I hope this article (From Chuck Westfall of Canon) on the 1D4 AF Expansion system, (which is as far as I know the only EOS body that allows expansion to jump up to the entire array of AF points, in the 1D4's case, that's all 45 af points of expansion) can solve your debate?
http://learn.usa.canon.com …_auto_focus_article.htmlp
Although this Article and excerpt are discussing the 1D4, most of the behavior explaining AF expansion applies to all Canon bodies with expansion. The only thing totally specific to the 1D4 is CFN 8-3, ie; the part where expansion will track beyond the initial selected and limited surrounding AF points. CFN 8-1 an 8-2 are identical in behavior to other bodies.
Chuck Westfall
Canon USA Professional Products Engineering & Solutions Division. wrote:
AF point expansion
With C.Fn III -8 and a manually-selected AF point, AF can be expanded by using the AF points adjacent to the selected AF point. If the manually-selected AF point cannot achieve focus, focus can be achieved with one of the adjacent (expanded) AF points. The range of expandable AF points is shown in the diagram given here:
These settings can be effective for shooting a moving subject that is difficult to track with just one AF point. The [1: Left/right AF point] and [2:Surrounding AF points] options are the same as the EOS-1D Mark III. The [3: All 45 points area] setting is a very powerful option that’s new to the 1D series. It effectively allows the photographer to manually select the starting point for focus detection while gaining the ability to track the subject anywhere it moves within the 45-point Area AF ellipse indicated on the focusing screen. (By comparison, when automatic focusing point selection is combined with AI Servo AF, the subject must be initially detected with the center AF point.) Moreover, C.Fn III-8-3 allows the camera to display the active focusing point in the viewfinder as it follows the subject during a burst shooting sequence.
Here’s how it works: When C.Fn III-8-3 is combined with AI Servo AF, up to 18 AF points adjacent to the manually-selected AF point will become active. Each time the subject moves to another AF point, the expanded AF points will also shift to surround the AF point covering the subject. In response to the subject's movement, the AF point lit in red also shifts continuously. Since the clump of active focusing points can shift automatically according to subject movement, all 45 focusing points are effectively available for burst mode shooting.
As long as the subject is within the Area AF frame, autofocusing of the subject will continue. This is the same logic as with automatic AF point selection. However, since the focusing is concentrated over a smaller area than during automatic AF point selection, focusing a particular subject is easier. Also, if the main AF point loses the subject by a considerable amount during automatic AF point selection, all 45 AF points are used to search for the subject.
With C.Fn III-8-3, the 18 expanded AF points are instead used to search for the subject. For example, even if there is an object closer than the main subject, the camera can still quickly and securely track the main subject.
If One-Shot AF is set and focus cannot be achieved with the manually-selected AF point, focus is executed with an expanded area of up to 44 AF points. Subject detection is done in three instant steps, with 6, 18, then 44 AF points. Because the manually-selected AF point is where the subject is supposed to be, focusing is easier with the AF points closer to the main AF point. Better yet, DPP 3.7.3 and higher can display the active focusing point for each image when C.Fn III-8-3 is used together with manual focusing point selection, even when the active focusing point differs from the manually selected focusing point.
C.Fn III-8-1, III-8-2, or III-8-3 must be on for C.Fn III-4 [AI Servo AF tracking method] to become fully effective. The exception occurs when using Automatic Focusing Point Selection, in which case both settings for C.Fn III-4 are effective.
Regardless of which C.Fn III-4 setting is used, the C.Fn III-2 setting [AI Servo tracking sensitivity] generally takes priority. However, if a subject blocks the main subject when the C.Fn III-4 setting is 0, the camera refocuses on the new subject regardless of the C.Fn III-2 setting.
