personally I've found this causes a delay and would prefer to select my own focus mode, so I never use AI Focus. AI Servo is designed to track moving subjects, as long as you keep the shutter release button half-pressed the camera will continuously update focus... but because it never locks focus you don't get focus confirmation in AI Servo.
The three focus modes are selectable when you have the camera in Av, Tv, P or M exposure modes. When using Movie Mode, it's the same as Live VIew.
In the "Scene" modes such as the "running man" for sports or the "mountain" for a scenic shot, etc., you might not be able to select a focus mode. The focus mode and other things may be dictated by the scene mode (varies, depending upon the particular scene mode).
In Live View, there also is a Quick Focus mode. This is set up using the Q button. In this focus mode, the camera needs to already be set to One Shot, then to focus the camera momentarily stops Live View so that the main AF system can perform the focusing (faster than Live View), then returns to Live VIew screen so you can finish taking the shot.
One thing to note, the camera can overheat while in Live View. The reason this happens is that the mirror is being held up, the shutter is being held open and the imaging sensor is running continously while in Live View. If overheating occurs, there is a warning symbol displayed. If that symbol starts blinking, it's getting more serious, until it reaches a point where the camera shuts down to prevent damage. After it cools down, the camera is usable again. Maybe this is why you can take a shot one way, but later cannot take another shot with the camera oriented differently. Maybe the the camera has gotten overheated and shut down temporarily.
Now, all cameras do have an orientation sensor in them. This is used to properly rotate the images when reviewing them on the back of the camera or on our computer screen, by the Image Stabilization system of some lenses, and for some other purposes. It is possible, though unlikely, that something is wrong with the camera. I'd suggest testing it in both orientation in good daylight with a nice, contrasty subject, such as a detailed brick wall or similar. It should focus the same in either landscape or portrait orientation, whether using Live View or the viewfinder.
So, I'm not saying it's not the fault of the camera. Something could be wrong with it. However, it's usually not the camera, but something we users are doing wrong with them, that causes problems. Hopefully this will help you test the camera some more and see if it's actually a fault with it, or if it's something else.
***********
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
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