Auto Focusing my camera

When using my camera I like to have the auto focus use only one point (so I know where it will focus) and I compose my image around that point.  Normally I have been using points in the upper left or upper right (so that my picture uses the rule of thirds and is composed nicely.  I have felt for a while that my set up (canon t1i + 70-200mm F4 L lens) was having a hard time getting the focusing spot on.  I don't use a tripod as much as I should, but this week I was playing with my camera and moved the auto focus point to the center spot and I felt like a much larger percentage of my photos were tact sharp.  So I started looking online.  Sure enough, the center point is a much better auto focusing point.  In short it detects focus by using both horizontal and vertical information while the rest of the points on my camera use either horizontal or vertical, but not both.  The below information comes from http://www.imaging-resource.com/PRODS/T1I/T1IA4.HTM.  This is a great website with lots of great information in their review.

"The Rebel T1i uses the same AF sensor as the previous XSi, which features enhanced precision at the center point. This AF sensor design achieves more precise focus when used with lenses with apertures of f/2.8 or larger.

SLR autofocus accuracy is governed in part by how far apart you can space the sensor elements for a single AF point from each other. The wider this "baseline," the more accurately the AF point can determine focus. What limits the AF baseline spacing in a camera system is the lens aperture. You can build sensors with wider baselines, but that also restricts the range of lenses they can be used with. By and large, camera manufacturers have set f/5.6 as the minimum aperture their AF systems will work with. Lenses with wide-open apertures smaller than f/5.6 just won't focus. But once the baseline spacing is set, using a faster lens doesn't improve focus accuracy any. That is, unless you add a larger baseline sensor, as is built into the Canon T1i's center AF point. (Note that only the center AF point shows this increased accuracy at f/2.8: The surrounding AF points are all conventional f/5.6 ones.)

The AF points on the Canon T1i cover about 60% of the frame width, and about 50% of the frame height. The illustration above shows the relative spacing between the various AF points, with the distances between them marked in millimeters, at the focal plane. (Illustration courtesy Canon USA.)

While the AF sensor used in the Canon T1i appears to be the same as in the XSi, Canon did tell us that the Rebel T1i's AF system is better at handling extreme out-of-focus conditions than was its predecessor. They didn't have (or couldn't share) details as to how this was accomplished, but there should be fewer situations in which the Canon T1i will have to rack the lens across its entire focus range to figure out where optimum focus is.

While the image sensor in the Canon T1i appears to be nearly identical to that in the 50D (the principle difference being fewer readout channels, yielding a lower continuous-mode frame rate), the AF system is one area where the 50D justifies its higher price. In the 50D, there's an additional cross-type (diagonal) f/2.8 AF sensor for the central point, and all points are cross-type, meaning they're sensitive to both horizontal and vertical detail. Other than its central point, all of the Canon T1i's AF points are line-type, with most being sensitive to horizontal detail, but the top-center and bottom-center ones being sensitive to vertical lines."

http://www.imaging-resource.com/PRODS/T1I/T1IA4.HTM

So know, I will almost always be shooting with the center point, and not the edge ones near as much. I will have to get better at composing my image from the center.